@@motoiq they are to ignorant to realize the effect is "exhaust pulse scavenging" the exhaust pulses traveling through the pipe pulls th next pulse behind it.
Sound is a form of pressure, and the closer that pressure is to the cylinder the quicker the charge in the cylinder is induced at above atmospheric levels much like a turbocharger. So technically it is a form of back pressure from my understanding. Two cycles most certainly can be tuned for torque as well, and not all expansion chambers are created equally.
@@louisedayao1562 No just think of how a speaker works, like a subwoofer. The sound is made from the speaker cone moving in and out, together with the speaker enclosure (box) it changes the pressure inside the enclosure. If you have ports on the box the pressure is forced out the holes and it gives the low bass more punch. Its all to do with air pressure.
I knew a guy that made chambers in his garage. He would test them, then do it over, and over again. Later, Honda hired him to design pipes for them. Expansion chambers important! Another guy I knew from the same small beach town worked on shocks. Now he is a renowned shock guy. All this started from them doing motocross.
@@motoiq Name drop. For pipes, it was Paul Turner (sp) and for shocks it was Mike McAndrews. A search using expansion pipes, and shocks with each respective name will reveal. We all lived in a tiny beach town called La Selva Beach, CA Be good Stan, I love your stuff! My freshly rebuilt Honda A20A3 will roar again soon with an MFactory diff. and I got the last bearing and synchro kit in the USA for that trans!! Good old machine guy from Portland, OR did the work. He is retired now and doing a limited amount of work, because he just can't quit! I am a very lucky man! Who knows? Maybe one day I will send you an EJ25! It still runs great in my 2003 Forester with 218,000 miles!
Is that the Mike McAndrews from mountain bike stuff? He was the first guys to do digressive damping on bikes. I designed the Elsworth ICT suspension. ellsworthbikes.com/
Used such tubes over 40y ago with some success, as my then police report had said, but never before did I hear someone describing their functioning in such easy sentences. 👍🏻🙏🏻 Can’t imagine what I would have done with that understanding. It’s similar to my guitar playing. Back then I was in a band, but clueless. Now I‘m only playing at home, but understand the music theory behind.
2 strokes love pulse tuning. My 125 iame Leppard was a different animal . Run a 42 teeth rear sprocket with a 65 mm exhaust sleeve. Or run it with a 70 mm sleeve. Placing the engine in its sweet spots due to load and rpm.
It reality the wave from the pipe makes”back pressure” in itself to help push unburned fuel and air back into the chamber. So it kinda does need “back pressure” just not in the sense people usually think of it.
As the exhaust pulse expands and compresses in with the cones It also supercharges the following combustion cycle continuously, which gives the “power band”…
The amplitude of the return “pressure” (there’s that word again) wave is dictated by the angle of the conical baffle. The return pressure wave occurs long before the end of the conical baffle.
its all about the exhaust sound wave from the reverse cone that creates a force field and blocks the exhaust port holding the supercharged air/fuel mix from the crankcase from escaping the exhaust port allowing the piston to compress the mixer the 2nd time in the combustion chamber...the exhaust gases are force out i of the cylinder by the supercharged incoming air/fuel mix from the crankcase..the powerband starts when the frequency of the sound wave equals the firing of the incoming from the transfer ports..
Which is a pressure wave that pushes the mixture BACK into the cylinder. ALSO CALLED BACKPRESSURE. This guy ain't a 2 stroker, he should stick to 4's until he actually knows his stuff. He's got no excuse other than lack of actual experience or interest.
Its all about port velocity and scavenging, in 1981 vamaha came out with the first variable exhaust port for mx bikes powered by centrifugal force creating a dual range power band called ypvs
If you want to really see how an expansion chamber works, take a pipe like that and sing into it at different notes and when you find the right one, the pipe will suck the wind right out of you. It's wild!
What are your thoughts on pipe pressure at high rpm speeding up the wave velocities and helping widen the effective rpm range that the pipe is effective for? Same reason a rich main jet kills top end, rich, cool exhaust gases slowing the wave speeds.
To be more clear, For a properly designed expansion chamber the size of the orifice at the end of the conical baffle, (what you call the back cone) controls the pressure and pressure bleed in the chamber.
It controls the back pulse and a properly designed exit is done to maximize the strength of the reflected wave and minimize the mean backpressure. That is the function of a stinger or nozzle style exits.
@@motoiq The amplitude of the return “pressure” (there’s that word again) wave is dictated by the angle of the conical baffle. The return pressure wave occurs long before the end of the conical baffle.
The length of the end cone, diffuser and belly and the overall volume of the pipe are more critical than the angle for tuning the pipe, the angle is just a function of the other dimensions.
@@motoiq I'm sorry but you're just plain wrong here, see Gordon Blairs SAE work iirc on two stroke expansion chambers for example. A certain gas density and pressure in the pipe is important, if lower was simply better we'd all just use multiple stingers from the mid section or a perforated mid section to bleed everything off as freely as possible. But that won't work well, because contrary to four strokes a certain amount of pressure in the pipe is necessary for proper function, you actually want even more with high performance engines tuned for a narrow peak power band than with general use engines as a rule of thumb. Besides increasing the strength of the pulse there's also an important "plugging" effect and a lot of sht going on even major manufacturers weren't easily able to wrap their engineers heads around. Everybody who has designed and tested pipes knows that after a certain outlet diameter there's losses. Substantial losses at that. And it's definitely not due to the Stinger somehow interfering with the pulse generation by the baffle, exhausts with mid section stinger for packaging reasons and "suspended baffle" designs that bleed through a small annular gap suffer from the exact same effect and constructions with different baffle arrangements than cone-cone do so as well. All in all the fluid dynamics and sht in a two stroke are orders of magnitude more complicated and intricate than people usually think. But it's absolutely certain that lower pressure doesn't just equal more power. Therefore - in two strokes backpressure is indeed important.
@@heinzhaupthaar5590motoiq was on about volume and saying the angles don't matter?? Ive never heard any renowned 2 stroke/expansion chamber expert mention volume. You calculate what size of pipe you need based on powerband average temp yata yata, volume is just the outcome i don't think theres any inference to come from it at all. volume is shapeless, And the principles depend exactly on shape and geometry.
It does use backpressure... The first cone increases in volume which creates a lower pressure which allows mixture from the crankcase to push up through the transfers, the belly section inbetween the two cones is a intermediary section that stops a sudden change in pressure, the next cone reduces volume and increases pressure which is utilised when the pulse bounces off the end of the stinger to create a higher pressure wave (backpressure) to push mixture leaking out of the exhaust back into the cylinder along with some dregs of fuel in the exhaust fumes. Backpressure is half of the action of a expansion chamber.. to say its unwanted or not how it works is beyond wrong.
Actually via Bernoulli's principal the pressure increases in a cone shaped diffuser. Also we are taking about mean backpressure, not periodic pressure pulses are we explained simply in the video. Mean pressure raises the floor of the gas exchange portion of the two stroke cycle which reduces the amount of work that can be recovered as power.
@@motoiq i see. I had it backwards. I was thinking without bernoulli's principle which would mean that mixture would be sucked from the tranfers due to the low pressure at the diffuser and then the wave reflected back squeesed going into the baffle would push leaking mixture and leftover mixture back in but it must be the opposite. Diffuser causes high pressure which prevents leakage and then the baffle causes a lower pressure to pull mixture from the crankcase. If thats not it then im having a word with god and his mad fluid dynamics he created with expansion chambers.
You got it half right! The cone diffuser thing is counterintuitive but you are trading velocity for pressure which is what Bernoulli's principle is all about.
Not true, the exhaust chamber tuning is separate from the back pressure requirements. 35 years or building Yamaha and Kawasaki two strokes, we found that eliminating back pressure completely caused a drop in roughly 150 RPMs and inconsistent bottom end response. Factory Pipe Products, mass manufacturer of aftermarket and stock exhaust for Kawasaki Yamaha Polaris they used to make mufflers and people racing in limited class found that they were losing top speed after installing it, and they did a mass recall where they installed this insert to bring back up the back pressure. Top Speed Returned. What you can also notice is that most aftermarket exhaust have the end section of the pipe significantly smaller tube ID than the one at the exhaust port, like in this go kart you are showing. This down size Increases back pressure. Back pressure does matter on a two stroke, at least for top their competition and power development, On a four stroke it does not matter and you want none if sound levels are an issue.
What I said is totally true, no piston engine works better with retained backpressure. it limits the amount of work that can be recovered from the cycle. Adding a restrictor to that pipe was a bandaid for improper pipe design, back pressure increases the speed of the reflected wave in the chamber. Pipe design uses wave propagation and timing, it's all about that, you want all of that with the least amount of backpressure possible. The exit of the expansion chamber is called the stinger, a rule of thumb is that its length is 6x or more of its diameter, It not only serves as the exhaust exit but also is part of the pressure wave tuning. If the stinger exit area is less than the exhaust port, it is that the exhaust gas diffuses, loses a lot of energy, and greatly cools down. It no longer needs as big of an exit cross-section. If that pipe was designed correctly and tested correctly, it would work better with no restrictors.
@@motoiq I agree with all the principles and theory’s your laying out, but I can’t go against the evidence I have seen with my own eyes. Another example i had was in 2001 when Yamaha recently the GPR 1200 watercraft, We started r&d on one of them, we started with the exhaust as we always do on a new vehicle, and started eliminating back pressure by modifying the muffler, and what do you know we yet again dropped RPM and top speed. Now this was completely stock, so Yamaha designed the exhaust system, yet when you went to get rid of the back pressure, we lost performance. Maybe Yamaha didn’t design the exhaust correctly? Possibly? I was only part of the race team so we weren’t involved in the actual development of the power plant and only received new models a few month before the public, but I trust that OEM’s don’t purposely do things to lose power (aside from Meeting sound/pollution regulations) because on the showroom floor they are competing with all the other OEMs and even saying you have 5 more horsepower helps sales. I really enjoy all your video and in-depth break down on lots of things. But on this subject we’ll just have to agree to disagree.
You have to understand the principles of how two stroke pipes and physics work, getting rid of a muffler and back pressure, if the chamber was designed for the back pressure of the muffler you will loose power. That being said a chamber designed only to make power and not to comply with noise regulations with minimal back pressure is going to make even more power. Look up the pressure volume cycles and the thermodynamics of two strokes and see. I am not making this stuff up. Of course your observations are correct but the reasons why are not.
www.mecholic.com/2016/02/working-of-two-stroke-engine-with-pv-diagram.html, if you retain pressure inside the engine due to back pressure, you raise the bottom of the PV curve and loose area inside the PV loop, the area inside is the work or power. Two stroke expansion chambers are like musical instruments, if you do stuff to change something, they can get off tune very easily. By reducing the backpressure by removing the muffler, you are reducing the density and temperature of the gas inside the chamber, this means that the acoustic and pressure waves will travel slower getting the pipe off tune. A simple way of getting it back without adding backpressure may be by playing with the stinger length or adding a stinger before the new muffler. A lot of times this won't package in the space you have though.
Yes and no. The stinger is there to keep the pressure wave from just yeeting itself out the back without ever getting reflected. We could call that backpressure of a sort. But importantly, the stinger must absolutely not be allowed to restrict bulk gas flow and cause an increase in average chamber pressure. That results in a loss of top-end power and, more importantly, a very aggressive increase in engine temperature. Most pipe makers will leave just a bit of low-end power on the table in order to ensure the stinger isn't too small, with a margin. Especially when a muffler is involved, that restriction (and additional restriction over time caused by "the spooge") is accounted for. I'm guessing you know all this, but it's good information for future visitors to these comments.
@@motoiq it does this by restriction, allowing pipe pressure above atmosphere for the blocking wave to supercharge the cylinder. This blocking wave is literally backpressure! @JETZcorp did a pretty good job explaining, but it worth mentioning that larger diameter restrictor actually tends to improve bottom end power by weakening this blocking wave, which is destructive at 2/3 peak RPM. tighter tends to drive peak power higher until you become thermally limited as @JETZcorp wisely said
It does it by acting like a wave interference device, that's why the ratio of diameter to length is important. If it was a restrictor, they would not use a stinger but a hole. What JETZcorp said was correct as well.
@@motoiq I know Honda's power valve and Kawasaki KIPS do it. They have a chamber by the exhaust port that is either opened or closed to exhaust flow, effectively varying the tuned length of the header pipe. Honda's earliest PV did this only with no change to port timing or geometry. I believe KIPS was the first to combine the chamber system with a variable-height port. Obviously they weren't doing anything as radical as tromboning the pipe, although I've read some PWCs inject water into the pipe to change temperature and therefore the pulse timing.
The lengths and sizes of them cones And the exhaust port timing Dictate the rpm the ending makes its power In that karts example 125cc and about 50hp And rev over 14000rpm Back pressure is needed the end of the pipe called the stinger is only about 20mm To help contain the glasses so the pulses can work It’s a fine balancing act To big and the pulses are not strong enough to work. To small and to much exaust remains and the engine melts Normally eroding the piston over the exhaust port So saying they don’t like back pressure is not 100% right
At certain RPM you need a little back pressure to prevent over scavenging. The great tuners from the 50s onward realized this including factory engineers and started regulating back pressure. Read Blairs engineering book on two stroke engines. Sorry dude.
You never add backpressure, at least if you are correctly designing the pipe, you should change the pipe tuning instead. Adding backpressure can be a quick and dirty way to change the rpm range that a pipe is in tune by speeding the reflected wave by changing the gas density and temperature but it's not the best way. Adding backpressure reduces the amount of work that can be recovered by each cycle, if you don't believe me look up the PV cycle of a two-stroke engine, it is thermodynamics, sorry dude.
All engines like that…. It’s called the scavenging effect….. using spent exhaust gasses to create a negative pressure wave behind them helping to pull the next charge of exhaust out of the cylinder making the upward work of the exhaust stroke less work on the engine freeing up HP and increasing efficiency. No engine likes back pressure but a good exhaust that helps scavenging will make more power than open headers
Since you are seemingly unwilling to test and understandably you are unwilling to take my word for it I have pasted a paragraph from Gordon Jennings (Two stoke tuners handbook page 64) whom mentioned in an earlier response. Having gotten past the business of diffusers and baffle-cones, we can proceed onward to the lengths and diameters of lead-in and outlet pipes. The latter will, if the rest of the expansion chamber is proportioned fairly closely in accord with the advice I have given, have a diameter between .58- and .62-times that of the lead-in pipe, and a length equal to 12 of its own diameters. It is simply a pressure-bleed resister, which prevents the free escape of exhaust gases from inside the expansion chamber and thereby creates a backpressure to enhance the port-plugging efforts of the wave reflected by the baffle- cone.
I am not even talking about pipe design theory, you are, I am talking about mean back pressure. You understand mean means average right? I am talking about VE, thermodynamics and gas exchange of the engie being hampered my excessive mean backpressure, not debating the importance of pulse timing, etc. I am talking about the entire engine, you are taking about just the pipe.
@@motoiq Wow what a disconnect, your entire short titled “2-strokes don’t need backpressure” was about the pipe and pulses within it and now to avoid admitting your title was inaccurate as I said, when confronted with evidence from a source you cited, you deflect to VE? And FYI the mean pressure in the pipe is positive thus creating back pressure. I’d be happy to show you pressure traces I’ll be at Rok Fest in Vegas next week look for me in the scrutineering tent I’ll be the on wearing a shirt that say’s “Official”
I am not deflecting, I am just stating facts. What I said is accurate. Instead of trying to argue with me, your time would be better spent studying how the rest of the engine works. Look up the pressure-volume diagram for a two-stroke engine and you can easily see that if you raise the pressure base, less work or power can be recovered from each cycle. That is just one of the reasons but it's the simplest and easiest to visualize and there should be many examples for you to look at online. You understand how the chambers work 100% but are not looking at the entire engine. I didn't make this stuff up, every second-year engineering student learns this and it was well-understood over 100 years ago. If you look at the pressure profile of a chamber, you can see that pressure goes negative in significant parts of the cycle, especially in the diffuser area of the pipe. You probably know this. If you squeeze down the pipe exit to the point where you are intentionally increasing mean pressure, the timing of the pipe becomes faster due to the increased density and temperature of the gas inside and sometimes this gives favorable results. It's more of a tuning Band-Aid as a pipe with the desired tuning with lower mean pressure is going to be better for power recovery and production. Like I said, it's not just me making something up but well well-understood phenomenon that I learned in school and applied later in life.
@@motoiq ah gotcha. You guys definitely know what you're doing with a kart that pristine. This is a great video and explains this better than I can. I'll definitely be sending this to my driver's that don't understand the value of a clean pipe
But the smaller stinger you have the larger effect that pulse has but with the consequence of extra heat in the combustion chamber that could be potentially catastrophic.
@@motoiq I read a very interesting old article where they inverted the stinger (put it entirely inside the chamber). They noticed almost no change in performance, but it made the engine as quiet as a typical MX style silencer. I believe this didn't catch on because cleaning the inside of the pipe became extremely difficult, and it looked doofy, and silencers were required most places anyway.
Okay yes, but there's more to it. With this pipe we see a very skinny end pipe going to the muffler. Back in the days of anarchy, they ran those "stingers" with no muffler at all. Free fliwing, right? Well, it was found that the stinger needs to be a certain diameter and/or length to get the correct back pressure (wider and shorter provides more flow, thinner and longer provides less. Why would this be? When the exhaust pulse arrives at the rear cone, that pressure wave can be largely lost if the exit is too free-flowing, resulting in a loss of low-end power. If the stinger is too restrictive, then it will fail to provide enough gas flow, in a very simplistic 4-strokish way, which causes loss of top-end power AND an extremely pronounced increase in engine heat. My dad has an old '73 100cc Yamaha MX with a pipe that someone fitted a too-large stinger back in the day. The bike is impossibly weak in the low-end, even for a 100, but finally finds it's power again right around 1.2 million rpm.
rough formula to start is stinger length is 6x diameter or more. Another trick for a larger collector is to have it extend into the chamber, this way you can get away with a bigger one.
@@motoiq *lol* I DO know how it works, but i seriously doubt you know. I´ve been tuning twostrokes for about 35years now, and if i would go by your example and NOT having backpressure the pipe would be very very hard to tune at correct rpm as the heat would be lost. The 'same' pipe length would tune at very low rpm. And by that, trying to tune at say 7500rpm(250 dirbike) it would be so short that you would need to have such abrubt angels on the different sections that you would actually make it EVEN HARDER to tune it due to the pulse is 'getting lost' 0.2-0.3bar pressure is a normal value to aim on, this allows the pulses to fluctuate above and below atmospheric pressure, and by this makes the engine breathe properly due to pulses. And getting this correct, the pressurelevel actually helps at higher rpm(above tuned point) to push back some of the fresh charge for a longer duration where he pulses have started to fade(above tuned rpm) Hence many misunderstands this. 'Higher backpressure gives more power' Yes it might, but less torque down low. Correct backpressure is the key!
You don't understand thermodynamics and have only a practical knowledge of how a chamber works. Purposely increasing the mean backpressure to do something is a crutch to proper pipe/port/intake design.
@@motoiq do not judge me! I know a lot but might have problems with language. But i do see that you are trying to use conquer technique in this conversation.
@@motoiq so, the pulse going back has pressure? If yes is it negative or positive?(for sure positive). If not, what is that pulse. There no such pulse of nothing. Like pulse of blood, electric, water, air etc. so what is that. If you can name it what is that pulse, i will ask again if it has pressure.
@@motoiq with reference to what or where? When the pulse coming back what’s the element of the pulse? Is it plasma, air, water? Did it bring or it has pressure with that pulse coming back?
Yes but you're wrong in a way, the stinger pipe is a restrictor, and it needs to be tuned for best performance. Too big and the engine just flows a lot of air and makes no power out of it. It's about controlling the density of the charge trapped in the cylinder.
The stinger is made to reduce the shock wave energy leaving the chamber so more can be reflected. It does not serve as a restrictor. If you restrict an expansion chamber with too small of an outlet, the EGT's go up and you can seize the motor. The stinger can be smaller than the exhaust port because the exhaust diffuses and cools and thus shrinks in volume in the chamber. It's good to have a low mean pressure in the chamber with strong properly timed pulses for the best powerband and overall power. You can use a smaller in diameter stinger to increase the mean pressure in the chamber and speed the wave up for tuning, but that can reduce overall engine efficiency. I can't get into this detail in the 60 seconds you have for a reel.
Interesting for sure. There's more to this exhaust system though. Must be California. After all that tuned exhaust it has another muffler or sound suppressor.
No direct intrinsic increase to cylinder pressure, but a proper exhaust-scavenging technique will effectively reduce how much 'pushing' the piston has to do, as well as 'pull' fresh fuel/air mixture into the cylinder (4stroke speak here - I'm not 100% certain how it translates to 2stroke) ... so, more/better combustion will probably create more cylinder pressure sustained if not peak
@@davidsandberg3109 The pulses from the expansion chamber can be somewhere in the neighborhood of 5-8psi (positive or negative) depending on pipe design. When the negative pressure wave arrives, all the ports are open, and that negative pressure can make it all the way to the intake/carb on a reed-valve engine. You can imagine how useful 8psi of suction at the intake port would be. In really hot engines with big port timing, this drives intake volumes more than crankcase pumping, and is the reason reed valves finally beat rotary valves for absolute performance. This huge negative pulse deliberately over-scavenges the cylinder at low RPM, and then the 5-8psi positive pulse from the baffle cone comes in and crams the air back into the exhaust port. In ideal circumstances this does act like a sort of resonance supercharger. It's worth noting that the guy who invented the expansion chamber, Walter Kaaden, was one of the main engineers behind the V1 pulse-jet program in WW2. Using the very same principles, pulse jets make pretty significant compression with resonance alone and no moving parts.
So close but here's what you missed lol. So as far as back pressure goes you actually are balancing the stinger diameter with ignition advance to keep egts around 1150°f. 1200 means a broken ring land normally. For the most power after the expansion chamber and ports are set up you do actually play with the stinger size to make the most and it can make a pretty big difference. I know in a few iv played with we were able to gain good gains throughout the rpm range by going a bit smaller on the stinger but doing so raises egts same as ignition advance. The best way to do it is with a ignition computer that monitors combustion chamber pressure to modulate the timing to a stinger that's too small egt wise at peak power. This would give you the most under the curve and not loose anything up top to a point. Ot works by raising the pressure in the pipe and the density of the exhaust gases and also the over scavenged air fuel mix. You do less over scavenging before it and harder boost through the exhaust. Different animals to the 4strokes we are so used to.
@@mitchellgould7405 oh I'm far from a 2stroke expert I just know a few and know theres more to the story. Honestly 2stroke theory is one of those subjects that hurts my brain after the first hr. I'm a more boost kinda guy. But I do remember Alex at 2strokestuffing went into some detail about it and tuning it back in the day. I understand more than most about them but I'm not even close to Alex's level. I just know these guys are amazing at getting a small hint and learning alot for a vid down the road. Frankly this guys probably forgot more than I learned and I have a feeling you might aswell lol.
You can only say so much in the 60 seconds you tube lets you have to make a short! Where do you put the EGT probe? For Karting we monitor cylinder head temp but not egt commonly. It does make sense though because temp makes a big difference in pipe tuning.
"acKtcuaLly" 🤓, in the case of back pressure in an expansion chamber, the part behind the reflection cone, often referred to as a "stinger" (the part where it decreases to a relatively very small diameter pipe), will give you more power, the smaller the outlet is. This is because it stops the pressure from bleeding out as quick, thus enhancing the effect of the resonance by building more pressure. This will obviously only work to a point, because if you restrict it too much, your engine temperature will rise catastrophically, melting a hole in the piston 😂
That us not necessarily true all the time, what is most important is the ratio of stinger diameter to length. The right diameter is usually 50-70% of the head pipe diameter and its length 12-13x it's diameter. Most of the time this will get you in the ballpark.
it likes 1/2 to 1 psi back pressure, not exactly nothing. If you don't believe me then try different stinger sizes and measure the pressure at the belly and see which pressure the engine likes the best
@@motoiq back pressure is the baseline that the pipes return waves ride on and so is very important. There could be engines that don't want any back pressure but each engine would have to be tested with different pressures to see what it likes. All normal 2 stroke pipes (expansion chambers) have a stinger that allows some back pressure. Must be a reason for that.
All engines don't want backpressure, you raise the bottom of the PV Curve due to backpressure there is less energy or power to recover, don't mistake having some backpressure speeding the reflected wave and the engine liking it as the engine liking backpressure. More power can be found by improving the pipe design to have the proper pulse timing with less backpressure. The stinger isnt there to increase backpressure but to make it more difficult for the reflected wave to leave the chamber while allowing exhaust gas to easily pass through. That is why the minimum dimension of the stinger is 6x the exit hole size.
Not narrow but the correct description of what is going in regards to extracting work from the pressure volume cycle. If the pressure at the bottom of the cycle is higher due to backpressure, less work or power is extracted. vocal.media/journal/what-is-the-difference-between-pv-diagram-of-two-and-four-stroke-engines
The man enriches the automotive knowledge greatly!
2 stroke has almost nothing to do with automative tho 😅 not alot of 2 stroke cars
I never understood this till now. Thanks for oversimplifying it!
No internal combustion engine "likes" backpressure.
A lot of people think that based on the tons of dumb comments on our exhaust videos!
lord yes
I got friends thst can not grasp that an engine is just an air pump and they want to breathe.
@@AtomicReverend There are still people out there who believe the "kadency" theory of exhaust flow. This was disproven many, many years ago.
@@motoiq they are to ignorant to realize the effect is "exhaust pulse scavenging" the exhaust pulses traveling through the pipe pulls th next pulse behind it.
Resonance is everything in a 2 stroke!
Sound is a form of pressure, and the closer that pressure is to the cylinder the quicker the charge in the cylinder is induced at above atmospheric levels much like a turbocharger. So technically it is a form of back pressure from my understanding. Two cycles most certainly can be tuned for torque as well, and not all expansion chambers are created equally.
This is an exhaust, intake resonance tuning is something else.
@@motoiq Please do a video on intake resonance tuning 🙏
"sound" is in fact the human interpretation of pressure :)
@@davidsandberg3109 don’t you mean vibration not pressure
@@louisedayao1562 No just think of how a speaker works, like a subwoofer. The sound is made from the speaker cone moving in and out, together with the speaker enclosure (box) it changes the pressure inside the enclosure. If you have ports on the box the pressure is forced out the holes and it gives the low bass more punch. Its all to do with air pressure.
I never understood what those cones did. Now I do! Thanks
I knew a guy that made chambers in his garage.
He would test them, then do it over, and over again.
Later, Honda hired him to design pipes for them.
Expansion chambers important!
Another guy I knew from the same small beach town worked on shocks. Now he is a renowned shock guy. All this started from them doing motocross.
Was it Toomi in Lawndale I think? He used to make pipes for factory teams. Old school cut and try. EC Burt too.
@@motoiq Name drop. For pipes, it was Paul Turner (sp) and for shocks it was Mike McAndrews. A search using expansion pipes, and shocks with each respective name will reveal. We all lived in a tiny beach town called La Selva Beach, CA Be good Stan, I love your stuff! My freshly rebuilt Honda A20A3 will roar again soon with an MFactory diff. and I got the last bearing and synchro kit in the USA for that trans!! Good old machine guy from Portland, OR did the work. He is retired now and doing a limited amount of work, because he just can't quit! I am a very lucky man! Who knows? Maybe one day I will send you an EJ25! It still runs great in my 2003 Forester with 218,000 miles!
So you are the one that got the last parts! People been talking about you.
Is that the Mike McAndrews from mountain bike stuff? He was the first guys to do digressive damping on bikes. I designed the Elsworth ICT suspension. ellsworthbikes.com/
@@motoiq YES! I remember him toying with shocks when I was a "kid". He and Paul were heavy into motocross. You mean Ellsworth stole your design!
Best explanation I've ever heard. Finally makes sense.
Used such tubes over 40y ago with some success, as my then police report had said, but never before did I hear someone describing their functioning in such easy sentences.
👍🏻🙏🏻
Can’t imagine what I would have done with that understanding.
It’s similar to my guitar playing. Back then I was in a band, but clueless. Now I‘m only playing at home, but understand the music theory behind.
2 strokes love pulse tuning. My 125 iame Leppard was a different animal . Run a 42 teeth rear sprocket with a 65 mm exhaust sleeve. Or run it with a 70 mm sleeve. Placing the engine in its sweet spots due to load and rpm.
Is there anything this guy doesn’t know? 👍. Great video straight to the point.
Motor professor Mike must be awesome to drink a beer with.
Mike looks like the happiest man alive!
Has a chill tone in his voice. Can’t imagine him being angry. Lol
I love the way you explain things
best explanation of pulse tuning that I've ever heard.
Thank you for the knowledge 💯
& they say you learn something new everyday, thanks dude ✌️😎
We do this with 2 stroke radio control airplanes. It’s called a tuned pipe. Works really well.
I grew up riding dirt bikes and I never knew understood this concept until I started playing with gas Rc cars.
It reality the wave from the pipe makes”back pressure” in itself to help push unburned fuel and air back into the chamber. So it kinda does need “back pressure” just not in the sense people usually think of it.
Backpressure in the sense of mufflers isn't good for it.
Dynamic > static.
Always wondered why two strokes had the bulge in the exhaust. Now I know 😊
Very informative. Glad to be following you sir...
This is the only channel where I have notifications turned on
I love shifter karts, they are brutal .
true... the most simplest explaination
This guy is an encyclopedia
Saving this under point provers
that man always smile, cool man
Very well explained. A plethora of information. Mike your brain is more valuable than moon rocks. Lol
I've tried to explain this to an idiot and he refuses to believe it.😂😂😂😂😂😊❤
i just want to hang out with this guy.
As the exhaust pulse expands and compresses in with the cones It also supercharges the following combustion cycle continuously, which gives the “power band”…
Awesome I’d love to see more kart content !
Well explained 🤙
That go cart looks badass, that could be alotta fun,
Sound waves and pressure waves have a lot of correlations in exhaust tuning. You don’t have to be loud to be fast.
This is the way 🔥
The amplitude of the return “pressure” (there’s that word again) wave is dictated by the angle of the conical baffle. The return pressure wave occurs long before the end of the conical baffle.
The angle, length, and exit geometry are all very important, do some research, I didn't make this stuff up.
its all about the exhaust sound wave from the reverse cone that creates a force field and blocks the exhaust port holding the supercharged air/fuel mix from the crankcase from escaping the exhaust port allowing the piston to compress the mixer the 2nd time in the combustion chamber...the exhaust gases are force out i of the cylinder by the supercharged incoming air/fuel mix from the crankcase..the powerband starts when the frequency of the sound wave equals the firing of the incoming from the transfer ports..
Homie looks baked
Can we reduce the camera man's back pressure?
Sonic resonance is the term you are looking for👍
Which is a pressure wave that pushes the mixture BACK into the cylinder.
ALSO CALLED BACKPRESSURE.
This guy ain't a 2 stroker, he should stick to 4's until he actually knows his stuff. He's got no excuse other than lack of actual experience or interest.
So many "car people" really need to watch this. Countless times I've heard people say a engine needs back pressure to run.
if it needs back pressure why can it run without an exhaust
it will run but it will loose as much as 50% of the power. You need to watch and comprehend the video.
It needs an expansion chamber. The muffler will hurt it though.
@@fish_R_stinky69 muffler wont hurt it
@@11sjv It'll make less power with a muffler lol.
I prefer the 2 strokes with reeds and powervalves :)
By the way it is actually rocket science or really pulse jet science from WW2
I cut the stock muffler on our zy250 and but a different homemade pipe and it gained so much power that it would blow the tires loose easily
Bro you teach real shit.
Its all about port velocity and scavenging, in 1981 vamaha came out with the first variable exhaust port for mx bikes powered by centrifugal force creating a dual range power band called ypvs
You learn stuff every day....
If you want to really see how an expansion chamber works, take a pipe like that and sing into it at different notes and when you find the right one, the pipe will suck the wind right out of you. It's wild!
Resonance 🙂
Shock wave re-entry.
Exactly
Oversimplified for those who don't know anything yet but it's correct and i like that
For shorts, you only have 60 seconds.
@@motoiq true that
Have had personal experience with 50cc two-stroke engines and a pipe makes a big difference
Yeah you can probably double the power.
@@motoiq way past double the power
As more knowledge is smoking the cameraman's breathing intensifies. 😂
What are your thoughts on pipe pressure at high rpm speeding up the wave velocities and helping widen the effective rpm range that the pipe is effective for? Same reason a rich main jet kills top end, rich, cool exhaust gases slowing the wave speeds.
That's a lot of how the pipe works
I have a chainsaw with no muffler that i run a couple times a week for over a year. Seems to be just fine albeit quite loud.
Wow ‼️
Like a pulsejet?
To be more clear, For a properly designed expansion chamber the size of the orifice at the end of the conical baffle, (what you call the back cone) controls the pressure and pressure bleed in the chamber.
It controls the back pulse and a properly designed exit is done to maximize the strength of the reflected wave and minimize the mean backpressure. That is the function of a stinger or nozzle style exits.
@@motoiq The amplitude of the return “pressure” (there’s that word again) wave is dictated by the angle of the conical baffle. The return pressure wave occurs long before the end of the conical baffle.
The length of the end cone, diffuser and belly and the overall volume of the pipe are more critical than the angle for tuning the pipe, the angle is just a function of the other dimensions.
@@motoiq
I'm sorry but you're just plain wrong here, see Gordon Blairs SAE work iirc on two stroke expansion chambers for example. A certain gas density and pressure in the pipe is important, if lower was simply better we'd all just use multiple stingers from the mid section or a perforated mid section to bleed everything off as freely as possible.
But that won't work well, because contrary to four strokes a certain amount of pressure in the pipe is necessary for proper function, you actually want even more with high performance engines tuned for a narrow peak power band than with general use engines as a rule of thumb. Besides increasing the strength of the pulse there's also an important "plugging" effect and a lot of sht going on even major manufacturers weren't easily able to wrap their engineers heads around.
Everybody who has designed and tested pipes knows that after a certain outlet diameter there's losses. Substantial losses at that. And it's definitely not due to the Stinger somehow interfering with the pulse generation by the baffle, exhausts with mid section stinger for packaging reasons and "suspended baffle" designs that bleed through a small annular gap suffer from the exact same effect and constructions with different baffle arrangements than cone-cone do so as well.
All in all the fluid dynamics and sht in a two stroke are orders of magnitude more complicated and intricate than people usually think. But it's absolutely certain that lower pressure doesn't just equal more power. Therefore - in two strokes backpressure is indeed important.
@@heinzhaupthaar5590motoiq was on about volume and saying the angles don't matter??
Ive never heard any renowned 2 stroke/expansion chamber expert mention volume. You calculate what size of pipe you need based on powerband average temp yata yata, volume is just the outcome i don't think theres any inference to come from it at all.
volume is shapeless, And the principles depend exactly on shape and geometry.
Supercharge!
I like when they suck at the Boddum end
It does use backpressure...
The first cone increases in volume which creates a lower pressure which allows mixture from the crankcase to push up through the transfers, the belly section inbetween the two cones is a intermediary section that stops a sudden change in pressure, the next cone reduces volume and increases pressure which is utilised when the pulse bounces off the end of the stinger to create a higher pressure wave (backpressure) to push mixture leaking out of the exhaust back into the cylinder along with some dregs of fuel in the exhaust fumes.
Backpressure is half of the action of a expansion chamber.. to say its unwanted or not how it works is beyond wrong.
Actually via Bernoulli's principal the pressure increases in a cone shaped diffuser. Also we are taking about mean backpressure, not periodic pressure pulses are we explained simply in the video. Mean pressure raises the floor of the gas exchange portion of the two stroke cycle which reduces the amount of work that can be recovered as power.
@@motoiq i see. I had it backwards.
I was thinking without bernoulli's principle which would mean that mixture would be sucked from the tranfers due to the low pressure at the diffuser and then the wave reflected back squeesed going into the baffle would push leaking mixture and leftover mixture back in but it must be the opposite.
Diffuser causes high pressure which prevents leakage and then the baffle causes a lower pressure to pull mixture from the crankcase. If thats not it then im having a word with god and his mad fluid dynamics he created with expansion chambers.
You got it half right! The cone diffuser thing is counterintuitive but you are trading velocity for pressure which is what Bernoulli's principle is all about.
Not true, the exhaust chamber tuning is separate from the back pressure requirements. 35 years or building Yamaha and Kawasaki two strokes, we found that eliminating back pressure completely caused a drop in roughly 150 RPMs and inconsistent bottom end response.
Factory Pipe Products, mass manufacturer of aftermarket and stock exhaust for Kawasaki Yamaha Polaris they used to make mufflers and people racing in limited class found that they were losing top speed after installing it, and they did a mass recall where they installed this insert to bring back up the back pressure. Top Speed Returned.
What you can also notice is that most aftermarket exhaust have the end section of the pipe significantly smaller tube ID than the one at the exhaust port, like in this go kart you are showing. This down size Increases back pressure.
Back pressure does matter on a two stroke, at least for top their competition and power development, On a four stroke it does not matter and you want none if sound levels are an issue.
What I said is totally true, no piston engine works better with retained backpressure. it limits the amount of work that can be recovered from the cycle. Adding a restrictor to that pipe was a bandaid for improper pipe design, back pressure increases the speed of the reflected wave in the chamber. Pipe design uses wave propagation and timing, it's all about that, you want all of that with the least amount of backpressure possible. The exit of the expansion chamber is called the stinger, a rule of thumb is that its length is 6x or more of its diameter, It not only serves as the exhaust exit but also is part of the pressure wave tuning. If the stinger exit area is less than the exhaust port, it is that the exhaust gas diffuses, loses a lot of energy, and greatly cools down. It no longer needs as big of an exit cross-section. If that pipe was designed correctly and tested correctly, it would work better with no restrictors.
@@motoiq MIKE DROPPED
@@motoiq I agree with all the principles and theory’s your laying out, but I can’t go against the evidence I have seen with my own eyes.
Another example i had was in 2001 when Yamaha recently the GPR 1200 watercraft, We started r&d on one of them, we started with the exhaust as we always do on a new vehicle, and started eliminating back pressure by modifying the muffler, and what do you know we yet again dropped RPM and top speed.
Now this was completely stock, so Yamaha designed the exhaust system, yet when you went to get rid of the back pressure, we lost performance. Maybe Yamaha didn’t design the exhaust correctly? Possibly? I was only part of the race team so we weren’t involved in the actual development of the power plant and only received new models a few month before the public, but I trust that OEM’s don’t purposely do things to lose power (aside from Meeting sound/pollution regulations) because on the showroom floor they are competing with all the other OEMs and even saying you have 5 more horsepower helps sales.
I really enjoy all your video and in-depth break down on lots of things. But on this subject we’ll just have to agree to disagree.
You have to understand the principles of how two stroke pipes and physics work, getting rid of a muffler and back pressure, if the chamber was designed for the back pressure of the muffler you will loose power. That being said a chamber designed only to make power and not to comply with noise regulations with minimal back pressure is going to make even more power. Look up the pressure volume cycles and the thermodynamics of two strokes and see. I am not making this stuff up. Of course your observations are correct but the reasons why are not.
www.mecholic.com/2016/02/working-of-two-stroke-engine-with-pv-diagram.html, if you retain pressure inside the engine due to back pressure, you raise the bottom of the PV curve and loose area inside the PV loop, the area inside is the work or power. Two stroke expansion chambers are like musical instruments, if you do stuff to change something, they can get off tune very easily. By reducing the backpressure by removing the muffler, you are reducing the density and temperature of the gas inside the chamber, this means that the acoustic and pressure waves will travel slower getting the pipe off tune. A simple way of getting it back without adding backpressure may be by playing with the stinger length or adding a stinger before the new muffler. A lot of times this won't package in the space you have though.
2 strokes need backpressure to function, ask yourself what the stinger/tailpipe restrictor part of the expansion chamber does
Yes and no. The stinger is there to keep the pressure wave from just yeeting itself out the back without ever getting reflected. We could call that backpressure of a sort. But importantly, the stinger must absolutely not be allowed to restrict bulk gas flow and cause an increase in average chamber pressure. That results in a loss of top-end power and, more importantly, a very aggressive increase in engine temperature. Most pipe makers will leave just a bit of low-end power on the table in order to ensure the stinger isn't too small, with a margin. Especially when a muffler is involved, that restriction (and additional restriction over time caused by "the spooge") is accounted for.
I'm guessing you know all this, but it's good information for future visitors to these comments.
@@JETZcorp yes, we call that backpressure
Its not a restrictor, it is a way to let the exhaust gas out and still get strong reflection off the back cone.
@@motoiq it does this by restriction, allowing pipe pressure above atmosphere for the blocking wave to supercharge the cylinder. This blocking wave is literally backpressure! @JETZcorp did a pretty good job explaining, but it worth mentioning that larger diameter restrictor actually tends to improve bottom end power by weakening this blocking wave, which is destructive at 2/3 peak RPM. tighter tends to drive peak power higher until you become thermally limited as @JETZcorp wisely said
It does it by acting like a wave interference device, that's why the ratio of diameter to length is important. If it was a restrictor, they would not use a stinger but a hole. What JETZcorp said was correct as well.
So many self proclaimed experts losing their minds right now.
Could you delve into 2 stroke "power valves"?
They change the port timing for a wider powerband, like variable cam timing in a 4-stroke.
@@motoiq Some of them also change expansion chamber volume and tuned length.
really what motors?
@@motoiq I know Honda's power valve and Kawasaki KIPS do it. They have a chamber by the exhaust port that is either opened or closed to exhaust flow, effectively varying the tuned length of the header pipe. Honda's earliest PV did this only with no change to port timing or geometry. I believe KIPS was the first to combine the chamber system with a variable-height port. Obviously they weren't doing anything as radical as tromboning the pipe, although I've read some PWCs inject water into the pipe to change temperature and therefore the pulse timing.
Resonance
The lengths and sizes of them cones And the exhaust port timing Dictate the rpm the ending makes its power
In that karts example 125cc and about 50hp And rev over 14000rpm
Back pressure is needed the end of the pipe called the stinger is only about 20mm
To help contain the glasses so the pulses can work
It’s a fine balancing act To big and the pulses are not strong enough to work.
To small and to much exaust remains and the engine melts Normally eroding the piston over the exhaust port
So saying they don’t like back pressure is not 100% right
the stingers function is to prevent the pulse from escaping while letting exhaust gas out.
Mike I'd love some of your insight on BMW M50 based motors
They are really strong!
At certain RPM you need a little back pressure to prevent over scavenging. The great tuners from the 50s onward realized this including factory engineers and started regulating back pressure.
Read Blairs engineering book on two stroke engines. Sorry dude.
You never add backpressure, at least if you are correctly designing the pipe, you should change the pipe tuning instead. Adding backpressure can be a quick and dirty way to change the rpm range that a pipe is in tune by speeding the reflected wave by changing the gas density and temperature but it's not the best way. Adding backpressure reduces the amount of work that can be recovered by each cycle, if you don't believe me look up the PV cycle of a two-stroke engine, it is thermodynamics, sorry dude.
All engines like that…. It’s called the scavenging effect….. using spent exhaust gasses to create a negative pressure wave behind them helping to pull the next charge of exhaust out of the cylinder making the upward work of the exhaust stroke less work on the engine freeing up HP and increasing efficiency. No engine likes back pressure but a good exhaust that helps scavenging will make more power than open headers
Since you are seemingly unwilling to test and understandably you are unwilling to take my word for it I have pasted a paragraph from Gordon Jennings (Two stoke tuners handbook page 64) whom mentioned in an earlier response.
Having gotten past the business of diffusers and baffle-cones, we can proceed onward to the lengths and diameters of lead-in and outlet pipes. The latter will, if the rest of the expansion chamber is proportioned fairly closely in accord with the advice I have given, have a diameter between .58- and .62-times that of the lead-in pipe, and a length equal to 12 of its own diameters. It is simply a pressure-bleed resister, which prevents the free escape of exhaust gases from inside the expansion chamber and thereby creates a backpressure to enhance the port-plugging efforts of the wave reflected by the baffle- cone.
I am not even talking about pipe design theory, you are, I am talking about mean back pressure. You understand mean means average right? I am talking about VE, thermodynamics and gas exchange of the engie being hampered my excessive mean backpressure, not debating the importance of pulse timing, etc. I am talking about the entire engine, you are taking about just the pipe.
@@motoiq Wow what a disconnect, your entire short titled “2-strokes don’t need backpressure” was about the pipe and pulses within it and now to avoid admitting your title was inaccurate as I said, when confronted with evidence from a source you cited, you deflect to VE? And FYI the mean pressure in the pipe is positive thus creating back pressure. I’d be happy to show you pressure traces I’ll be at Rok Fest in Vegas next week look for me in the scrutineering tent I’ll be the on wearing a shirt that say’s “Official”
I am not deflecting, I am just stating facts. What I said is accurate. Instead of trying to argue with me, your time would be better spent studying how the rest of the engine works. Look up the pressure-volume diagram for a two-stroke engine and you can easily see that if you raise the pressure base, less work or power can be recovered from each cycle. That is just one of the reasons but it's the simplest and easiest to visualize and there should be many examples for you to look at online. You understand how the chambers work 100% but are not looking at the entire engine. I didn't make this stuff up, every second-year engineering student learns this and it was well-understood over 100 years ago. If you look at the pressure profile of a chamber, you can see that pressure goes negative in significant parts of the cycle, especially in the diffuser area of the pipe. You probably know this. If you squeeze down the pipe exit to the point where you are intentionally increasing mean pressure, the timing of the pipe becomes faster due to the increased density and temperature of the gas inside and sometimes this gives favorable results. It's more of a tuning Band-Aid as a pipe with the desired tuning with lower mean pressure is going to be better for power recovery and production. Like I said, it's not just me making something up but well well-understood phenomenon that I learned in school and applied later in life.
That's a clean shifter. That seats verticality looks painful though.
Very short driver, my daughter.
@@motoiq ah gotcha. You guys definitely know what you're doing with a kart that pristine. This is a great video and explains this better than I can. I'll definitely be sending this to my driver's that don't understand the value of a clean pipe
Actually it's pretty new, she just moved up from a 125cc TAG.
🤯👍
But the smaller stinger you have the larger effect that pulse has but with the consequence of extra heat in the combustion chamber that could be potentially catastrophic.
You can have the stinger go into the pipe chamber. That way you can have a larger stinger for less backpressure but maximum reflection.
@@motoiq I read a very interesting old article where they inverted the stinger (put it entirely inside the chamber). They noticed almost no change in performance, but it made the engine as quiet as a typical MX style silencer. I believe this didn't catch on because cleaning the inside of the pipe became extremely difficult, and it looked doofy, and silencers were required most places anyway.
I can believe that. I know stingers have been put inside pipes to shorten the overall length for packaging reasons.
My experience is that if you remove the glassfiber filling in the last muffler after the cone you will not be able to hit rpm above 10 000rpm
It depends on a lot of things but generally you want the uninterrupted stinger to be 6XD
my 80cc 2 stroke ran better after i covered the exhaust a bit, more responsive throttle for some dang reason
'Needed Backpressure' is total internet fallacy B.S.
Okay yes, but there's more to it. With this pipe we see a very skinny end pipe going to the muffler. Back in the days of anarchy, they ran those "stingers" with no muffler at all. Free fliwing, right? Well, it was found that the stinger needs to be a certain diameter and/or length to get the correct back pressure (wider and shorter provides more flow, thinner and longer provides less. Why would this be? When the exhaust pulse arrives at the rear cone, that pressure wave can be largely lost if the exit is too free-flowing, resulting in a loss of low-end power. If the stinger is too restrictive, then it will fail to provide enough gas flow, in a very simplistic 4-strokish way, which causes loss of top-end power AND an extremely pronounced increase in engine heat. My dad has an old '73 100cc Yamaha MX with a pipe that someone fitted a too-large stinger back in the day. The bike is impossibly weak in the low-end, even for a 100, but finally finds it's power again right around 1.2 million rpm.
rough formula to start is stinger length is 6x diameter or more. Another trick for a larger collector is to have it extend into the chamber, this way you can get away with a bigger one.
Well, you need some backpressure to build heat in the pipe, to make it tune in wanted rpm band.
Without backpressure the heat(energy) is spilled away.
That is not how it works although heat has an effect on pipe tuning.
@@motoiq *lol*
I DO know how it works, but i seriously doubt you know.
I´ve been tuning twostrokes for about 35years now, and if i would go by your example and NOT having backpressure the pipe would be very very hard to tune at correct rpm as the heat would be lost.
The 'same' pipe length would tune at very low rpm.
And by that, trying to tune at say 7500rpm(250 dirbike) it would be so short that you would need to have such abrubt angels on the different sections that you would actually make it EVEN HARDER to tune it due to the pulse is 'getting lost'
0.2-0.3bar pressure is a normal value to aim on, this allows the pulses to fluctuate above and below atmospheric pressure, and by this makes the engine breathe properly due to pulses.
And getting this correct, the pressurelevel actually helps at higher rpm(above tuned point) to push back some of the fresh charge for a longer duration where he pulses have started to fade(above tuned rpm)
Hence many misunderstands this.
'Higher backpressure gives more power'
Yes it might, but less torque down low.
Correct backpressure is the key!
You don't understand thermodynamics and have only a practical knowledge of how a chamber works. Purposely increasing the mean backpressure to do something is a crutch to proper pipe/port/intake design.
@@motoiq do not judge me!
I know a lot but might have problems with language.
But i do see that you are trying to use conquer technique in this conversation.
Don't judge me either, I know a lot.
But if you didn't have the first cone, wouldn't you avoid overscavenging the fuel/air mixture?
You need the diffusion to help get the stuff out of the cylinder.
@@motoiq gotcha.
👍
What's that kinda exhaust?
Stock TM spec exhaust.
So would it matter if you got rid of the silencer?
The pipe is homolgated under the rules with it so that would be illegal for the class it runs in so I would never test it that way.
In short, it’s called back pressure.
No its a back pulse, not the mean backpressure in the exhaust, the same as headers.
@@motoiq so, the pulse going back has pressure? If yes is it negative or positive?(for sure positive). If not, what is that pulse. There no such pulse of nothing. Like pulse of blood, electric, water, air etc. so what is that. If you can name it what is that pulse, i will ask again if it has pressure.
I answered your question. Mean backpressure needs to be less.
@@motoiq with reference to what or where? When the pulse coming back what’s the element of the pulse? Is it plasma, air, water? Did it bring or it has pressure with that pulse coming back?
Yes but you're wrong in a way, the stinger pipe is a restrictor, and it needs to be tuned for best performance. Too big and the engine just flows a lot of air and makes no power out of it. It's about controlling the density of the charge trapped in the cylinder.
The stinger is made to reduce the shock wave energy leaving the chamber so more can be reflected. It does not serve as a restrictor. If you restrict an expansion chamber with too small of an outlet, the EGT's go up and you can seize the motor. The stinger can be smaller than the exhaust port because the exhaust diffuses and cools and thus shrinks in volume in the chamber. It's good to have a low mean pressure in the chamber with strong properly timed pulses for the best powerband and overall power. You can use a smaller in diameter stinger to increase the mean pressure in the chamber and speed the wave up for tuning, but that can reduce overall engine efficiency. I can't get into this detail in the 60 seconds you have for a reel.
Is the pulse that pushes some of the air/fuel mix back into the cylinder not considered a type of back pressure?
No
@@motoiq so the expansion chamber doesn’t use PRESSURE to push the mixture BACK into the cylinder?
That is not backpressure in the sense that we are talking about, we are talking about mean pressure.
Not back pressure. It's called scavenging.
Interesting for sure. There's more to this exhaust system though. Must be California. After all that tuned exhaust it has another muffler or sound suppressor.
You can see the whole exhaust very clearly. It's a muffler, its straight through perforated core, no backpressure.
When the exhaust diffuses doesnt it increase in pressure due to bernoulli’s principle? The cross section gets larger.
it does but we are talking about mean backpressure affecting the floor and energy recovery potential of the PV curve.
Does the pressure wave also increase cylinder pressure? I believe that helps combustion for more power and cleaner emissions.
It doesnt help combustion other than the reasons of preventing overscavanging during overlap
No direct intrinsic increase to cylinder pressure, but a proper exhaust-scavenging technique will effectively reduce how much 'pushing' the piston has to do, as well as 'pull' fresh fuel/air mixture into the cylinder (4stroke speak here - I'm not 100% certain how it translates to 2stroke) ... so, more/better combustion will probably create more cylinder pressure sustained if not peak
In this way yes!
@@davidsandberg3109 The pulses from the expansion chamber can be somewhere in the neighborhood of 5-8psi (positive or negative) depending on pipe design. When the negative pressure wave arrives, all the ports are open, and that negative pressure can make it all the way to the intake/carb on a reed-valve engine. You can imagine how useful 8psi of suction at the intake port would be. In really hot engines with big port timing, this drives intake volumes more than crankcase pumping, and is the reason reed valves finally beat rotary valves for absolute performance. This huge negative pulse deliberately over-scavenges the cylinder at low RPM, and then the 5-8psi positive pulse from the baffle cone comes in and crams the air back into the exhaust port. In ideal circumstances this does act like a sort of resonance supercharger. It's worth noting that the guy who invented the expansion chamber, Walter Kaaden, was one of the main engineers behind the V1 pulse-jet program in WW2. Using the very same principles, pulse jets make pretty significant compression with resonance alone and no moving parts.
So close but here's what you missed lol. So as far as back pressure goes you actually are balancing the stinger diameter with ignition advance to keep egts around 1150°f. 1200 means a broken ring land normally. For the most power after the expansion chamber and ports are set up you do actually play with the stinger size to make the most and it can make a pretty big difference. I know in a few iv played with we were able to gain good gains throughout the rpm range by going a bit smaller on the stinger but doing so raises egts same as ignition advance. The best way to do it is with a ignition computer that monitors combustion chamber pressure to modulate the timing to a stinger that's too small egt wise at peak power. This would give you the most under the curve and not loose anything up top to a point. Ot works by raising the pressure in the pipe and the density of the exhaust gases and also the over scavenged air fuel mix. You do less over scavenging before it and harder boost through the exhaust. Different animals to the 4strokes we are so used to.
your focus on egt is secondary to piston crown temp relation to stinger diametre
@@mitchellgould7405 oh I'm far from a 2stroke expert I just know a few and know theres more to the story. Honestly 2stroke theory is one of those subjects that hurts my brain after the first hr. I'm a more boost kinda guy. But I do remember Alex at 2strokestuffing went into some detail about it and tuning it back in the day. I understand more than most about them but I'm not even close to Alex's level. I just know these guys are amazing at getting a small hint and learning alot for a vid down the road. Frankly this guys probably forgot more than I learned and I have a feeling you might aswell lol.
@@oliverscorsim they are a bit of fun, and everything is a balancing act. Keep up the enthusiasm
You can only say so much in the 60 seconds you tube lets you have to make a short! Where do you put the EGT probe? For Karting we monitor cylinder head temp but not egt commonly. It does make sense though because temp makes a big difference in pipe tuning.
If you didnt have backpressure, then all the energy would just be lost in the first pulse overscavenging, leaving none left for the blocking wave
👽👍🏻✨
"acKtcuaLly" 🤓, in the case of back pressure in an expansion chamber, the part behind the reflection cone, often referred to as a "stinger" (the part where it decreases to a relatively very small diameter pipe), will give you more power, the smaller the outlet is. This is because it stops the pressure from bleeding out as quick, thus enhancing the effect of the resonance by building more pressure. This will obviously only work to a point, because if you restrict it too much, your engine temperature will rise catastrophically, melting a hole in the piston 😂
That us not necessarily true all the time, what is most important is the ratio of stinger diameter to length. The right diameter is usually 50-70% of the head pipe diameter and its length 12-13x it's diameter. Most of the time this will get you in the ballpark.
it likes 1/2 to 1 psi back pressure, not exactly nothing. If you don't believe me then try different stinger sizes and measure the pressure at the belly and see which pressure the engine likes the best
It's not the engine liking pressure, it's changing the tune of the pipe.
@@motoiq back pressure is the baseline that the pipes return waves ride on and so is very important. There could be engines that don't want any back pressure but each engine would have to be tested with different pressures to see what it likes. All normal 2 stroke pipes (expansion chambers) have a stinger that allows some back pressure. Must be a reason for that.
All engines don't want backpressure, you raise the bottom of the PV Curve due to backpressure there is less energy or power to recover, don't mistake having some backpressure speeding the reflected wave and the engine liking it as the engine liking backpressure. More power can be found by improving the pipe design to have the proper pulse timing with less backpressure. The stinger isnt there to increase backpressure but to make it more difficult for the reflected wave to leave the chamber while allowing exhaust gas to easily pass through. That is why the minimum dimension of the stinger is 6x the exit hole size.
They still need backpressure though.
No they don't
It's a tuned pipe
what would that high pressure wave be as it slams back into the cylinder? engineered back pressure at the moment u want it
It's not the mean pressure in the chamber which is the actual backpressure.
@@motoiq youre just creating a narrow definition of back pressure.
Not narrow but the correct description of what is going in regards to extracting work from the pressure volume cycle. If the pressure at the bottom of the cycle is higher due to backpressure, less work or power is extracted. vocal.media/journal/what-is-the-difference-between-pv-diagram-of-two-and-four-stroke-engines
How could you say that when all pipes always have a smaller stinger than header?
The exhaust gas looses a lot of its temperature, more than half, and its volume decrease.