I believe I have commented on a video in the past however I really have to give you credit here. I am currently in school for Automotive Technology and i've been watching these older videos to help me better understand the content in my engines class. Anyways just wanted to say that I love the videos as they have helped me alot!
Im in school to finish my Millwright apprenticeship and had a difficult time understanding valve overlap. This video made a huge difference for me, Thanks!
some the exaust gases will find their way out of the cylinder because of the inertia (high rpm) (basicly reverse inertia filling) and create a negative pressure a well tuned exaust will increase this effect. and then when the intake valve opens you accelerate your fuel air mixture because of the pressure differance a well tuned intake runner will increase this effect. the point im trying to make is overlap is not just about getting exuast out its also about starting the accelration of the fuel air mixture (which takes some time because of the mass of the air) before the piston is on its way down again. If you start the acceleration before TDC the piston will not start to create a negative pressure and thus decreaseing pumping loses and so on (affects port size where top speed of the fuel air mixture is). (english = 2nd language)
Also, the reason your vacuum goes to crap with more overlap is you are essentially creating a low-rpm vacuum leak. With the throttle nearly closed the only other place the engine can draw from is the still-open exhaust valve(s) and what it's trying to draw back in isn't air, it's exhaust, so it's even worse the a normal vacuum leak. This is why some racing engine with really large, aggressive camshafts can't idle below ~1000rpm. You must open the throttle more at idle (and usually give it more spark advance and a richer air/fuel mixture) so that the gases have more momentum to reduce this reversion and so the more of what is being drawn in is air so that the engine can produce enough power to idle. Basically, overlap makes you engine really inefficient at an idle but it does all sorts of wonderful things for torque and horsepower as engine speed increases. This is why variable valve timing is such a powerful tool. We can get the best of both without the drawbacks of either.
Great explanation. I've heard 2 engines that had so much overlap that you could hear it at idle. An Isuzu 4 cylinder (Chevy Luv pickup) and a Pantera (Ford 351 Cleveland). Both were really fast.
Well explained👍🏽, Also Intake And Exhaust Valve overlap occurs while the piston movement is minimal ,at about 15degrees before/after TDC Or BDC When the piston is barely moving.
Thanks for the HQ lesson, very educating. What do you think of 12.5:1 compression ratio 4T engine? Will it have smaller amount of "trapped" exhaust gas inside combusting chamber compared to a 10:1 compression ratio engine?.. if yes, do you think this is the reason why manufacturer produce higher compression ratio engine (to increase the volume efficiency)?
Hi, Thanks for explaining all this. The only one I came across was Jafmobile explaining cam separation angle, very good but He didn't do many drawings, and I like drawings ; - ) Actually I find things much easier to understand with diagrams and pictures. So have I got it right that a Narrow lobe seperation angle will let the valves stay open longer and give a Larger valve overlap? Or is it the other way round? Best Wishes Dee
The exhaust blow-down pressure is different because of the back pressure from the turbine interface. Its all a bit wishy washy when it comes to turbos - but we're gonna get there.
This is verry different from set up to set up if you look a stock cars with small turbos and small exaust housings the back pressure can be a lot larger than the intake pressure. this is reflected in the camshaft design! stock turbo cams usually have smaller amount of overlap on stock cars a nice curve is important so a small turbo and a mild cam i used. BUT! a really good thing if you are tuning an engine (like alot) is to have quite a really large turbo(in some cases to large) with a large exuast housing and a manifold with long tubes you will have a lower back pressure which might equal or is lower then intake pressure(boost!) then you can run a cam more similar to a NA motor. this will give you more power with lower boost. but i some cases its better with a short manifold and a small turbo with small houseing so you can get boost eariler in the powerband! which is good for track cars or milder street cars! the basics! (english = 2nd language)
@@dirtygarageguy yes, usually discussed more in marine applications because of the water cooled heat exchangers on exhaust, but I'm curious if reversion could explain my current situation where I have an engine with scored cylinder walls and metal dust above both valves in the cylinder head, but none in the intake manifold or catch can that's on the pcv system. My hypothesis is that with such a heavy cam profile (hotrod street cam) and the engine never having turned over 3000rpm, I never had the intake charge velocity to overcome reversion during overlap and that's how the fine powder ended up where I found it.
@@dirtygarageguy thanks for the response. It was a real head scratcher for me. Still baffled on exactly what went wrong in this engine but the physics involved are so neat to ponder. I'll have it torn down by the builder at the end of the week when I get back into town
How can purge be possible as the remaining exhaust gas will be positive pressure and the intake will be in vacuum, it won't push it out of the exhaust it will suck it back toward the vacuum (intake). At 2:07 you mention with the remainder of the unexhausted gas remaining it's still 1.5 bar.
Because of momentum, and the venturi effect. The exhaust gases are flowing out of the cylinder, this causes its own pressure system. If the engine is tuned correctly the intake has a resonance that promotes inertial filling from the intake side.
Why not split the intake to separate intakes and have a different cam profile for each side. One side opens early to allow purging but doesn't get fuel sprayed through it and then the other opens up later when the exhaust is closed and sprays fuel through it.
Building a high(er) compression VW 2.0 8v engine. Currently running a sport 260 cam with 1° overlap, and the engine lopes at idle a little. Planning on going with a 276 cam, with 7° overlap, but fear that itll be too much for an OBD2 engine with stock tune (for now). Driveability is not a concern since its a weekend cruiser, potential race car, aslong as it can idle. Is 7° overlap too much for a stock tune obd2 engine?
would a turbo help or hinder this condition? i mean, you have more back pressure against the turbine, yet forced air to help expel the gases, i hope that makes sense, seya in a bit ;)
This is verry different from set up to set up if you look a stock cars with small turbos and small exaust housings the back pressure can be a lot larger than the intake pressure. this is reflected in the camshaft design! stock turbo cams usually have smaller amount of overlap on stock cars a nice curve is important so a small turbo and a mild cam i used. BUT! a really good thing if you are tuning an engine (like alot) is to have quite a really large turbo(in some cases to large) with a large exuast housing and a manifold with long tubes you will have a lower back pressure which might equal or is lower then intake pressure(boost!) then you can run a cam more similar to a NA motor. this will give you more power with lower boost. but i some cases its better with a short manifold and a small turbo with small houseing so you can get boost eariler in the powerband! which is good for track cars or milder street cars! (english = 2nd language)
This is probably too involved for a simple comment, but how does a person calculate dynamic compression? One of my problems is the cam manufacturer doesn't have all the info on the cams I installed. The only part I know is that overlap was significantly more on the new cam. Trying to work out if I really need high octane fuel, or if the overlap is enough to let me use something like 93 octane. Currently running at about 105 with a mix of 91 and some MMT based additive, but was running real race fuel of 105 before. The race fuel was too difficult to get, so trying the Torco additive. Would be happier with 93 pump gas if I could safely run it. I've done web searches on dynamic compression, and never really found anything that explained what I needed to know to compute what my engine is really running right now.
the dynamic compression ratio change alot over the rpm range because of inertia filling at high rpm(which give you higher pressure) or leak out of the intake valve on low rpm! you also need to take ignition timing into account(it plays a huge part in what kind of fuel you can run). what you wanna do is calculate the maximum pressure in your cylinder.
jompa zx I'm hoping that the question and your reply will fuel Matt to explain it to us so I can maybe, finally grasp this concept. It does make sense that compression will change as the air gets flowing, the fuel/air mix does have inertia so the fuel charge can slightly raise the cylinder pressure at bdc before the valve closes. And the same sort of thing from the overlap as was mentioned in this video. My intake harmonics kind of suck, wasn't much room to get a longer intake so I think it is like 3rd or 4th order and pretty high in the rpms, kind of prevents good overlap performance at the lower rpms.
What you forget is the intake valve close after BDC this is necessary on high rpm otherwise the cylinder will not be full and on high rpm you might get some inirtia filling so maybe you get over 100% VE this will make you dynamic compression ratio higher. BUT on low rpm you will fill the cylinder at BDC or just right after BDC so if you have high rpm cams you will push some fuel/air mixture out of the intake valve again this will hurt your dynamic compression ratio!
+Mini GP Racing Dynamic or effective compression ratio is a complex subject with many variables. Obtain a dial indicator, degree wheel, evaluate and plot your exact camshaft(s) intake/exhaust lobe profiles, lift, duration, event timing, lobe separation angle, intake/exhaust lobe center-line and overlap. Static or dynamic or effective compression ratio is only one variable of many influencing fuel octane value requirement/choice. There several excellent articles and calculators available online. members.uia.net/pkelley2/DynamicCR.html ftlracing.com/dynamiccr.htm www.rbracing-rsr.com/camshaft.html www.rbracing-rsr.com/comprAdvHD.htm www.wallaceracing.com/dynamic-cr.php www.wallaceracing.com/eff-cr.php www.google.com/search?num=100&newwindow=1&ei=SrZaW8-DH9u_0PEP5fGv2Aw&q=dynamic+compression+calculation&oq=dynamic+compression+calculation&gs_l=psy-ab.12..0i22i30k1l4.7590.8683.0.11082.3.3.0.0.0.0.88.239.3.3.0....0...1c.1.64.psy-ab..0.3.238...35i39k1j0i13i30k1j0i13i5i30k1.0.K6Ti8_GvHA4 www.google.com/search?num=100&newwindow=1&ei=VQBaW4j1M8OezwLiqpLABA&q=dynamic+compression+ratio+calculator+and+camshaft+selection+utility&oq=dynamic+compression+calculator&gs_l=psy-ab.1.5.0i71k1l8.0.0.0.13397.0.0.0.0.0.0.0.0..0.0....0...1c..64.psy-ab..0.0.0....0.a3vCzBtPkDw
Pretty close, but exhaust doesn't close at TDC. Exhaust closing EC is usually around the same degree ATDC as intake opens BTDC in a 4 stroke petrol engine.
If you're going to discuss valve overlap then of course it matters and if you did 30 seconds of research on it before making the video you would have discovered that. You're a qualified engineer and should know better. And spare me the Delboy tantrum over the correction, I've been a mechanic too long to take any notice of that crap so suck it up princes.
LOL nice comeback. "You're a qualified engineer and should know better." - don't really know what this means? I should know better? What difference would it make if I draw it ending at 0deg? After rewatching the video bit - I do say "it'll close at TDC or something like that" On the spiral diagram the line slightly passes TDC. That fact is that is doesn't matter for this video. You will start shouting "the cirlce isn't perfectly round". I'm sure there are some engines out there where the closing of the exhaust is TDC - probably. Oh look - loads of 'theoretical diagrams' with TDC as the exhaust valve closing. (there's loads of 'correct' diagrams as well, don't get me wrong) www.researchgate.net/profile/Rupesh_Gupta12/publication/309011115/figure/fig8/AS:416252737671175@1476253794862/Valve-timing-diagram-shown.png image.slidesharecdn.com/24514675-2-internal-combustion-engine-120405123335-phpapp01/95/24514675-2internalcombustionengine-31-728.jpg?cb\u003d1333629439 dzmm.info/wp-content/uploads/valve-timing-diagram-for-diesel-engine-theoretical-of-4-stroke-cycle-present-gallery-likewise-553-640x346.png Until I do videos on the effect of valve opening and closing you're just being a padantic prick. Its the 'pretty close' patronising comment that is the problem, on a video that is the basics. This is all on the fly and you bang on about delbollocks but the guy is talking complete made-up bullshit. When I say 'something like that' what I'm not saying is ALWAYS TDC. Regardless, you wanna knit-pick then that fine, but there are people out there that this is all new to.
Well that was just a load of noise and 3 useless images. Theoretical valve opening my foot. As an engineer if you cant spend a matter of seconds to put the words valve overlap into google to get it right then WTF are you doing teaching mechanics. All those speeches you give about correcting all the bad mechanical advice on TH-cam and getting the facts right for viewers and saying time and time again and then you go do something is lame as that. Stop whining and just admit you cocked it up.
Kind effort but the explanation contains mistakes. Valve overlap allows the low pressure caused by the exhaust gases leaving the cylinder to help the fresh charge enter the cylinder. Not what this video describes saying that the fresh air pushes the exhaust gases out. When the intake valve opens before TDC there is not yet any dynamics to the inlet charge to enter and push the exhaust away. Inlet charge will start entering the cylinder either by the negative pressure of the exhaust gases ( so that the exhaust helps suck the inlet charge) or by the negative pressure created by the piston motion after TDC. To understand it better, Imagine that if you open the intake valve before TDC at low RPM sometimes you get the opposite. Exhaust gases enter the inlet manifold and pollute the charge. This often happens when you do not install a proper exhaust system tuned for your particular valve timing. So to summarize Valve overlap is there to use the exhaust dynamics ( created by your exhaust system) to suck air charge before the piston goes downwards. Effectively increasing the air charge induction period. This has also a small effect on cleaning the cylinder better from exhaust gases since the increased air charge sucked inside the cylinder occupies more space and pushes any left overs out. That is also the reason the exhaust valve after TDC delays to close.
There is, it the inertia of the intake air through timing of the intake resonance. Do you honestly think a hot gas around 400degC would drop below atmospheric pressure?
So it does seem to me you really know what you talk about and you definately know a lot more than i do or prolly will. I tried to look in your "about" section to your channel to figure what your qualifications are. Like are you a educated machine worker or mechanic or have you simply taken apart every single moped, car or motorcycle you have ever had for the past 10 years and then buggered it up and then researched it to its limits and then fixed them? It does seem like it is primarily the first and ever so slightly the latter as well. Love that you demyth the tarts of youtube, but it would be lovely to just get rid of the last doubt because we are bloody muppets when it comes to knowing and know-how. Perhaps that is just me then but still. I would definately feel more comfident with the knowledge you share where you have your knowledge from. No prick-attitude intended. Cheers
I've done a video about this - th-cam.com/video/Ja1QLU6oz8A/w-d-xo.html But in a nut shell. I'm a mechanical engineer (HNC, HND) I started out in the British Army and attended the ATFC. I worked on WAH-64s which the British Army had just taken delivery of at the time. I've worked for several aviation companies, and mechanical design companies. I also worked for Perkins and Europak - designing systems for diesel engines.
@@dirtygarageguy You know nothing about camshafts lobe separation angle, valve overlap TDC, BDC, degrees, or anything. Calculate valve overlap which is one of the main items for a camshaft. It takes four numbers to do it.
I believe I have commented on a video in the past however I really have to give you credit here. I am currently in school for Automotive Technology and i've been watching these older videos to help me better understand the content in my engines class. Anyways just wanted to say that I love the videos as they have helped me alot!
Very helpful 6 yrs later, If only they would teach it like you did. Brilliant Explanation!!
Im in school to finish my Millwright apprenticeship and had a difficult time understanding valve overlap. This video made a huge difference for me, Thanks!
im searching for research purposes and i found a stand up comedy , what a legend
That was a great explanation. I was having some trouble and getting confused. Like your presentation👍
brilliant breakdown mate
some the exaust gases will find their way out of the cylinder because of the inertia (high rpm) (basicly reverse inertia filling) and create a negative pressure a well tuned exaust will increase this effect. and then when the intake valve opens you accelerate your fuel air mixture because of the pressure differance a well tuned intake runner will increase this effect. the point im trying to make is overlap is not just about getting exuast out its also about starting the accelration of the fuel air mixture (which takes some time because of the mass of the air) before the piston is on its way down again. If you start the acceleration before TDC the piston will not start to create a negative pressure and thus decreaseing pumping loses and so on (affects port size where top speed of the fuel air mixture is). (english = 2nd language)
Also, the reason your vacuum goes to crap with more overlap is you are essentially creating a low-rpm vacuum leak. With the throttle nearly closed the only other place the engine can draw from is the still-open exhaust valve(s) and what it's trying to draw back in isn't air, it's exhaust, so it's even worse the a normal vacuum leak. This is why some racing engine with really large, aggressive camshafts can't idle below ~1000rpm. You must open the throttle more at idle (and usually give it more spark advance and a richer air/fuel mixture) so that the gases have more momentum to reduce this reversion and so the more of what is being drawn in is air so that the engine can produce enough power to idle.
Basically, overlap makes you engine really inefficient at an idle but it does all sorts of wonderful things for torque and horsepower as engine speed increases. This is why variable valve timing is such a powerful tool. We can get the best of both without the drawbacks of either.
Great explanation. I've heard 2 engines that had so much overlap that you could hear it at idle. An Isuzu 4 cylinder (Chevy Luv pickup) and a Pantera (Ford 351 Cleveland). Both were really fast.
Excellent video and solves the mystery for me... Gonna read the cam specs on 3.6L pcar and see if I can draw a spiral diagram that makes sense to me 👍
Well explained👍🏽, Also Intake And Exhaust Valve overlap occurs while the piston movement is minimal ,at about 15degrees before/after TDC Or BDC When the piston is barely moving.
depending on the cam. mine opens 26° before TDC. 66° Overlap. Stock cam is 6° overlap :D
i love the way you explain it
What a great explanation. As far as I can see on a stock basic push rod engine the only way to change overlap would be to change the camshaft profile?
Correct.
It's a proud moment when u fix your own car and set it's timing perfectly
Big fan of your content Matt thanks for sharing!
Brilliant video thank you 👀❤️👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍
Thanks for sharing
hello when changing the timing chain in my motorcycle should i put the camshaft in compression position?
Hey there ! You are awesome.
Very good! GOD bless you
Thanks for the HQ lesson, very educating. What do you think of 12.5:1 compression ratio 4T engine? Will it have smaller amount of "trapped" exhaust gas inside combusting chamber compared to a 10:1 compression ratio engine?.. if yes, do you think this is the reason why manufacturer produce higher compression ratio engine (to increase the volume efficiency)?
High compression = thermal efficiency
very good
Hi, Thanks for explaining all this. The only one I came across was Jafmobile explaining cam separation angle, very good but He didn't do many drawings, and I like drawings ; - ) Actually I find things much easier to understand with diagrams and pictures. So have I got it right that a Narrow lobe seperation angle will let the valves stay open longer and give a Larger valve overlap?
Or is it the other way round? Best Wishes Dee
yes that's correct, people like to put it that way, but it doesn;t actually mean anything.
@@dirtygarageguy thanks
What camshaft lift macthing the big valve 25 29 and what rocker arm ratio
how is the valve open at TDC and no engine damage?
Hi Matt!
What do you think about opposed piston engines?
What is the turn used to refer when both valve open position? Lead , lang, overlap or none ?
Overlap
Is it possible to overlap in 2 strokes diesel engines ?
Does the overlap change with a turbo system seeing as there is an intake charge pressure that is higher than the exhaust pressure.
The exhaust blow-down pressure is different because of the back pressure from the turbine interface. Its all a bit wishy washy when it comes to turbos - but we're gonna get there.
This is verry different from set up to set up if you look a stock cars with small turbos and small exaust housings the back pressure can be a lot larger than the intake pressure. this is reflected in the camshaft design! stock turbo cams usually have smaller amount of overlap on stock cars a nice curve is important so a small turbo and a mild cam i used. BUT! a really good thing if you are tuning an engine (like alot) is to have quite a really large turbo(in some cases to large) with a large exuast housing and a manifold with long tubes you will have a lower back pressure which might equal or is lower then intake pressure(boost!) then you can run a cam more similar to a NA motor. this will give you more power with lower boost. but i some cases its better with a short manifold and a small turbo with small houseing so you can get boost eariler in the powerband! which is good for track cars or milder street cars! the basics! (english = 2nd language)
How does this work on an interference engine? Intake opens, closes, opens again? Does the exhaust valve open and close twice as well?
So the momentum of the escaping exhaust gases helps to pull fresh air into the cylinder?
If the timing is correct - this is why exhaust set-up is important - we'll be going down this rabbit hole soon.
can you explain about compression ratio with the cam overlap
No mention of tappet clearance to increase or decrease overlap that is what I was looking for.
Enjoyed it though 👍
Say what now?
I just discovered your video, have yet to check your list of other videos.... I would really love to hear your thoughts on reversion
Flow reversion?
@@dirtygarageguy yes, usually discussed more in marine applications because of the water cooled heat exchangers on exhaust, but I'm curious if reversion could explain my current situation where I have an engine with scored cylinder walls and metal dust above both valves in the cylinder head, but none in the intake manifold or catch can that's on the pcv system. My hypothesis is that with such a heavy cam profile (hotrod street cam) and the engine never having turned over 3000rpm, I never had the intake charge velocity to overcome reversion during overlap and that's how the fine powder ended up where I found it.
Or it could be running down your valve stems from the head, or maybe your valve aren't closing properly
The airflow doesn't have enough lifting capability during reversal to lift metal flakes into the airstream.
@@dirtygarageguy thanks for the response. It was a real head scratcher for me. Still baffled on exactly what went wrong in this engine but the physics involved are so neat to ponder. I'll have it torn down by the builder at the end of the week when I get back into town
thats odd, i didnt get a notification for the vid, good thing i knew it was time for one ;)
whoop, there it is
Where does the momentum of air came from? Isn't that gona push the exhaust gas to the intake port because the atmosphere difference?
The exhaust gas itself hs momentum, as for the intake, this is where resonance of the intake charge comes in.
How can purge be possible as the remaining exhaust gas will be positive pressure and the intake will be in vacuum, it won't push it out of the exhaust it will suck it back toward the vacuum (intake). At 2:07 you mention with the remainder of the unexhausted gas remaining it's still 1.5 bar.
Because of momentum, and the venturi effect. The exhaust gases are flowing out of the cylinder, this causes its own pressure system. If the engine is tuned correctly the intake has a resonance that promotes inertial filling from the intake side.
In other words pressure is not the only thing to calculate, hence why this is a difficult subject...
Awesome!!! Is this what happens in BMW's R1250 ShiftCam engines?
th-cam.com/video/Z0tYMiAnQfU/w-d-xo.html
And... I’m subscribed
Why not split the intake to separate intakes and have a different cam profile for each side. One side opens early to allow purging but doesn't get fuel sprayed through it and then the other opens up later when the exhaust is closed and sprays fuel through it.
Building a high(er) compression VW 2.0 8v engine. Currently running a sport 260 cam with 1° overlap, and the engine lopes at idle a little. Planning on going with a 276 cam, with 7° overlap, but fear that itll be too much for an OBD2 engine with stock tune (for now). Driveability is not a concern since its a weekend cruiser, potential race car, aslong as it can idle. Is 7° overlap too much for a stock tune obd2 engine?
Are your overlap degrees @0.050” lift? No overlap = no high performance
How much fresh intake charge enters during overlap as compared to piston pull and inertia?!? According to an expert more enters during overlap...
Fisting easter egg at 3:56
My banshee has BADDDD overlap. It happened ever since I replaced the purple power band with a red one.
a banshee is a 2 stroke
would a turbo help or hinder this condition? i mean, you have more back pressure against the turbine, yet forced air to help expel the gases, i hope that makes sense, seya in a bit ;)
This is verry different from set up to set up if you look a stock cars with small turbos and small exaust housings the back pressure can be a lot larger than the intake pressure. this is reflected in the camshaft design! stock turbo cams usually have smaller amount of overlap on stock cars a nice curve is important so a small turbo and a mild cam i used. BUT! a really good thing if you are tuning an engine (like alot) is to have quite a really large turbo(in some cases to large) with a large exuast housing and a manifold with long tubes you will have a lower back pressure which might equal or is lower then intake pressure(boost!) then you can run a cam more similar to a NA motor. this will give you more power with lower boost. but i some cases its better with a short manifold and a small turbo with small houseing so you can get boost eariler in the powerband! which is good for track cars or milder street cars! (english = 2nd language)
This is probably too involved for a simple comment, but how does a person calculate dynamic compression? One of my problems is the cam manufacturer doesn't have all the info on the cams I installed. The only part I know is that overlap was significantly more on the new cam. Trying to work out if I really need high octane fuel, or if the overlap is enough to let me use something like 93 octane. Currently running at about 105 with a mix of 91 and some MMT based additive, but was running real race fuel of 105 before. The race fuel was too difficult to get, so trying the Torco additive. Would be happier with 93 pump gas if I could safely run it. I've done web searches on dynamic compression, and never really found anything that explained what I needed to know to compute what my engine is really running right now.
the dynamic compression ratio change alot over the rpm range because of inertia filling at high rpm(which give you higher pressure) or leak out of the intake valve on low rpm! you also need to take ignition timing into account(it plays a huge part in what kind of fuel you can run). what you wanna do is calculate the maximum pressure in your cylinder.
jompa zx I'm hoping that the question and your reply will fuel Matt to explain it to us so I can maybe, finally grasp this concept. It does make sense that compression will change as the air gets flowing, the fuel/air mix does have inertia so the fuel charge can slightly raise the cylinder pressure at bdc before the valve closes. And the same sort of thing from the overlap as was mentioned in this video. My intake harmonics kind of suck, wasn't much room to get a longer intake so I think it is like 3rd or 4th order and pretty high in the rpms, kind of prevents good overlap performance at the lower rpms.
What you forget is the intake valve close after BDC this is necessary on high rpm otherwise the cylinder will not be full and on high rpm you might get some inirtia filling so maybe you get over 100% VE this will make you dynamic compression ratio higher. BUT on low rpm you will fill the cylinder at BDC or just right after BDC so if you have high rpm cams you will push some fuel/air mixture out of the intake valve again this will hurt your dynamic compression ratio!
+Mini GP Racing
Dynamic or effective compression ratio is a complex subject with many variables.
Obtain a dial indicator, degree wheel, evaluate and plot your exact camshaft(s) intake/exhaust lobe profiles, lift, duration, event timing, lobe separation angle, intake/exhaust lobe center-line and overlap.
Static or dynamic or effective compression ratio is only one variable of many influencing fuel octane value requirement/choice.
There several excellent articles and calculators available online.
members.uia.net/pkelley2/DynamicCR.html
ftlracing.com/dynamiccr.htm
www.rbracing-rsr.com/camshaft.html
www.rbracing-rsr.com/comprAdvHD.htm
www.wallaceracing.com/dynamic-cr.php
www.wallaceracing.com/eff-cr.php
www.google.com/search?num=100&newwindow=1&ei=SrZaW8-DH9u_0PEP5fGv2Aw&q=dynamic+compression+calculation&oq=dynamic+compression+calculation&gs_l=psy-ab.12..0i22i30k1l4.7590.8683.0.11082.3.3.0.0.0.0.88.239.3.3.0....0...1c.1.64.psy-ab..0.3.238...35i39k1j0i13i30k1j0i13i5i30k1.0.K6Ti8_GvHA4
www.google.com/search?num=100&newwindow=1&ei=VQBaW4j1M8OezwLiqpLABA&q=dynamic+compression+ratio+calculator+and+camshaft+selection+utility&oq=dynamic+compression+calculator&gs_l=psy-ab.1.5.0i71k1l8.0.0.0.13397.0.0.0.0.0.0.0.0..0.0....0...1c..64.psy-ab..0.0.0....0.a3vCzBtPkDw
patw52pb1 Thanks for the links, I'll check them out.
Nr 1
damn im slackin tonight only 2nd lol
Pretty close, but exhaust doesn't close at TDC.
Exhaust closing EC is usually around the same degree ATDC as intake opens BTDC in a 4 stroke petrol engine.
intake opens before TDC exaust closes after TDC
Yes, that's what I just wrote
EC is ATDC and IO is BTDC
If you're going to discuss valve overlap then of course it matters and if you did 30 seconds of research on it before making the video you would have discovered that.
You're a qualified engineer and should know better.
And spare me the Delboy tantrum over the correction, I've been a mechanic too long to take any notice of that crap so suck it up princes.
LOL nice comeback. "You're a qualified engineer and should know better." - don't really know what this means? I should know better?
What difference would it make if I draw it ending at 0deg?
After rewatching the video bit - I do say "it'll close at TDC or something like that"
On the spiral diagram the line slightly passes TDC. That fact is that is doesn't matter for this video. You will start shouting "the cirlce isn't perfectly round".
I'm sure there are some engines out there where the closing of the exhaust is TDC - probably.
Oh look - loads of 'theoretical diagrams' with TDC as the exhaust valve closing. (there's loads of 'correct' diagrams as well, don't get me wrong)
www.researchgate.net/profile/Rupesh_Gupta12/publication/309011115/figure/fig8/AS:416252737671175@1476253794862/Valve-timing-diagram-shown.png
image.slidesharecdn.com/24514675-2-internal-combustion-engine-120405123335-phpapp01/95/24514675-2internalcombustionengine-31-728.jpg?cb\u003d1333629439
dzmm.info/wp-content/uploads/valve-timing-diagram-for-diesel-engine-theoretical-of-4-stroke-cycle-present-gallery-likewise-553-640x346.png
Until I do videos on the effect of valve opening and closing you're just being a padantic prick. Its the 'pretty close' patronising comment that is the problem, on a video that is the basics.
This is all on the fly and you bang on about delbollocks but the guy is talking complete made-up bullshit. When I say 'something like that' what I'm not saying is ALWAYS TDC.
Regardless, you wanna knit-pick then that fine, but there are people out there that this is all new to.
Well that was just a load of noise and 3 useless images.
Theoretical valve opening my foot.
As an engineer if you cant spend a matter of seconds to put the words valve overlap into google to get it right then WTF are you doing teaching mechanics.
All those speeches you give about correcting all the bad mechanical advice on TH-cam and getting the facts right for viewers and saying time and time again and then you go do something is lame as that.
Stop whining and just admit you cocked it up.
Kind effort but the explanation contains mistakes. Valve overlap allows the low pressure caused by the exhaust gases leaving the cylinder to help the fresh charge enter the cylinder. Not what this video describes saying that the fresh air pushes the exhaust gases out. When the intake valve opens before TDC there is not yet any dynamics to the inlet charge to enter and push the exhaust away. Inlet charge will start entering the cylinder either by the negative pressure of the exhaust gases ( so that the exhaust helps suck the inlet charge) or by the negative pressure created by the piston motion after TDC. To understand it better, Imagine that if you open the intake valve before TDC at low RPM sometimes you get the opposite. Exhaust gases enter the inlet manifold and pollute the charge. This often happens when you do not install a proper exhaust system tuned for your particular valve timing. So to summarize Valve overlap is there to use the exhaust dynamics ( created by your exhaust system) to suck air charge before the piston goes downwards. Effectively increasing the air charge induction period. This has also a small effect on cleaning the cylinder better from exhaust gases since the increased air charge sucked inside the cylinder occupies more space and pushes any left overs out. That is also the reason the exhaust valve after TDC delays to close.
There is, it the inertia of the intake air through timing of the intake resonance. Do you honestly think a hot gas around 400degC would drop below atmospheric pressure?
It's a bit of both, but not purely exhaust. For instance, look at inertial filling
@@dirtygarageguyyes, it drops way below atmospheric, much more than in any other part of the cycle.
Hot exhaust gases drop below atmospheric? Lol please explain. Do you know what pv = rnt is?
21 Subs to go... =) =) =)
fuckin awsome expression
So it does seem to me you really know what you talk about and you definately know a lot more than i do or prolly will. I tried to look in your "about" section to your channel to figure what your qualifications are. Like are you a educated machine worker or mechanic or have you simply taken apart every single moped, car or motorcycle you have ever had for the past 10 years and then buggered it up and then researched it to its limits and then fixed them? It does seem like it is primarily the first and ever so slightly the latter as well. Love that you demyth the tarts of youtube, but it would be lovely to just get rid of the last doubt because we are bloody muppets when it comes to knowing and know-how. Perhaps that is just me then but still. I would definately feel more comfident with the knowledge you share where you have your knowledge from. No prick-attitude intended. Cheers
I've done a video about this - th-cam.com/video/Ja1QLU6oz8A/w-d-xo.html
But in a nut shell. I'm a mechanical engineer (HNC, HND) I started out in the British Army and attended the ATFC. I worked on WAH-64s which the British Army had just taken delivery of at the time. I've worked for several aviation companies, and mechanical design companies. I also worked for Perkins and Europak - designing systems for diesel engines.
fuck it im subbin
ok, mody camshaft to get this. adjust cam puley u got nothing. am i alright? sorry im not english. hihihiiiii... hope understand.
Very clear explanation. Thank you brother. Free subscribe and thumb up from me👍
Complete waste of time !!!! Never tells us valve overlap at all !!!!!!!!!!!!!!!!
Obviously didn't watch the video
@@dirtygarageguy Okay, how much valve overlap an lobe separation angle ??? How much vacuum do you have at say idle
There four numbers one needs to know in degrees, calculate valve overlap.
That isn't what overlap is... your comment is like "how long is a piece of string"
@@dirtygarageguy You know nothing about camshafts lobe separation angle, valve overlap TDC, BDC, degrees, or anything. Calculate valve overlap which is one of the main items for a camshaft. It takes four numbers to do it.