By "cigaro" I assume you mean old style none expansion chamber type mufflers. In general as the baffles in those old pipes clog up over time performance drops so no increasing back pressure on them would not help and keep in mind that too much back pressure will cause over heating that will increase the chances of seizing due to piston overheating in addition to inducing detonation.
At 0:57, this is confusing. The previous video you mentioned 3.5bar gave 350deg. Here you mention 5bar gives 350deg. Also your software shows it's at 600degrees with 2.56bar. Then you change bmep to 10.75 and it stayed at 600deg. Can you please clarify? Thank you for all your efforts!
The conversion from BMEP to exhaust temperature is not via any equation it is just an estimation and not that accurate. So it falls within a range of BMEP and I am using the estimation provide by Prof Blair in his SAE papers where he provides 3 basic range values of BMEP that equate to an approximate exhaust temperature. The only accurate way to know exhaust temps is to use an EGT gauge. Also keep in mind that even in an engine that has for example 600 C exhaust temp can fall to as low as 450 C at lower RPM so the aim is to test for the maximum temp at peak RPM in top gear too.
Also if you have a look at my expansion chamber software it has a tables of estimated ex temp based on BMEP Some softwares have a function to convert BMEP to ex temp but they don't use an equation they just do a straight line estimation of the same table I use but for transparency purposes I dont do that. I think its better people are not under some false impression the software is doing some accurate conversion of BMEP to ex temp. Also there are other factors that influence ex temp not just BMEP for example changes in the outlet pipe of the chamber and water cooled engines compared to air cooled, air fuel ratio, ignition timing and more.
@@AuMechanic Thank you; If i get time I will pull the pipe off at the diffuser and put a thermocouple in there at peak power to see how close the estimate is.
@@mitchell5828 Generally you can measure at the header end to get peak (which would be more representative), yes it will get cooler farther down the pipe but best to go with max temp, keeping in mind that a lower temps (slower wave speed) will result in a longer tuned length that fits lower RPM range and you don't want a pipe that "cuts off" below peak RPM, best to have the ability for a bit of over rev. Also keep in mind that in lower gears and lower speed temps are cooler and the pipe tune fits lower RPM which is favorable out of the corners on a track.
@@AuMechanic This application is for a self launching motor glider needing the engine for maximum climbrate for ~10min then shut off. Temps are easy enough to measure so I'll try a couple places to see how they differ. I might then build different pipes based on different temps and see which ones work best. Thank You
Those values represent the composition of the exhaust gas depending on the type of fuel used and the Air fuel ratio that both effect the speed of sound and thus the tuned length which is calculated using the speed of sound inside the pipe. ICG is the Individual Gas Constant and RSH is the Ratio Specific Heats. The default values are for Gasoline at an average AF ratio and temperature. If you hold your mouse over them the help will pop up explaining them.
In short both. You can adjust the values to make the pipe have a wider or more narrow power range. For a CVT a narrow power band would suffice and no need to sacrifice peak output.
Because this is using Blairs formula (1996) to design the pipe that specifies that point to point to the end of the baffle cone.. The formula using half the angle of convergence of the baffle cone is from the Gordon Jennings formula from the 70's that Graham Bell also used later.
AuMechanic I was thinking the end of the cone measurement would be the end of the wave as well...where the the pressure drops back off to atmospheric pressure and the beginning of the cone is where it just starts to raise pressure in the wave, timing wise, and mid cone is the peak of the wave where it hits it's hardens so could you not use the beginning of the baffle to time where that pipe starts to come in and the end of the cone where it drops off..using the middle of the cone for where you want the most hit in the rpm range?
There is no "sine" wave in the pipe, the wave it refers to is the initial compression wave that comes out of the exhaust port as it opens, it then travels the length of the pipe to the opposing baffle cone taper and bounces back toward the exhaust port arriving at the port just as the exhaust port closes to force intake charge spilling out of the cylinder. The outlet pipe is the pressure bleed for the chamber that's all. This is all covered in the earlier series covering the scientific fundamentals of pipe operation as per Professor Blairs papers..
The Jennings equation appears to be based on a peak pressure centre point of the compression wave where it is reflected and not the far end of the wave.
6.33 minutes. Should it be rpm divided by 60000 and not 60000 divided by rpm? Am trying my best to get rd400 tuned length and only getting 380mm. Mmm keeps me on my toes
That exhaust is port time in milliseconds. 1 minute = 60,000 milliseconds What the equation is doing is finding out what division of 360 degree is the port timing. So if its 180 degree then it 1/2 or 0.5 of 360 degrees. So the equation is 0.5 multiplied by the time the port is open at during one crank revolution in milliseconds based on the RPM. So if its 10,000 RPM then the equation is 60,000 / 10,000 = 6 So the port that is open for 180 degree each revolution is open for 6 milliseconds when the engine is spinning at 10,000 RPM And based on the speed of sound inside the pipe at the given temperature which is "Velocity" at metres per second. So we convert metres per second Velocity multiplied by 1000 equals millimetres per second. Velocity in millimetres per second X Exhaust port time in milliseconds. So that is converted to seconds by dividing it by 1000. Then we divide it by 2 allowing for the fact the wave goes to the end of pipe and back to the port and we only want to know how long it takes in one direction to find out the tuned length of the pipe.
If you are struggling with the equations it might be easier to just download the free software and use it to get tuned length. See link in description section under video or in later videos.
Yes Hank, in that case the exhaust timing is taken with the Exhaust valve fully open position as it is at top end RPM. Covered that also in the earlier video th-cam.com/video/SD0gy7jYdT4/w-d-xo.html
Using the tuned length formula you will see that with the ex fully open timing at peak RPM, when you reduce the ex timing by closing the valve as it would at lower RPM, and then change the peak RPM value in the equation to render the same tuned length you started with that will show you how the effective peak of the pipe has shifted down the RPM range.
Hi Dave, what target RPM for pipe design would you suggest for a 15-ish hp, 50cc cylinder with 190 deg exhaust duration (with auxiliary ports) and 122 transfer duration? I've looked up charts but they range from 7000 all the way to 14000. Your help would be really helpful! Thanks!
The target RPM for the tuned length calculation is always the maximum RPM of the engine. This is the point you want to pipe to go from producing gains to dropping power rapidly. It works like a bit of a rev limiter in effect.
@@AuMechanic I absolutely love how dedicated you are on educating people. I always read the comments and try to learn something from here as well. Thank you a thousandfold.
@@AuMechanic What if you won't reach the max published RPM? I run RC boats. Stated max RPM is 28,000 at @ 5HP on some of my engines, but I wouldn't expect to see more that 20k under load (large props with lots of pitch). I never know which RPM to design for, but it seems like the max *achievable* RPM would be the figure to design around. Is this incorrect thinking?
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Great video. Would be possible to refine/improve pipe back pressure performance on old two stroke 'cigaro' type exhausts?
By "cigaro" I assume you mean old style none expansion chamber type mufflers.
In general as the baffles in those old pipes clog up over time performance drops so no increasing back pressure on them would not help and keep in mind that too much back pressure will cause over heating that will increase the chances of seizing due to piston overheating in addition to inducing detonation.
At 0:57, this is confusing. The previous video you mentioned 3.5bar gave 350deg. Here you mention 5bar gives 350deg. Also your software shows it's at 600degrees with 2.56bar. Then you change bmep to 10.75 and it stayed at 600deg. Can you please clarify?
Thank you for all your efforts!
The conversion from BMEP to exhaust temperature is not via any equation it is just an estimation and not that accurate.
So it falls within a range of BMEP and I am using the estimation provide by Prof Blair in his SAE papers where he provides 3 basic range values of BMEP that equate to an approximate exhaust temperature.
The only accurate way to know exhaust temps is to use an EGT gauge.
Also keep in mind that even in an engine that has for example 600 C exhaust temp can fall to as low as 450 C at lower RPM so the aim is to test for the maximum temp at peak RPM in top gear too.
Also if you have a look at my expansion chamber software it has a tables of estimated ex temp based on BMEP
Some softwares have a function to convert BMEP to ex temp but they don't use an equation they just do a straight line estimation of the same table I use but for transparency purposes I dont do that.
I think its better people are not under some false impression the software is doing some accurate conversion of BMEP to ex temp. Also there are other factors that influence ex temp not just BMEP for example changes in the outlet pipe of the chamber and water cooled engines compared to air cooled, air fuel ratio, ignition timing and more.
@@AuMechanic
Thank you; If i get time I will pull the pipe off at the diffuser and put a thermocouple in there at peak power to see how close the estimate is.
@@mitchell5828 Generally you can measure at the header end to get peak (which would be more representative), yes it will get cooler farther down the pipe but best to go with max temp, keeping in mind that a lower temps (slower wave speed) will result in a longer tuned length that fits lower RPM range and you don't want a pipe that "cuts off" below peak RPM, best to have the ability for a bit of over rev. Also keep in mind that in lower gears and lower speed temps are cooler and the pipe tune fits lower RPM which is favorable out of the corners on a track.
@@AuMechanic This application is for a self launching motor glider needing the engine for maximum climbrate for ~10min then shut off. Temps are easy enough to measure so I'll try a couple places to see how they differ. I might then build different pipes based on different temps and see which ones work best.
Thank You
Another query from me... The IGC and SHR values must be left the defaults that the program gives us (1.36, 300)?
Those values represent the composition of the exhaust gas depending on the type of fuel used and the Air fuel ratio that both effect the speed of sound and thus the tuned length which is calculated using the speed of sound inside the pipe.
ICG is the Individual Gas Constant and RSH is the Ratio Specific Heats.
The default values are for Gasoline at an average AF ratio and temperature.
If you hold your mouse over them the help will pop up explaining them.
are these calculations for geared bike or for cvt
In short both.
You can adjust the values to make the pipe have a wider or more narrow power range.
For a CVT a narrow power band would suffice and no need to sacrifice peak output.
Before you were saying tuned length is taken to the mean point of reflection.mid cone? Why now are you saying to the end of the cone.?
Because this is using Blairs formula (1996) to design the pipe that specifies that point to point to the end of the baffle cone..
The formula using half the angle of convergence of the baffle cone is from the Gordon Jennings formula from the 70's that Graham Bell also used later.
AuMechanic I was thinking the end of the cone measurement would be the end of the wave as well...where the the pressure drops back off to atmospheric pressure and the beginning of the cone is where it just starts to raise pressure in the wave, timing wise, and mid cone is the peak of the wave where it hits it's hardens so could you not use the beginning of the baffle to time where that pipe starts to come in and the end of the cone where it drops off..using the middle of the cone for where you want the most hit in the rpm range?
Because noe I'm confused as to why they would have different ideas of where to measure the tuned length to
There is no "sine" wave in the pipe, the wave it refers to is the initial compression wave that comes out of the exhaust port as it opens, it then travels the length of the pipe to the opposing baffle cone taper and bounces back toward the exhaust port arriving at the port just as the exhaust port closes to force intake charge spilling out of the cylinder.
The outlet pipe is the pressure bleed for the chamber that's all.
This is all covered in the earlier series covering the scientific fundamentals of pipe operation as per Professor Blairs papers..
The Jennings equation appears to be based on a peak pressure centre point of the compression wave where it is reflected and not the far end of the wave.
6.33 minutes. Should it be rpm divided by 60000 and not 60000 divided by rpm?
Am trying my best to get rd400 tuned length and only getting 380mm. Mmm keeps me on my toes
That exhaust is port time in milliseconds.
1 minute = 60,000 milliseconds
What the equation is doing is finding out what division of 360 degree is the port timing.
So if its 180 degree then it 1/2 or 0.5 of 360 degrees.
So the equation is 0.5 multiplied by the time the port is open at during one crank revolution in milliseconds based on the RPM.
So if its 10,000 RPM then the equation is 60,000 / 10,000 = 6
So the port that is open for 180 degree each revolution is open for 6 milliseconds when the engine is spinning at 10,000 RPM
And based on the speed of sound inside the pipe at the given temperature which is "Velocity" at metres per second.
So we convert metres per second Velocity multiplied by 1000 equals millimetres per second.
Velocity in millimetres per second X Exhaust port time in milliseconds. So that is converted to seconds by dividing it by 1000.
Then we divide it by 2 allowing for the fact the wave goes to the end of pipe and back to the port and we only want to know how long it takes in one direction to find out the tuned length of the pipe.
If you are struggling with the equations it might be easier to just download the free software and use it to get tuned length.
See link in description section under video or in later videos.
I have a desert aircraft DA35 and I plan on building a tuned pipe. I was wondering if you have ever built a pipe out of carbon fiber?
No I cant help with that.
I can do the carbon fiber part I just need to know how to make a curved or U shaped tuned pipe. Will the calculations work the same ?
@@BowDwn2 The material used wont effect the dimensions in the pipe equations.
Also exhaust port time is in ms to bring it up to seconds you would multiply it by 1000 not divided ? Am i correct?
No to convert milliseconds into seconds divide by 1000 because there are 1000 milliseconds in each second.
Does BMEP equal to compression ?
BMEP is "Brake Mean Effective Pressure"
This explains it
en.wikipedia.org/wiki/Mean_effective_pressure
Here is a BMEP calculator
www.omnicalculator.com/physics/brake-mean-effective-pressure-bmep
I have an rz 250 does the ypvs make a difference in measuring the port thanks
Yes Hank, in that case the exhaust timing is taken with the Exhaust valve fully open position as it is at top end RPM.
Covered that also in the earlier video th-cam.com/video/SD0gy7jYdT4/w-d-xo.html
AuMechanic I did watch it I will watch it again
Using the tuned length formula you will see that with the ex fully open timing at peak RPM, when you reduce the ex timing by closing the valve as it would at lower RPM, and then change the peak RPM value in the equation to render the same tuned length you started with that will show you how the effective peak of the pipe has shifted down the RPM range.
AuMechanic thanks Dave
Hi Dave, what target RPM for pipe design would you suggest for a 15-ish hp, 50cc cylinder with 190 deg exhaust duration (with auxiliary ports) and 122 transfer duration? I've looked up charts but they range from 7000 all the way to 14000. Your help would be really helpful! Thanks!
The target RPM for the tuned length calculation is always the maximum RPM of the engine.
This is the point you want to pipe to go from producing gains to dropping power rapidly.
It works like a bit of a rev limiter in effect.
@@AuMechanic I absolutely love how dedicated you are on educating people. I always read the comments and try to learn something from here as well. Thank you a thousandfold.
@@BK-it6wg
And thank you for the comment.
Cheers
@@AuMechanic What if you won't reach the max published RPM? I run RC boats. Stated max RPM is 28,000 at @ 5HP on some of my engines, but I wouldn't expect to see more that 20k under load (large props with lots of pitch). I never know which RPM to design for, but it seems like the max *achievable* RPM would be the figure to design around. Is this incorrect thinking?
Also, thanks a lot for all of the videos. Very informative, and useful, for a nitro-nerd such as myself.