- Dec 31, 2007
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Just to add something to the discussion. This article is a comprehensive guide for anyone who is trying to achieve a race spec exhaust system with considerably low and street noise friendly.
Please note the info below applies only to straight flow setup. For sflow/ reverse flow setup, i am not sure whether the result is 100% satisfactory.
1st thing 1st, ur exhaust is highly influenced by ur camshaft profile, so it is better to to do ur cam first then match it with the exhaust system. U might hear some people did a cam swap but maintained stock exhaust design and lost a lot of usable power, this is why.
these are the sequence of importance when it comes to exhaust tuning:
a) collector / downpipe length (until first bullet/resonator)
b) colector / downpipe diameter
c) header primary length
d) header primary diameter
note that collector length and diameter plays much more important role than ur header. This means that trying too hard to get the exact equal length might not get as much power as getting ur collector length and diameter the correct dimension. Collector length is more sensitive to the powerband.
take a look at this video for proof:
Size Matters...Exhaust Collector Testing Part 1 - YouTube
There are 2 parts in exhaust tuning - diameter tuning and length tuning and these 2 parts almost work independently.
diameter tuning is what have been discussed by Iszo and Drexchan and this mostly concerns with the flow volume of exhaust. Basically, we want the smallest possible size of pipe that can flow enough for our application without suffering from significant restriction.
Length tuning concerns with more of the manipulation of pressure wave that comes along with the gasses at much higher speed.
When exhaust valve opens, apart from the exhaust gasses, there is also pressure wave travelling along to the tailpipe at the speed of sound. This pressure wave is a form of energy so it can be harnessed to aid exhaust scavenging, in fact pressure wave has much more impact on exhaust scavenging than exhaust gas velocity due to the fact waves travel at a speed of sound (380m/s), 10 times much faster than exhaust gas speed itself
The characteristics of pressure wave:
- if this high pressure pulse encounter an increase in area of piping, it will reflect back a low pressure wave back to the cylinder
- if high pressure wave encounter a decrease in area of piping, it will reflect back a high pressure wave back to the cylinder
We want the low pressure wave to come back during valve overlap to further reduce the pressure at exhaust valve so more exhaust gas can be expelled and more intake can be filled in. We dont want the high pressure wave to be returned back to cylinder because it will push exhaust back in and push the intake out, this is what we call exhaust reversion or backpressure.
Yes, backpressure is caused by pressure wave, having small pipe choke the flow but does not necessarily reverse the flow in direction because gas only move when there is pressure difference, and cylinder pressure is always higher than the exhaust system so in principle, the flow is always from engine to tailpipe. High pressure reflection is main culprit in causing backpressure and this is due to contraction in crush bent and so on.
Note that we can only get low pressure wave by expanding the pipe. That is why u see header pipe is sized a bit larger than exhaust port and claim to avoid reversion, this is true. Also this is why they use merge collector which has bigger area than the primary pipes, also to induce low pressure wave reflection. Why 4-2-1 gives a wider range of torque compared to 4-1? because 4-2-1 has multiple expansion areas, at 4-2 and 2-1. The bigger the expansion, the stronger the wave, the stronger the boost we get from scavenging. 4-2-1 has multiple smaller expansion so the scavenging sustain longer but smaller giving good overall torque with no special peak power. 4-1 has only 1 expansion at the merge collector but the expansion is bigger so giving shorter scavenging duration but a much stronger reflection which is observed at the peak power RPM.
How do we calculate this length to time this low pressure wave to come at RPM that we want to?
use this formula:
0.5*1500*((180+Exhaust Open BBDC)/360)*(60/RPM)*12
Look at ur engine manual for exhaust open BBDC or cam spec
RPM is the rpm that u want to tune for
example:
kelisa BBDC = 36.5 degree
RPM to tuned for = 4500 since i like to have a lot of power in this range
substitute the figures and i get 72.2 inch of header and collector. Note that this length is from the exhaust valve all the way to the end of collector/downpipe therefore u have to consider the exhaust port length in the cylinder head too.
But this exhaust length is too short to be used and will not be street legal, only good for racing with open header. Thats when pressure wave termination box comes in.
Pressure wave termination box is basically a larger cross sectional area of piping large enough to resembles the atmosphere. The box is used to separate ur tuned header and collector away from the rest of exhaust system. The rest of the exhaust system does not matter in terms of wave tuning, we only need to make sure it flows well enough.
Once u have got the desired header and collector length, install the biggest bullet u can possibly fit under the chassis so this big space difference will be treated as the "atmosphere" and ur header and collector will stay tuned. Rule of thumb is the volume of termination box should be twice the size of ur engine displacement, i use 3" diameter 19" length oval bullet to get me volume of around 2.2 litre for my 1.0 kelisa.
The rest of ur exhaust system can be sized approriately small that flows enough for ur application, wave tuning does not apply after the termination box.
This are some guides for piping and muffler after the termination box:
- dont use too big of a pipe at the end of exhaust system because exhaust gas cools down and lose a lot of energy as it travels to the tailpipe. U can still touch the tailpipe after long drive but definitely not the header right? thats the proof. Small piping at the end tend to keep the velocity high to compensate for this energy loss. This also quiets down the exhaust note.
- Your back muffler pipe diameter should not be too big from ur piping size. if u can match the diameter (2" piping to 2" straight flow muffler, 1.6" piping to 1.6" straight flow muffler) that would be the best because u dont want unnecessary expansion anymore at the back which can induce turbulence and impeding flow. Plus a 1.6" straight flow does not resembles same flow as straight 1.6" pipe, take a look at the pic below:
as you can see, a 1.6" straight flow muffler will act like expanding pipe with 1.6" inlet, this is because as gas pass through the muffler, the energy is being absorbed more and more by the fibre packing so the energy loss is same like gas flowing through an expanding pipe. so u can see why this can cause unwanted turbulence at high rpm and sometime lose top speed. The expansion cools down the exhaust further and slow it down more and more. so size it according to ur tailpipe size.
- get the biggest body/canister muffler u can fit under ur chassis because the bigger the body, the higher the muffling capability. Muffler noise can be estimated from the ratio of body size to pipe diameter size. Bigger body with small pipe will muffle the best (straight flow muffler with sflow big body), small body with big pipe muffle the worse (trumpet style, bullet)
I am quite tired so i might have typed wrongly or raised up confusion but the general idea is there. So do ask questions if u find something does not add up :D
The info above is a summary of my continuing studies of exhaust system performance from several tuning books, the internet and students and lecturers of fluid mechanics and wave principle. This study is a non-profitable one and purely for passion and knowledge. All of the above was targeted to achieve on designing a zero-loss exhaust system with street legal noise without having to pay a fortune :D
I dont have dyno to prove each of the above but i have proved it by doing tests on my own car which has cost me thousands of ringgit on exhaust system only and my ride is not an expensive one, its a kelisa :D
thank you
Please note the info below applies only to straight flow setup. For sflow/ reverse flow setup, i am not sure whether the result is 100% satisfactory.
1st thing 1st, ur exhaust is highly influenced by ur camshaft profile, so it is better to to do ur cam first then match it with the exhaust system. U might hear some people did a cam swap but maintained stock exhaust design and lost a lot of usable power, this is why.
these are the sequence of importance when it comes to exhaust tuning:
a) collector / downpipe length (until first bullet/resonator)
b) colector / downpipe diameter
c) header primary length
d) header primary diameter
note that collector length and diameter plays much more important role than ur header. This means that trying too hard to get the exact equal length might not get as much power as getting ur collector length and diameter the correct dimension. Collector length is more sensitive to the powerband.
take a look at this video for proof:
Size Matters...Exhaust Collector Testing Part 1 - YouTube
There are 2 parts in exhaust tuning - diameter tuning and length tuning and these 2 parts almost work independently.
diameter tuning is what have been discussed by Iszo and Drexchan and this mostly concerns with the flow volume of exhaust. Basically, we want the smallest possible size of pipe that can flow enough for our application without suffering from significant restriction.
Length tuning concerns with more of the manipulation of pressure wave that comes along with the gasses at much higher speed.
When exhaust valve opens, apart from the exhaust gasses, there is also pressure wave travelling along to the tailpipe at the speed of sound. This pressure wave is a form of energy so it can be harnessed to aid exhaust scavenging, in fact pressure wave has much more impact on exhaust scavenging than exhaust gas velocity due to the fact waves travel at a speed of sound (380m/s), 10 times much faster than exhaust gas speed itself
The characteristics of pressure wave:
- if this high pressure pulse encounter an increase in area of piping, it will reflect back a low pressure wave back to the cylinder
- if high pressure wave encounter a decrease in area of piping, it will reflect back a high pressure wave back to the cylinder
We want the low pressure wave to come back during valve overlap to further reduce the pressure at exhaust valve so more exhaust gas can be expelled and more intake can be filled in. We dont want the high pressure wave to be returned back to cylinder because it will push exhaust back in and push the intake out, this is what we call exhaust reversion or backpressure.
Yes, backpressure is caused by pressure wave, having small pipe choke the flow but does not necessarily reverse the flow in direction because gas only move when there is pressure difference, and cylinder pressure is always higher than the exhaust system so in principle, the flow is always from engine to tailpipe. High pressure reflection is main culprit in causing backpressure and this is due to contraction in crush bent and so on.
Note that we can only get low pressure wave by expanding the pipe. That is why u see header pipe is sized a bit larger than exhaust port and claim to avoid reversion, this is true. Also this is why they use merge collector which has bigger area than the primary pipes, also to induce low pressure wave reflection. Why 4-2-1 gives a wider range of torque compared to 4-1? because 4-2-1 has multiple expansion areas, at 4-2 and 2-1. The bigger the expansion, the stronger the wave, the stronger the boost we get from scavenging. 4-2-1 has multiple smaller expansion so the scavenging sustain longer but smaller giving good overall torque with no special peak power. 4-1 has only 1 expansion at the merge collector but the expansion is bigger so giving shorter scavenging duration but a much stronger reflection which is observed at the peak power RPM.
How do we calculate this length to time this low pressure wave to come at RPM that we want to?
use this formula:
0.5*1500*((180+Exhaust Open BBDC)/360)*(60/RPM)*12
Look at ur engine manual for exhaust open BBDC or cam spec
RPM is the rpm that u want to tune for
example:
kelisa BBDC = 36.5 degree
RPM to tuned for = 4500 since i like to have a lot of power in this range
substitute the figures and i get 72.2 inch of header and collector. Note that this length is from the exhaust valve all the way to the end of collector/downpipe therefore u have to consider the exhaust port length in the cylinder head too.
But this exhaust length is too short to be used and will not be street legal, only good for racing with open header. Thats when pressure wave termination box comes in.
Pressure wave termination box is basically a larger cross sectional area of piping large enough to resembles the atmosphere. The box is used to separate ur tuned header and collector away from the rest of exhaust system. The rest of the exhaust system does not matter in terms of wave tuning, we only need to make sure it flows well enough.
Once u have got the desired header and collector length, install the biggest bullet u can possibly fit under the chassis so this big space difference will be treated as the "atmosphere" and ur header and collector will stay tuned. Rule of thumb is the volume of termination box should be twice the size of ur engine displacement, i use 3" diameter 19" length oval bullet to get me volume of around 2.2 litre for my 1.0 kelisa.
The rest of ur exhaust system can be sized approriately small that flows enough for ur application, wave tuning does not apply after the termination box.
This are some guides for piping and muffler after the termination box:
- dont use too big of a pipe at the end of exhaust system because exhaust gas cools down and lose a lot of energy as it travels to the tailpipe. U can still touch the tailpipe after long drive but definitely not the header right? thats the proof. Small piping at the end tend to keep the velocity high to compensate for this energy loss. This also quiets down the exhaust note.
- Your back muffler pipe diameter should not be too big from ur piping size. if u can match the diameter (2" piping to 2" straight flow muffler, 1.6" piping to 1.6" straight flow muffler) that would be the best because u dont want unnecessary expansion anymore at the back which can induce turbulence and impeding flow. Plus a 1.6" straight flow does not resembles same flow as straight 1.6" pipe, take a look at the pic below:
as you can see, a 1.6" straight flow muffler will act like expanding pipe with 1.6" inlet, this is because as gas pass through the muffler, the energy is being absorbed more and more by the fibre packing so the energy loss is same like gas flowing through an expanding pipe. so u can see why this can cause unwanted turbulence at high rpm and sometime lose top speed. The expansion cools down the exhaust further and slow it down more and more. so size it according to ur tailpipe size.
- get the biggest body/canister muffler u can fit under ur chassis because the bigger the body, the higher the muffling capability. Muffler noise can be estimated from the ratio of body size to pipe diameter size. Bigger body with small pipe will muffle the best (straight flow muffler with sflow big body), small body with big pipe muffle the worse (trumpet style, bullet)
I am quite tired so i might have typed wrongly or raised up confusion but the general idea is there. So do ask questions if u find something does not add up :D
The info above is a summary of my continuing studies of exhaust system performance from several tuning books, the internet and students and lecturers of fluid mechanics and wave principle. This study is a non-profitable one and purely for passion and knowledge. All of the above was targeted to achieve on designing a zero-loss exhaust system with street legal noise without having to pay a fortune :D
I dont have dyno to prove each of the above but i have proved it by doing tests on my own car which has cost me thousands of ringgit on exhaust system only and my ride is not an expensive one, its a kelisa :D
thank you
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