Exhaust System Flow Capacity
Exhaust System Flow Capacity
(OP)
Just a question. Is there a rule of thumb for sizing exhaust systems for passenger cars? I assume the main concern is the avoidance of condensation by keeping velocities up. Second would be low back pressure.





RE: Exhaust System Flow Capacity
Regards
Pat
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RE: Exhaust System Flow Capacity
RE: Exhaust System Flow Capacity
Then you need to think about exhaust sound quality.
- Steve
RE: Exhaust System Flow Capacity
Re water collection and rust, it depends on muffler position and presence or not of small drain holes as well as exhaust gas velocity.
From vague memories I would think for a normal family sedan the pipe dia is typically around 2" for 200hp but it is a SWAG
Regards
Pat
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RE: Exhaust System Flow Capacity
- Steve
RE: Exhaust System Flow Capacity
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RE: Exhaust System Flow Capacity
"Schiefgehen will, was schiefgehen kann" - das Murphygesetz
RE: Exhaust System Flow Capacity
Pipe design with no water traps until the muffler also helps a lot.
Drain holes in the muffler also helps, but size is an issue. To small and they block up. To big and they make an objectionable noise and arguably release toxic gas where it might be sucked into the cabin.
Regards
Pat
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RE: Exhaust System Flow Capacity
- Steve
RE: Exhaust System Flow Capacity
The engine boys will bitch and moan as the backpressure rises.
Further down the system the ratio of pipe diameter to expansion chamber diameter defines the max attenuation from simple expansion chambers, but there aren't many of those around as OEM any more.
I've never seen much acoustic effect with tailpipe diameter, but I know that when styling want pretty chrome tips added they can cause problems.
I suspect that it is easier to design a durable exhaust with larger pipes, but never really paid much attention to that.
Cheers
Greg Locock
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RE: Exhaust System Flow Capacity
I certainly got a droning noise problem with an increase in exhaust pipe dia. I think I went from something like 1.75 to 2.5" from the stock catalytic converter back on a 3 litre Toyota Soarer turbo MZ21 model.
It was whisper quiet OEM but raise your voice somewhat to talk and be heard at WOT after the umm upgrade. It sure went better though although a few other minor mods where also done. like boost turned up a few # and mildly ported head and slightly opened up exhaust scroll on the turbo and polished and ceramic coated exhaust manifold and turbo hot side housing. I would guess it went from the factory rated 185kw to maybe 210 to 220.
Regards
Pat
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RE: Exhaust System Flow Capacity
RE: Exhaust System Flow Capacity
- Steve
RE: Exhaust System Flow Capacity
Regards
Pat
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RE: Exhaust System Flow Capacity
Silencer volume should be about 10 to 12 times the engine capacity.
If the engine has a short exhaust- such as a middle or rear engined layout and the entire range of resonators and 3 pass silencing must be accomodated within one big muffler can- then aim to increase your silencer volume to about 16 times engine capacity.
If you're aiming to make a high performance engine that really can't afford high back pressure then also aim for bigger diameter pipes and more silencer volume-again say 15 times eng capacity- to get the Db level back down-however this takes up more packaging space.
If you're package limited for some reason or the manufacturers priority and impetus to reduce back pressure isn't there you can go lower in silencer volume and use the back pressure to silence. But remember high back pressure is a cheap and lousy (read inelegant) way of silencing
Some exceptionally low back pressures from the top of my head
BMW E39 M5 about 250 mbar at peak power
Honda S2000 about 250 mbar at peak power
Aston Martin V8 Vantage coupe about 350 mbars at peak power
Euro BMW M3 3 litre (286 bhp version) about 240 mbars
BMW M3 3.2 "S54"- about 330 mbars
Jaguar X308 XJR about 750 mbars (supercharged means there's more airflow than the peak powerfigure would suggest- due to the parasitic losses of the Rootes blower at full wack)
2002 Ford Mondeo V6 (european) about 650 mbars (600-700 seems to be typical for a run of the mill cost compromised vehicle with no particular sporting pretentions
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RE: Exhaust System Flow Capacity
Cheers
Greg Locock
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RE: Exhaust System Flow Capacity
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RE: Exhaust System Flow Capacity
I have no real data, just anecdotal evidence and suspicions.
1) Does number of cylinders have an influence. I suspect more cylinders for the same displacement can get by with smaller exhaust, but on a diminishing scale? I ask because I am doing some design work on a 3 litre 2 cylinder antique engine refurbishment but with no original pattern or data
2) Can a catalytic converter be considered as part of the silencing system?
3) On economy cars is the cat the most likely part to be undersized from a performance point of view? I ask because I am building a high performance hot rod Honda after market turbo. I think using a much bigger cat is the main step to increased exhaust flow.
Regards
Pat
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RE: Exhaust System Flow Capacity
The rules of silencer volume to engine capacity were rough rules of thumb. Number of cylinders DOES have an effect. It is my understanding that the silencer volume rule of thumb is usually used and then to attenuate booms, orders or certain frequencies (that are usually inherent to a particular engine cylinder configuration) it is decided what KIND of silencing should be use (3 pass silencer, absorbtion silencer, resonator etc).
Cats and Turbos certainly DO effect the required level of silencing. My rules of thumb apply to cars with CATs as that's all i've ever really been involved in, but with no turbos.
So yes, I have considered CATS as part of the system- and Turbos should be considered part of the system too.
Cheaper cars can have more restrictive cats, for sure, but what is more common is more modern cars meeting ever more stringent emissions standards are fitted with denser cats. 600 cells per sq in cat void fractions were common in the late nineties, giving way to 900 cells per sq in in the early 2000s to meet Euro2 and 3 emissions standards.
Oh and 'cheaper cars' will have more restrictive systems and cats, but not neccessarily due to cost cutting, but also because a small engine that flows say a peak engine air flow number of 600 kg/hr is ok with a certain exhaust system that suddently becomes a retriction if that engines performance is raised or a transplant raises the peak engine airflow to 1100 kg/hr!
I should also mention the tuning aspect to exhaust if you have a naturally aspirated engine- the diameter of the exhausts system pipes will effect low speed torque quite significantly about as far back as just behind the front seats on a typical mid size saloon. Behind that the tuning range will be very low and usually out of the engine rev range.
On a boosted engine I would say the predominant factor is restriction and getting the lowest back pressure is more beneficial than designing for exhaust tuning.
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RE: Exhaust System Flow Capacity
Most helpful.
Confirms a few thoughts and creates a few ideas.
Regards
Pat
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RE: Exhaust System Flow Capacity
- Steve
RE: Exhaust System Flow Capacity
I seem to recall David Vizard was suggesting if the back pressure measured at a pipe wall bung was 2.5 psi or less, leave it alone.
Dan T
RE: Exhaust System Flow Capacity
My figures are vehicles that meet emissions, complete with catalytic converters and meeting drive by noise regulations
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Sideways To Victory!
RE: Exhaust System Flow Capacity
RE: Exhaust System Flow Capacity
Exhaust back pressure melts pistons.
- Steve
RE: Exhaust System Flow Capacity
Regards
Pat
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RE: Exhaust System Flow Capacity
I guess I should have said I was offering David Vizard's number as a number I recalled, maybe incorrectly, just as another, interesting (to me, anyway) data point.
(Does he still sport the hairdo he wore in "How to build horsepower". For a while I thought he and Clive Tricky were the same person)
I'm hoping to persuade/convince an impatient young engineer friend to weld a few bungs onto his old Corvette's exhaust before re-sizing the exhaust system for his passenger car.
The high performance values you provided will be quite useful.
thanks
Dan T
RE: Exhaust System Flow Capacity
I didn't ask about his hairstyle
I think he lives in NC.
I think for your own project his figures look good and low, the lower you can go the better.
Back pressure isn't really good for anything- although some folks get confused and talk about BP being better for low speed tuning- what they probably mean is that smaller diameter pipes benefit low speed tuning which just happens to have higher back pressure unfortunately.
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RE: Exhaust System Flow Capacity
Stationary engines typically have even lower backpressure requirements. One application comes to mind, IIRC, in the vicinity of 10" H2O. This is due to the free-floating turbocharger - wastegates are not accepted in the mainstream powergen market.
"Schiefgehen will, was schiefgehen kann" - das Murphygesetz
RE: Exhaust System Flow Capacity
RE: Exhaust System Flow Capacity
"Schiefgehen will, was schiefgehen kann" - das Murphygesetz