Exhaust System Flow Capacity 1

[140Airpower]

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.

 

[patprimmer]

I would think acceptable noise levels are a major consideration.

Regards
Pat
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[140Airpower]

Pat, yes, noise of course and costs. Those may outweigh everything else. Even so, there may be some empirical sweet spot re longevity vs restriction and back pressure. I remember when Dodge had 3" sewer systems on their big block cars. Is their a liability with that (other than noise)?

 

[SomptingGuy]

Firstly you need to pass the prevailing noise legs for you market.
Then you need to think about exhaust sound quality.


- Steve

 

[patprimmer]

Noise and power are at odds, so the market the car is aimed for has a considerable influence.

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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[SomptingGuy]

... it's quite common for replacement exhausts to not have those drain holes and/or tubes to assist with draining. (Gasoline) mufflers have a short life and don't cost much.

- Steve

 

[dgallup]

Every OE system I've seen in recent years was 100% stainless and last a decade+. Neither cheap or short life.

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[Lou Scannon]

From data I've collected myself, and very limited OEM data that I've managed to get a glance of, I'd say that mainstream OE exhaust systems have peak backpressure of up to 6psig at rated power. Performance vehicles tend to have lower backpressure. I suspect the trend is toward lower backpressure, so my 6psig figure could be a little dated. I'm not so sure that velocity is the preferred countermeasure for condensation, as backpressure correlates with the square of velocity.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[patprimmer]

Velocity certainly helps keep the tailpipe end hotter thus increasing the chance of evaporating water off.

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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[SomptingGuy]

From an acoustic point of view, volume and packaging are the biggest constraints. If your engine is in the front and tailpipe(s) in the rear, you have a lot of variables to play with. If it's a rear/mid engined vehicle, squeezing the required expansion volumes will be a challenge.

- Steve

 

[GregLocock]

Downpipe minimum diameter is set by the need to keep gas speeds low enough to avoid mach related turbulence. Otherwise you have to go to a twin wall downpipe, but these days you might be doing that anyway.

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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[patprimmer]

Greg

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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[Fordz]

What also helps is if you want a free flowing exhaust but not the wrong noise make sure both mufflers are of a different design. Example is the front muffler could be a baffled muffler that flows good good but is way to noisy but by placing a side absorption muffler at the back the sound usually is a good note but not noisy. How it works is the front muffler knocks out certain frequencies and the back muffler will clean up the frequencies that are left.Usually no droning. The mufflers don't need to be huge, just flow and different type is all you will need to worry about.

 

[SomptingGuy]

I could promote my employer and their services and products in this field. But I feel that unethical. They are good though. Farty noises are fun.

- Steve

 

[patprimmer]

Steve. I wish I could even afford the down payment on your employers products. I spend under $500 on simulation software and keep my fingers crossed.

Regards
Pat
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[Marquis]

Rules of thumb for vehicles designed to meet stringent European 74 Db drive by regs are the following:

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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[GregLocock]

Hi Marquis are those upstream of the cat or downstream?

Cheers

Greg Locock


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[Marquis]

Upstream in the catalyst cone (so including the cat monoliths contribution to back pressure)

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[patprimmer]

Marquis

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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[Marquis]

Hello Pat.
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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