HP dependent on exhaust length? - Tilted Forum Project Discussion Community

BeBogus · 09-17-2003, 02:36 PM

From what i have noticed and tell me if i am wrong, but I thought the length of your exhaust has some effect on the horsepower an engine can produce. As far as what i have seen is that the longer and straighter the exhaust passage is the more HP can be produced. For example and M3 produces 343 HP and the M roadster, which had a shorter car length and pipe length prodcued less horses with the same engine.

If this is the case, then why would the new McLauren SLR have the exhaust tips right behind the front wheels and not chanelled to the rear of the car?

help me out in understanding whats goin on

DEI37 · 09-17-2003, 02:49 PM

Part of the difference in the M3 and the M Roadster may NOT lie in the difference in length. It may be that the different engine bays necessitate different intake and/or exhaust manifolds or plumbing. It's the same as the C5 Corvette making 345hp from the original LS1 and the F-body cars making 320hp from the same engine. The exhaust manifolds were different, and the exhaust plumbing in the engine bay was different. That is just my observations. Length probably does have some effect, though.

Peryn · 09-17-2003, 04:02 PM

i think backpressure effects the engine and performance, no necesarily exhaust length, though they are often related.

FastShark85 · 09-17-2003, 04:51 PM

Lenght is important with headers, but that's directly related to equalizing pressure for the exhaust. Peryn is right that backpressure is more important than exhaust length.

Some cars need a little backpressure, or else unburnt fuel can be sucked out of the exhaust valves before it is burnt as well as its intended. This partially depends on valve timing, too.

I have an "X" shaped crossover that equalizes backpressure behind my exhaust manifolds. I suspect headers would help, too, but just adding the crossover increased my HP noticeably. The guy who made the x pipe dyno'ed it and claims an increase of about 30 HP. I don't know if it's that dramatic, but I did notice it immediately.

theguyondacouch · 09-17-2003, 05:00 PM

Its not so much lenght, as it is back pressure.

stingc · 09-17-2003, 06:30 PM

Exhaust length is actually important, although it is not as simple as longer being better. Your engine does not spit out air at a constant rate. Each time an exhaust valve opens, a discrete pressure pulse (sound wave) is sent down the pipe. Much of this reflects back to the valves. If the wavefront gets back just as the exhaust valve is about to close, then extra exhaust is pulled out of the cylinder, and some extra intake is sucked in (both exhaust and intake valves are open at this point). So it acts like a mild supercharger, but only works over a narrow rpm band. A very similar thing is done to optimize reflection timing in the intake. Some engine nowadays actually have variable-length intake runners (don't ask me how) to exploit the effect over a wider rpm band.

spacecowboy · 09-18-2003, 01:42 AM

OK explain this lucy!!

If a longer pipe was the thing, why don't dragesters have long pipe??

It's got to be back pressure!

JusticeForPhat · 09-18-2003, 02:21 PM

Stingc took the words out of my mouth. Its called "Hemoltz Resonance Theory". Its an interesting phenomena and can be used to squeeze a couple extra horespower out of engines.

As far as variable intake goes there are several varieties. The two runner design is common on some older cars as well as multiple plenum volumes. GM has done that on some 6 cylinders where there is a butterfly valve connecting 2 different plenums. At a certain rpm the valve opens and connects the plenums making a differnt volume. BMW has a continuosly variable intake that works on an interesting concept, but is a bitch to explain.

09-17-2003, 02:36 PM	#1 (permalink)
BeBogus Upright	HP dependent on exhaust length? From what i have noticed and tell me if i am wrong, but I thought the length of your exhaust has some effect on the horsepower an engine can produce. As far as what i have seen is that the longer and straighter the exhaust passage is the more HP can be produced. For example and M3 produces 343 HP and the M roadster, which had a shorter car length and pipe length prodcued less horses with the same engine. If this is the case, then why would the new McLauren SLR have the exhaust tips right behind the front wheels and not chanelled to the rear of the car? help me out in understanding whats goin on

09-17-2003, 02:49 PM	#2 (permalink)
DEI37 Go faster! Location: Wisconsin	Part of the difference in the M3 and the M Roadster may NOT lie in the difference in length. It may be that the different engine bays necessitate different intake and/or exhaust manifolds or plumbing. It's the same as the C5 Corvette making 345hp from the original LS1 and the F-body cars making 320hp from the same engine. The exhaust manifolds were different, and the exhaust plumbing in the engine bay was different. That is just my observations. Length probably does have some effect, though. __________________ Generally speaking, if you were to get what you really deserve, you might be unpleasantly surprised.

09-17-2003, 04:02 PM	#3 (permalink)
Peryn Junkie Location: San Diego, CA.	i think backpressure effects the engine and performance, no necesarily exhaust length, though they are often related. __________________ Dont cry kid, It's not your fault you suck.

09-17-2003, 04:51 PM	#4 (permalink)
FastShark85 Psycho Location: In the garage, under the car.	Lenght is important with headers, but that's directly related to equalizing pressure for the exhaust. Peryn is right that backpressure is more important than exhaust length. Some cars need a little backpressure, or else unburnt fuel can be sucked out of the exhaust valves before it is burnt as well as its intended. This partially depends on valve timing, too. I have an "X" shaped crossover that equalizes backpressure behind my exhaust manifolds. I suspect headers would help, too, but just adding the crossover increased my HP noticeably. The guy who made the x pipe dyno'ed it and claims an increase of about 30 HP. I don't know if it's that dramatic, but I did notice it immediately. Last edited by FastShark85; 09-17-2003 at 04:54 PM..

09-17-2003, 05:00 PM	#5 (permalink)
theguyondacouch salmon? Location: Outside Providence	Its not so much lenght, as it is back pressure. __________________ "Lick my frozen metal ass!"

09-17-2003, 06:30 PM	#6 (permalink)
stingc Psycho Location: PA	Exhaust length is actually important, although it is not as simple as longer being better. Your engine does not spit out air at a constant rate. Each time an exhaust valve opens, a discrete pressure pulse (sound wave) is sent down the pipe. Much of this reflects back to the valves. If the wavefront gets back just as the exhaust valve is about to close, then extra exhaust is pulled out of the cylinder, and some extra intake is sucked in (both exhaust and intake valves are open at this point). So it acts like a mild supercharger, but only works over a narrow rpm band. A very similar thing is done to optimize reflection timing in the intake. Some engine nowadays actually have variable-length intake runners (don't ask me how) to exploit the effect over a wider rpm band.

09-18-2003, 01:42 AM	#7 (permalink)
spacecowboy Spacecadet Location: out thair	OK explain this lucy!! If a longer pipe was the thing, why don't dragesters have long pipe?? It's got to be back pressure! __________________ ILLEYITIMUS NON GARBURUNDOM

09-18-2003, 02:21 PM	#8 (permalink)
JusticeForPhat Crazy Location: Philly	Stingc took the words out of my mouth. Its called "Hemoltz Resonance Theory". Its an interesting phenomena and can be used to squeeze a couple extra horespower out of engines. As far as variable intake goes there are several varieties. The two runner design is common on some older cars as well as multiple plenum volumes. GM has done that on some 6 cylinders where there is a butterfly valve connecting 2 different plenums. At a certain rpm the valve opens and connects the plenums making a differnt volume. BMW has a continuosly variable intake that works on an interesting concept, but is a bitch to explain.