# Exhaust Backpressure whats the deal?



## Pontiachris (Oct 30, 2009)

i keep hearing from ricers that you need good back pressure to make power...now it might be soo for like turbo applications or something like that but i always thought the less restriction the better that's why i run a 3in ex/system some kid even told me when i had open headers that i would burn up my exhaust valves i told him hes crazy somebody please disprove this myth


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## Rukee (Feb 8, 2007)

It works both ways. The only way the exhaust valves get burned with open headers is after shut down. Cold air can come back up the headers and damage the valve, but it's extremely rare and usually happens only after an extreme high RPM run like with a truck puller or something and then gets shut right off with no cool down time. I don't think you have to worry about that.
On the back pressure it's my understanding that back pressure both helps and hurts HP. Some back pressure helps the bottom end, but too much and you kill the top end with the restricted exhaust. With the torque of the LSx motor you shouldn't have to worry about that either.


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## jpalamar (Jul 22, 2008)

Don't rev the piss out of it right before you turn it off and it is virtually impossible to damage it from running no exhaust. I still wouldn't do it becajuse it just plain sounds like crap and drones horribly. There is a difference between loud and sounding good.

I think Rukee is on the right track about back pressure. I think there was a discussion in the exhaust section and someone posted some info from an exhaust company explaining in detail how it all works.


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## fergyflyer (Apr 18, 2005)

A friend of mine had a 2000 Camaro SS. He put an SLP Loudmouth on it. Basically a straight through exhaust with just a resonator. You could drop a golf ball in it and it came right through. 

He decided to put headers on and he went with 1-7/8" headers. Everyone told him they were too open for a stock motor. With just headers, no cats and the SLP Loudmouth and a tune he put 353 to the wheels. 

The guy that owns the speed shop was amazed. He the took the 1-7/8 headers off and put on a set of 1-3/4 that he was trying to convince my friend were the right set up for the car and did another pull and tuned it for those headers. He gained 2 ft-lbs of torque with the smaller headers but lost 14 hp. The 1-7/8" went right back on and the shop started recomending 1-7/8 for all LS1's stock or not. 

Here's a little test I found. 

GM Performance Parts LS1 Engine Header and Manifold Test - Car Craft Magazine


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## Gunslinger (Jan 5, 2010)

I always say its like breathing out of your mouth. With your mouth wide open it all comes out real fast but its distruptive to NORMAL breathing, close your mouth a little and it makes the whole act more efficient. The back pressure actually helps things run more smoothly, but this all depends on how hard you blow out, just like in an engine. The more power you develope the less restriction you need to balance things out. Top Fuel= no real exhaust Honda civic with no exhaust= powerless crap machine.


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## Poncho Dan (Jun 30, 2009)

I've heard it has to do more with the velocity of the exaust gas pulses rather than "back pressure", and that in scavenging terms (like headers do) you don't want the pulses to slow down as they travel through the system.


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## svede1212 (Nov 1, 2005)

Dan's right on. You really don't want any back pressure, that's a restriction. Smaller pipes do better "down low" because you aren't moving as much volume at lower RPM. The smaller pipe has higher velocity than a larger one and yet it is big enough to not have back pressure. That's also why it loses HP "up top" because at high RPM you're moving a lot more volume thru the pipe and the smaller one now is a restriction. A bigger pipe can handle the volume with little back pressure and the velocity is high because of that volume too.


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## GM4life (Mar 18, 2007)

Everybodys wong. You need backpressure.


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## 05GTO (Oct 6, 2004)

It would be hard to eliminate 100% of the back-pressure, but, by reducing the exhaust back pressure as with headers and high flow exhaust, engine power is increased in four-stroke engines.

The opposite is true for a two-stroke engine, a bulge in the exhaust pipe known as an expansion chamber uses the pressure of the exhaust to create a pump that squeezes more air and fuel into the cylinder during the intake stroke. This provides greater power and fuel efficiency.


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## Lautinjr (Mar 27, 2010)

The truth about exhaust backpressure and torque

The myth of exhaust backpressure [Archive] - My Pro Street

I really like this one since it's from a honda forum.
Backpressure: The Myth and Why It's Wrong - HondaCivicForum.com

This one is most relavent to us.
Exhaust back pressure myth - Mechanical Database


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## Poncho Dan (Jun 30, 2009)

05GTO said:


> The opposite is true for a two-stroke engine, a bulge in the exhaust pipe known as an expansion chamber uses the pressure of the exhaust to create a pump that squeezes more air and fuel into the cylinder during the intake stroke. This provides greater power and fuel efficiency.


:agree

The expansion chamber is there on a 2 stroker to allow the exiting, still burning, exhaust gas a place to slow/stop which is necessary to performance, otherwise most of the intake charge would get vacuumed right through the exhaust port.


Somebody _was_ making vacuum operated butterfly valves that you could install in your exhaust system, which would open and close like a TB. I don't know what ever came of it though.


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## mikes06goat (Jan 13, 2009)

So I was thinking about getting the DMH electric cutouts for my goat for track days but you guys are saying that would hurt performance?


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## svede1212 (Nov 1, 2005)

mikes06goat said:


> So I was thinking about getting the DMH electric cutouts for my goat for track days but you guys are saying that would hurt performance?


no but a change like that in the exhaust will affect the WOT tune somewhat.


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## danfigg (Sep 27, 2009)

*Re*

The faster the exhaust leaves the more power you make. Headers with say a set of glasspack mufflers should make more horse power than say headers threw an entire exhaust system with tailpipes out the back. Also I was told that when it comes to full exhaust cars muffler placement is important. Headers, glasspack, and full exhaust out the back and your car will sound like a harley. I dont think you will make as much horsepower as the headers, pipes out to the rear, and with the mufflers on the very end of the exhaust. Using the same Muffler closer to the headers, Mufflers before the rear axle and mufflers after the rear axle will all have a different sound and have a different horsepower curve. :confused Or am I full of it------------Danfigg


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## svede1212 (Nov 1, 2005)

With flow velocity being the same the muffler placement shouldn't make any difference in power after the H or X pipe. It does affect the sound. The main thing up toward the "front" part is the pressure waves which travel in both the forward and backward direction. Once they hit a place like a glass pack the pressure waves basically "see" an end to the pipe even tho the actual gas has a ways more to go. The scavenging is happening in that "front" part of the system so that's why placement there can affect things


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