backpressure - why, and how much? - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 02-23-2004, 03:08 AM   #1
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Default backpressure - why, and how much?

I've heard it said that a bit of backpressure from our exhaust systems is a good thing, at least for the NA'*. In other words, you want neither a terribly restrictive, nor extremely free-flowing exhaust system.

Why is this? It seems to me that the easier it is for the engine to pump out the spent gasses, the better. And that then the combustion chamber would then be as empty as possible, and ready to inhale the next charge of air/fuel without any remaining semi-pressurized exhaust gasses working against it.

I don't doubt the general consensus that backpressure can be a good thing. I just want to know the theory behind it.

Question #2: How much is ideal? Obviously, too much pressure is bad. But will a high flow muffler or cat get us closer to the ideal pressure, or farther from it?

How long will it be before someone comes up with an exhaust pressure regulator (to keep the pressure in a 'good' range throughout the rpm range)?
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Old 02-23-2004, 03:24 AM   #2
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Ok...we'll try the helicopter approach.......

A helicopter pushes the air down to create 'lift' in a generic sense. At lower altitudes, where the air is more dense, it'* able to perform better because it has something to 'push against'. At higher altitudes, it has less to push against, and performance decreases before the engines are starved for air. This is why helicopters have lower service ceilings than most fixed-wing aircraft.

Your engine is much the same. The pistons need something to push against. Too little or too much is bad for performance. With too little, the pistons are just slapping around.

How much is too little? Trial and error. I'm no scientist. This is a very generic analogy, but it should help.
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Old 02-23-2004, 10:31 AM   #3
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good answer ive always wondered this myself
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Old 02-23-2004, 10:35 AM   #4
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In a V6 likes ours that only pumps so much air, we need a certain amount of backpressure to perform. In a V8, which moves around a ton more air than ours, can be set up with a nice exhaust kit and gain a ton of power, only because that engine is restrictive in the first place, and too much backpressure is made.

Although I did hear that in 4cyl engine, backpressure isn't an issue, so you can get the biggest exhaust you want on it, and be fine. (Is this true?)
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Old 02-23-2004, 07:02 PM   #5
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The 4 cylinders are pushing less weight, and creating less horsies. We are right in the middle of the pack with our V6, so we need to be right in the middle with tuning.
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Old 02-23-2004, 09:32 PM   #6
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well i heard that with a forced induction car you want to have the intake and exhaust really free flowing, i also talked to jeff about this and thought he agreed, what do you guys think? will?
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Old 02-23-2004, 09:33 PM   #7
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There'* still a point where you have too little backpressure. You need to decide what you want. If you want high end horsepower, open it up all the way. If you want to balance the two (high hp and low tq), you have to be very careful.
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Old 02-23-2004, 10:25 PM   #8
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That'* the biggest thing, the amount of torque you are robbing from the engine when you open an exhaust too much. Like Bill said, open it up all the way and you will gain more high end hp (but lose a lot of low end tq, which is our advantage). That'* why Stock cars don't have mufflers.
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Old 02-25-2004, 02:29 PM   #9
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I still don't understand why we need backpressure. I'm sure there'* a logical reason. The helicopter analogy (while appreciated), doesn't do it for me, because while the copter loses lift as it goes into thinner air, the rotors spin more freely. More freely = more power. Our bonnies are trying to make power, not lift.

As for the 4 cylinder vs. 6 cyl. vs. 8 cyl thing; 4 cylinders are usually multivalve wonders, and mostly make horsepower at high rpm'*. Our 3800'* (and most american v8'*, I think), make gobs of torque down low in the rpm'*. While an open exhaust can help the maximum hp numbers, like Will said, it can also hurt torque, if taken too far.

I just don't understand how. And I want to.
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Old 02-25-2004, 02:40 PM   #10
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http://web.tampabay.rr.com/redroby/3liter/exhaust.html

The technical stuff behind backpressure.
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