OK guys i have a Gready header with a DC sports exauste with no resinator or cat so pretty much a muffler? ALSO i have a set of stage 2 crower cams. Do i need back pressre or not? People say yes you need back pressure, other say no what is it? If back pressure is need it is for low end power or high end?

 

I always hear that you need some backpressure...however, a few weeks ago I saw a dyno of a honda (forgot where and the other details, though I think it might've been on http://www.autospeed.com) where they just took off a performance exhaust and had a sidepipe behind the front wheel. It had power gains over the whole band, not a loss in the low end at all. I think you might lose some throttle response in the low end and that's why people think power is lost...though, all cars are different...

 

"Backpressure" is a myth. The cause of "power losses" people refer to is when they don't have enough air velocity and the exhaust pulses become muddled on the way out. If you have aftermarket cams on the car, you'll be fine. Yes, you will actually gain power if you unbolt the exhaust.

 

I was always a couple tenths faster on the track when I unbolted my exhaust. I would run a supertrapp muffler and mess with different back pressures and such, but my car would always be faster with no exhast.

Exhaust and intake pipes should be thinner for low end and wider for high end power, to keep velocity at optimum. This might or might not increase backpressure, but the backpressure is not what increases power. I don't see how backpressure by itself, that is, a pressure opposing the engine on the exhaust stroke, would make you faster in any instance.

No one could ever offer me either a decent explanation or evidence as to how soley increasing backpressure would yield more power. Until someone can do that, I will refuse to believe it.


[Modified by Lsos, 4:49 PM 4/17/2002]

 

OK, backpressure is not what is needing to be discussed here. Just like someone said, its the velocity that is important. When you start getting bigger exaust diameters(or shorter like a side pipe, or less restriction like cat's and mufflers), the air just slowly (not slowly, but slower) makes it way through the exaust. When you decrease the diameter of the exaust, the air is forced to move faster because there is less room inside of the exaust. This faster movement causes a slight vacuum or "pull" on the air in the headers right outside the exaust ports. So when the exaust valves open the burnt gases are sucked out in a sense rather then being pushed out by the incoming air (to a certain extent). Therefore making the engine more efficient. Of course there is a medium in there, you can just make the exaust tiny, then there is too much restriction.

 

Good call. Makes perfect sense. As a moronic side-note...I once took the exhaust off my car at the headers in the parking garage of my apartment complex...I bet the neighbors never heard VTEC so good

 

Yeah, there is an ideal internal diameter for exhaust for each rpm/throttle position so that the exhaust gasses move with highest velocity but the lowest backpressure. I b18c5 at 9000 rpms at full throttle with big cams will make the most power with a much bigger exhaust than an LS b18b at 2000 rpms with 25% throttle. The c5 will just be flowing WAY more air. Any exhaust you buy is a compromise. Your stock exhaust is probably best in the 2500-3500 range but if you drag race you probably want the most power in the 7000-8000+ range (to go with your skunk IM). It's all about where you want your torque peak to be.

-Chris

 

Not back pressure......exhaust flow velocity is what is needed.