exhaust back pressure

[chopped66impala]

hey y'all, i would like to know some of you guys opinions on this topic, it got me thinking and you guys have alot more experience and would have a better idea. i was havin a yarn to a guy at work about exhaust back pressure in performance cars, he reckons it helps engines run better but couldnt tell me exactly why and was stumbling with his words then started mumbling and trailing off about different rev ranges and sound pulses or whatever and mufflers making the car quieter etc which didnt help convince me its beneficial, i thought the idea was to get air in and out of an engine as quick as possible and how can back pressure from mufflers or whatever be helping get air in and out of the engine if its a restriction that holds spent exhaust gas in the exhaust system and when the exhaust valve is open the gas aint fully leaving the combustion chamber and is diluting the incoming fuel air charge, why else would people drive their car to the track then uncap the headers to race then cap them back up to go home, wouldnt they just race with the exhaust system in place?

so are there any circumstances where exhaust back pressure helps engines run better

[BigTruckDriver]

Back pressure does work for you moving around town at lower rpms . I would think in a high perfomance vehicle (like a race car only) back pressure should be kept at a minimum. People only use mufflers because the rules call for them.

oh ya, mufflers are for wimps

[chopped66impala]

how does it help out down low round town in a streeter? whats it actually doin to help move along? just wonderin

[glennsexton]

Ideally, we need to eliminate all of the burned gasses from the cylinder when the piston gets to top dead center (TDC) of the exhaust stroke. The more backpressure there is in the exhaust system (headers, catalytic converter, and mufflers), the higher the pressure will be inside the cylinder, lessening the exhaust gas that is expelled.

While not wanting to “over simplify” things, an engine is just a big air pump - more air in and out typically results in more usable power. As such, we don’t want spent gasses inside the cylinder, so we want the backpressure to be as low as possible.

There are several other factors, i.e., cam and valve timing that the pros use to tune and tweak just when the spent gases are released and new, fresh air/fuel charges are drawn into the cylinder. There are several such pros on this site who will, I'm certain, chime in with their expertise.

Also remember that atmospheric pressure has an effect on backpressure, especially when tuning for the last little bit of flow.

Headers are one of the best and easiest bolt-on accessories you can use to improve an engine's performance. The primary goal of headers is to make it easier for the engine to expel the spent exhaust gases out of the cylinders. Exhaust headers eliminate some of the back pressure created by the stock manifolds by providing each cylinder with its own exhaust pipe. Typically, the individual pipes are cut and bent so that each one is the same length. These individual pipes come together in a larger pipe (collector). The equal length allows each cylinder's exhaust gases to arrive in the collector in a manner that eliminates any back pressure generated by the cylinders sharing the collector.

Think dragsters and funny cars – short very open exhaust!

G’day and Happy New Year,
Glenn

[Dave Severson]

We played with exhaust and headers on the dyno once.... Actually, it seemed that scavenging had a bigger effect on low end torque and top end horsepower more then the back pressure. We'd made up some different collectors and tried them, mainly trying to peak the torque numbers within the normal operating rpm range. My favorite to play with is the 4 into 2 into 1 collectors, and the Tri-Y headers.... But, as Glenn said the type of exhaust and what the engine likes and dislikes has a lot to do with the cam specs and the flow rates of the heads.... About all we've determined so far is that, (1) in higher rpm's, the 4 into 2 into 1 collectors had the best numbers--whether it was with open headers or with a big tube exhaust system....and (2) dyno time gets expensive when you're just playing around with minimal changes in the power.......

The 4 into 2 into 1 showed some very minor increases (like 8 ft. lbs torque, 5 hp) by pairing the pipes with consideration given to the 2 cylinder's relationship in the firing order....

Anyway, we spent a bunch of time and money and didn't really have much to show for it.....

I'm next going to try the merge collectors, and possibly a tri-y header with a merge collector....To do much more then that, I'd have to own my own dyno!!!!!!

[chopped66impala]

interesting

[Dave Severson]

interesting

Yeah, we get bored out here in the sticks and mess around with all kinds of insignificant, meaningless stuff!!!!!