I've heard that some engines make more power with a muffler that is not a (straight through) race type. In other words, the engine needs more backpressure.

I've seen enough dyno results to know that most aftermarket exhausts for the S give a small increase in HP. So this backpressure thing doesn't appear to be true for the S.

I'm actually trying to find out if the same holds true for the engine in my Scion xA (without buying a lot of parts and testing).

I thought a more free-flowing exhaust would ALWAYS produce more HP than a more restrictive one.

Can anyone enlighten me on this topic?


-Pete

 

From my understanding, back pressure is most closely associated with low rpm torque.

 

i've read that its not true in either super street or modified magazine.. just can remember which.

 

What's not true and what are the online references?

 

Its not backpressure that cars need its exhaust velocity

You basically want to optimize your exhaust system so you have the correct exhaust velocity for the given engine. Having it exit to fast or to slow will result in loss of HP. Its more techinal then that, but I think thats it in lamen's terms.

 

2nd the velocity principal.

Many people think of it as more back pressure since smaller can be better than larger, but it is the appropriate size to effectively scavenge the cylinder.

The gas should flow the whole way thru, not stagnate or reduce velocity.

 

+1 on exhaust velocity. Something about "scavenging".

 

Requiring backpressure is fiction. You always want the least amount of backpressure as possible to reduce wasted HP the engine has to use to push out exhaust gas. Simple as that.

The subtle thing where people get confused about is to think a larger exhaust pipe always equals lower backpressure. If the pipe is to wide and long and there isn't enough exhaust flow, the exhaust gas is slow to exit the exhaust, cools down gets dense and slows down. It ends up being a virtual plug increasing the effective backpressure. This is where the idea that you NEED backpressure for low RPM torque comes from, but really its an improperly over sized exhaust with poor exhaust velocity that CAUSES a backpressure problem. The trick is to choose the exhaust size to optimize the RPM where you want the torque/power or pick something in between to have OK torque/power over the entire RPM range.

Scavenging is an effect that properly designed headers have. They are tuned to a resonate frequency where pressure waves to build up and help suck out exhaust gas from the engine, freeing up some HP at certain engine RPM ranges.

 

^^yes.

also, a small amount of backpressure is desired for the cat to work best. you need the exhaust gasses to stay in the cat longer for the chemicals in the cat and exhaust to mix. this is from an emmision point of view. not for engine power.

 

Backpressure occurs when the piping is too small to accommodate the amount of exhaust flow at a given rpm. It is true that lower rpm operation will benefit from a tuned manifold and smooth length of exhaust piping. Scavanging can be thought of in the same sense as a siphon...at low rpm the (properly tuned) exhaust acts like a siphon to remove spent exhaust. At high rpm the same exhaust may not work well, because it's too restrictive. A compromise is usually reached for street cars between low rpm operation and high rpm restriction (not to mention noise). Like everything else on a street car it's a compromise.