a_lopez1975

Bear with me because I'm still new to the Exhaust System concept. Now I've been doing some reading on "back pressure" and from what I understand, Back-pressure forces exhaust back into the engine, reducing performance and/or it's the resistance that the piston has to push against to force the exhaust gases out, thus power being wasted. Please correct me if I'm on the wrong track here. So with this being said, reducing back pressure should creates more power for the engine.

Now my three questions are:
1. Is it good to have no back pressure at all?
2. Or is it good to have at least some back pressure?
3. Are there any benefits to having back pressure?

Any input would be greatly appreciated.

On another note, I'll be leaving Afghanistan next month. Got rid of my shorty headers and bought a set of Long Tube Headers with an O/R Xpipe (in which some friends of my are installing for me right now), so I can't wait to drive my Mustang when I get back...

Norm Peterson

Back pressure by itself isn't a good thing for making power - it leaves more spent charge in the cylinders and "steals" a little torque because it takes more work to move the exhaust gas out the pipe(s). One rule of thumb (from Mark's Mechanical Engineers' Handbook) suggests that 1 psi backpressure corresponds to about a 2.5% reduction in peak power output.

But back pressure also has an implication about exhaust gas velocity in the pipes, with higher exhaust gas velocity in any given exhaust pipe causing more back pressure. Higher gas velocity can be a good thing, at least in moderation.


If you were to size your exhaust system to provide high gas velocity in the midrange - which you'd be hoping would give some extraction or resonance tuning benefits in that rpm range - your exhaust system would tend to be a bit undersize (with too much backpressure) at high revs. This is probably where the "you need to have some backpressure" arguments are coming from, where the higher amount of backpressure is just an unavoidable consequence of tuning for the midrange. Velocity is what you want, so corking a system up with a stuffy muffler to "increase backpressure" is not the hot tip here (you'll definitely get the disadvantages of backpressure while generally not getting much tuning benefit).


Norm

a_lopez1975

Thanks Norm for that breakdown!!!