My understanding of back-pressure is that it is necessary for low-end grunt, but bad for high-end power. I came to this understanding after reading a paper about it that described the effect of back-pressure on cylinder flow during valve overlap. Basically, the paper explained that during low 'flow" periods (i.e. low RPMS), low backpressure (such as that created by aftermarket exhausts) allowed some portion of the intake charge to spill into the exhaust header during valve overlap, thus causing the power decrease. At the high end of the RPM spectrum, the lower back-pressure of an aftermarket exhaust allowed the engine to breathe more easily, but since the pressure in the exhaust header is higher than at low RPM, the intake charge loss is prevented. The result is that the engine breathes easier without losing intake charge during valve overlap (i.e. there is enough pressure in the exhaust header to keep the intake charge inside the cylinder, but an ability to flow higher exhaust gas volumes more efficiently).

Can any of our "engine experts" out there chime in on this and shed some light on the subject?

What I've written above seems to be true given that my GReddy EVO2 has clearly cost me some low-end power, but I have great power at the top.

What do you guys think?

 

nice find!