Exhaust flow talk

[BenR]

I think there is alot of confused information here.

Backpressure itself, in performance terms, is not wanted. That is a given for all engine tuners strive to reduce pumping losses.

However, the use of back pressure is beneficial when you take it out of context for peak performance as JTA is speaking of.

When you run a larger exhuast diameter you reduce back pressure and restriction, however there is a fine line between optimum and too big.

As previously stated, the exhuast manifold is used to tune an area of efficiency in the exhaust range. With any exhuast manifold you need to design it to operate at a specific rpm range, but you will always loose in another area, and good exhaust manifold design will put this loss in an area which is used least. And the idea between true equal length headers is that you want each exhaust event to correspond with the next one 180 degrees later, and every event is 180 deg apart from each other.

Each exhuast event travels in the exhaust system as a 'slug' which can be broken down into 3 parts. The front is a high pressure area, middle, and rear most is a low pressure area. As this slug travels you need to time the rear of the slug to correspondingly meet the front of the next slug, and this timing depends on rpm, EGT, bore size, cam timing, cylinder head specs etc etc. This is the basis of manifold tunning

Anyway, i'm going completely off topic.

The discussion about back pressure. In regards to bore size, if you have too big a bore you will simply generate non linear flow which will tuble over itself, velocity is needed. Too small and there is plain straight forward restriction. In regards to back pressure reduction resulting in lost torque at lower torque, its quite simple. Larger bore exhausts gain peak power as back pressure and pumping losses are reduced, however at lower rpm where events have a longer timeframe, this low back pressure is not beneficial as it is 'too efficient'. At lower rpm the window of time during valve overlap is too long, and with high specific output engines you always run generous overlap. Where at high rpm the overlap helps, at low your simply reducing the dynamic CR by bleeding off cylinder filling, and this reduces torque. The lower back pressure allows the bleeding to happen more efficiently in the same time frame, hence you feel less torque.

As with all tuning, if you gain one area, you always loose in another....unless you start supercharging.