Exhaust scavenging and valve overlap...


$upraman
08-01-2005, 05:38 PM
Just to clear things up...from what I read, a little backpressure in the exhaust system is supposed to help with scavenging and ridding the cylinder of exhaust gases during valve overlap, at least in your everyday production car.

However lets say you have a valvetrain with zero or very little overlap, this scavenging becomes pretty much useless, right? In which case, the objective is to reduce backpressure to pretty much zero, correct?

MagicRat
08-01-2005, 08:22 PM
This gets complicated, but this is the simple version.
Someone may post a more detailed/correct explaination.
Scavanging is not accomplished by backpressure. More accuratly its an exhaust pulse. In a tuned exhaust system, when one exhaust valve opens a pulse travels down the runner (tube) for that valve. At the end of the runner, such as where it ends in a 'collector' (where several runners end) the pulse is reflected or reverses and travels back up the runner. Depending on how its tuned, this effect can occur on an adjacent runner. At a certain rpm level, this pulse can travel back up the runner just as the exhaust valve is opening on the next exhaust stroke. The pulse helps draw gases (scavange) out of the combustion chamber.

Therefore, the scavanging effect has nothing to do with valve overlap, and will occur even when there is no overlap.

One does want pulses that are correctly timed. This is an effect of the exhaust design and rpm range. One does not want any back
pressure cause by a restriction in the exhaust.

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