Think of an exhuast pulse (one puff af exhuast from one cylinder) flowing down
a pipe. Behind the
pusle is an area of low pressure (relative to the pulse high pressure) created
in/by the wake of the
pulse. This low pressure helps suck the next pulse out and is called
scavanging.
If the pipe is to small it restricts the
flow of the pulse (slowing it down). If the pipe is to large the pressure of
the pusle dissipates
and there is no scavanging. I would suspect the stock exhaust size is pretty
close to correct
as the original engineers knew about this issue. They did make compromises in
the exhuast manifold
(for cost, longevity, etc.) so a set of headers will help performance.
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