Re: Exhaust Diameter/back pressure

["J. Denis" <vwbus@xxxxxxxxx>]

Stefano, my 3 cents ($Cdn) worth:
It's not "backpressure" alone that influences an engine's bottom-end
torque.  Backpressure affects cylinder filling and therefore whether
the engine runs rich or lean - which in turn, may (or may not) affect
bottom-end torque.  Ferrari manages exhaust back pressure on the F355
with a valve in the exhaust to control cylinder filling.  Exhaust gas
momentum, and thus cylinder evacuation, are also determined by primary
pipe diameter.

Everything in life is a compromise.  Scott is right to say that the
muffler is the governing factor in determining ultimate exhaust flow.
Except on vehicles without mufflers!  As far as a 3" pipe is
concerned, the advantage is less weight for the flow area, the main
disadvantage (on a street car) being clearances. Otherwise it doesn't
matter (flow area being roughly equal) whether single or twin. Except
that 6 cylinder engines prefer to operate as two separate 3 cyl
engines all the way to atmosphere.

In fact, given the effect of a muffler, it also doesn't really matter
whether the pipe bends are a little squished or not (except w.r.t.
tubing headers).  The closer one is to the exhaust port, the more
critical is the diameter of the pipe (to maintain velocity and
momentum of the gases), the smoothness of the bore (to avoid pressure
drop) and the shape of any transitions.  The length of a header pipe
up to the first joint can, only sometimes, be important to creation of
resonant sound pulse effects (the pressure changes from reflected
sound waves can help to extract gases from the exhaust port).

------------------------------