bernoulli's principle is not dependent on having a small exhaust dump to be
effective. the bernuolli equation shows that the decrease in pressure will
be the same ( IF the engine is shut off ) weather the exhaust dump has a
small diameter (area) or large diameter (area) for a given velocity of air
rushing by . where the trick comes in is how fast are you compensating for
(negating) this effect with the exhaust flow with the engine running. if
your engine is forcing out enough exhaust compared to the area of the dump
( a small collector), then the bernoulli principle looses its effectiveness
and will reach a point where it no longer applies as the exhaust gasses are
exiting the dump so forcefully that the air flowing around the car can no
longer flow over the end of the dump. actually, the bigger the exhaust dump
the better, since the venturi effect of the bernoulli principle can
possibly work its magic over a much bigger area to hopefully compensate for
the pressure and flow of gasses exiting the engine. besides the bernoulli
principle, other factors such as the shape and orientation of the dump
relative to the air flow over the end of it also play an effect on net
pressure inside the dump via frictional and momentum interactions between
the exhaust gas and the air flowing by the end of the dump. at wide open
throttle bernoullis principle probably does not play a role in reducing
exhaust pressure since exhaust will be exiting the dump very forcefully,
but im sure the frictional force of the air rushing by the end of the dump
may play an "extractive " or " venturi" role depending on the geometry and
relative orientation of the dump.
food for thought for anyone who cares, not presented as gospel physics -
regards-
doug @ black radon engineering
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