Exhaust system stuff

[gjkscruggs]

There's been some conversation regarding best exhaust system diameter for
our turbo cars and I'll throw in what I've found.

Although Corky Bell does say that the best exhaust for a turbo is no
exhaust, that doesn't suggest that the largest diameter exhaust system is
therefore the best.  Freestream atmospheric pressure at turbo exit is one
thing... a huge sewer pipe exhaust is quite another.  An exhaust pipe is an
enclosed column of hot gas that is rapidly cooling, being pushed from the
hot end of the pipe and has only one direction to go.  Given the wall
friction (drag) and the gas that is becoming more dense (due to cooling) as
it goes down the tube, the situation is not ideal.  Making the diameter of
the system larger makes the problem worse until the diameter gets so large
as to simulate freestream atmospheric volumes and that would likely have to
be larger than 10" in diameter.

Bell states in his exhaust system chapter on page 132, " 'The larger the
better' is not the case."  He also includes a table, 11-4 on page 133, where
he shows the recommended exhaust system diameter for a given horsepower.  He
shows that a 3" exhaust system is ideal for the exhaust flow of a 700hp
engine.  Not since the factory raced these cars has that number been
mentioned... and they essentially didn't have an exhaust system.

My 2 1/2" system is good for 450hp so there is little danger that I'll out
strip it's capability anytime soon.

Yeah, several folks in our community run 3" exhaust systems and they swear
by them... but no one has yet brought to the list dynoed differences between
a 3" and 21/2" system.

Regards, Gross Scruggs