Mark said:
Even a lot of high performance cars have a fairly restrictive
exhaust system. I have a 1994 Corvette that responded very
well to aftermarket headers and exhaust even though it was
already putting out 300 hp/300 lb-ft torque.
A more normal car has even more restrictive stock exhaust
and will certainly respond to a more free flowing exhaust...
I used to think that less restriction was always better, but people on
this and other newsgroups educated me on the subject. Here is a post
that I made on this ng after I was educated:
I'm no expert either, but know only what I've read on this subject. On
modern 4-strokes, there is generally considerable valve overlap (i.e.,
there are a few degrees of crankshaft rotation during which both the
intake and exhaust valves are both open. This has to do with dynamics
of the flow that improve performance at higher rpms (IIRC, this has to
do with "scavenging"?).
Without some backpressure, the overlap would cause flow disruption at
lower rpms, and low-end performance would suffer without that
backpressure. With just the right amount of backpressure to optimize
operation throughout the rpm range, the system is considered to be
"tuned"; removing the backpressure "detunes" it. Of course, with a
turbo charger, the turbo itself presents back pressure, so you would
generally want as little other backpressure as posible in a turbocharged
system.
There - that "exhausts" my knowledge on the subject. 8^)
Bill Putney
(to reply by e-mail, replace the last letter of the alphabet in my
address with "x")
