"Freeman, Noah" wrote:
> Hello all-
> As someone who is going to have to at least partially replace his exhaust,
> im interested in learning about this stuff, but find it confusing...
> I initially thought: Less obstruction (free flow muffler and wide pipes)
> leads to less resistance for the enginge to fight, so always more
> efficient/fast/powerful motor, with a lot more noise.
> But now im hearing that large pipes, etc can actually hurt low end power?
> Is this true? Why?
I think that applies to turbo cars. A too-large diameter means
there's a large volume for gas to fill, so flow slows down, and
turbos don't spool well. That said, big pipes downstream of the
turbo should help.
Aside from backpressure, changing pipe lengths can have
significant effect on power. Pressure waves emanating from the
cylinder head propagate down-pipe to some terminus, then reflect
back. When the reflection arrives back at the cylinder, it could
either help or hinder exhaust evacuation or charge intake,
depending on which valves are open at the time. The terminus
that reflects the wave could be a sharp bend, a change in pipe
diameter, a silencer/resonator, or the tailpipe outlet.
It could also be where pipes join, such as the collector on your
header. This is why a 4-2-1 header behaves differently than 4-1,
for instance. The joins are at different distances from the
cylinders, so the reflected pressure waves arrive at different
A reflection's time of arrival, expressed as a crankshaft
position, will obviously depend on engine speed as well as pipe
length and the speed of sound in exhaust gas (whatever that is).
This is why a 4-1 header will operate best at one engine speed
(all its downpipes are the same length), while an engine wearing
a 4-2-1 design (with two different lengths) has a wider but lower
power band. You see 4-1 headers on race cars a lot because a
race car typically operates at only one engine speed: redline.
Street cars usually wear 4-2-1 headers.
An exhaust pipe is an organ pipe: same physics apply.
Douglas Frank Compaq Computer Corp.
ZKO 110 Spit Brook Rd. The older I get,
603-884-0501 Nashua, NH USA 03062 the better I was.
/// firstname.lastname@example.org mailing list
/// To unsubscribe send a plain text message to email@example.com
/// with nothing in it but
/// unsubscribe triumphs