Listers,
A good book on the subject fleshes out Douglas's points. It is the third
edition of "Scientific Design of Exhaust & Intake Systems" by the late
Philip H. Smith and John C. Morrison (both professional automobile
engineers), pub. in 1971 by G.T.Foulis & Co.
....Pat Mullen, TR3A
Douglas Frank wrote:
>
> "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
> times.
>
> 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.
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