well to start with, unless the first quote is out of context, it's simply
wrong. Shorter primaries emphasize high RPM power, because the negative sonic
pressure wave arrives at the exhaust valve sooner (too early for mid- and low-
range power), if the pipe is short enough the exhaust valve won't be open yet.
But it's very true that there are too many variables for an effective thumb
rule.
-----Original Message-----
From: fot-bounces@autox.team.net on behalf of SHANE Ingate
Sent: Tue 1/2/2007 12:45 PM
To: fot@autox.team.net
Subject: [Fot] Header primary lengths
Amici,
I'm really confused by "rules of thumbs" pertaining to
header lengths. E.g., in a recent 6-Pack article by Kai Radicke,
he states that "..shorter equal-length primaries .. emphasise
mid-range" viz. longer primaries.
Yet, trolling through the Burns web site, I came across this
"Reader's Digest Version" of exhaust tuning theory:
http://www.burnsstainless.com/TechArticles/Theory/theory.html
One thing that stands out in the article:
'There are many ideas about header pipe sizing. Usually the
primary pipe sizing is related to exhaust valve and port size.
Header pipe length is dependent on wave tuning (or lack of it).
Typically, longer pipes tune for lower r.p.m. power and the
shorter pipes favor high r.p.m. power".
As Kas points out in his book(s), exhaust design can get
very complex, but the opposing comments (iterated by
many others) indicates that there are NO rules of thumb,
that all designs have to be tested on the dyno, and that
serendipity is your best friend.
Shane Ingate, terminally confused in Maryland
PS Happy New Year!
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