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Re: 4-2-1 Exhaust Header

To: Vic Whitmore <vicwhit@home.com>
Subject: Re: 4-2-1 Exhaust Header
From: Nolan Penney <npenney@erols.com>
Date: Tue, 19 Sep 2000 19:56:32 -0400
> Are there any real differences between a 4 into 2 into 1 and a 4 into 1?

Yes, there are some significant differences.  Basically, a header is a musical
instrument, a trumpet if you will.  A 4-1 header is tuned very tightly for a
specific frequency, and hence a specific engine rpm.  At that frequency and rpm,
this header will flow better then a 421 type will.  A 421 is tuned twice, first 
the 4-2 range, then in the 4-1 range.  This design allows the header to have a
broader power range then the more precisely tuned 4-1 header, but it does not 
the peak power the 4-1 has.

There is always the matter of the runner lengths.  Most headers sold for the 
have runners that are far too short.  You get a power increase with these 
but it has to do with the free flowing nature of the tubes, not the tune of the
header itself.  You can use shorter lengths tuned by fractional standing waves, 
you get progressively less performance this way.  A street header tuned on 
frequencies would have the collector out towards the differential.  The only 
people I know willing to do this are the rotary engine folks, otherwise it's 
only (look at a circle track engine header some time).  As a musical example, a
trombone is a low frequency instrument because of its long passages, while a 
is a high frequency instrument because of its short passages.  A trombone can 
high frequency sounds, and a trumpet can make low frequency sounds, but they 
sound good doing it.  They work best in their primary frequency range.  The same
with headers.

There is also the highly important matter of the collectors.  Most street 
have terrible collectors.  A collector should be at least a foot long, to 
blend the flowing gasses together, and to use the sound frequency for 
Instead, lots of street header collectors are made a few inches long, with the 
pipes  just smashed into them.  Really defeats the purpose of a header.  They 
work because they are often times better then the stock exhaust.  Now a Spitfire
doesn't have a horrible designed stock manifold, so you don't see the 
that you do with say a GM log manifold.

There are freeware computer programs that will allow you to experiment with 
pipe lengths and collectors, so you can see for yourself what sort of lengths 
should be looking at for your application.

Quick re-cap
4-1 = highest peak power header design, but narrowest rpm band
4-2-1 = broader rpm band, but not as high a peak.  All other things equal
Longer pipes = lower engine rpm power range
longer collectors = better overall performance

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