Here is the useful info on exhausts
In a message dated 20/09/00 14:19:21 GMT Daylight Time,
michael.graziano@csfb.com writes:
<< Just a bit on header differences that was on the Spitfire list.
ANyone care to comment?
Mike
-----Original Message-----
From: Nolan Penney [mailto:npenney@erols.com]
Sent: Tuesday, September 19, 2000 7:57 PM
To: Vic Whitmore
Cc: Spitfires Maillist
Subject: Re: 4-2-1 Exhaust Header
> 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 in
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 have
the peak power the 4-1 has.
There is always the matter of the runner lengths. Most headers sold for the
street
have runners that are far too short. You get a power increase with these
headers,
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, but
you get progressively less performance this way. A street header tuned on
primary
frequencies would have the collector out towards the differential. The only
street
people I know willing to do this are the rotary engine folks, otherwise it's
racers
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
trumpet
is a high frequency instrument because of its short passages. A trombone
can make
high frequency sounds, and a trumpet can make low frequency sounds, but they
don't
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
headers
have terrible collectors. A collector should be at least a foot long, to
gradually
blend the flowing gasses together, and to use the sound frequency for
scavenging.
Instead, lots of street header collectors are made a few inches long, with
the down
pipes just smashed into them. Really defeats the purpose of a header.
They still
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
improvements
that you do with say a GM log manifold.
There are freeware computer programs that will allow you to experiment with
header
pipe lengths and collectors, so you can see for yourself what sort of
lengths you
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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