Are there any real differences between a 4 into 2
into 1 and a 4 into 1 headers?
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 4-2-1 type
will. A 4-2-1 is tuned twice, first in the 4-2
range, then in the 2-1 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
4-1 = highest peak power header design, but
narrowest rpm band
4-2-1 = broader rpm band, but not as high a
All other things equal:
Longer pipes = lower engine rpm power range
Longer collectors = better overall performance
-Thanks Nolan Penney