Header/Exhaust Information

Q:
Are there any real differences between a 4 into 2 into 1 and a 4 into 1 headers?

A:
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 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

-Thanks Nolan Penney