okay, whats the difference (in power) between a 4-1 and 4-2-1 header and their collector diameters? like where does a 4-1 give more power and why? and if the collector diameter was 3" where would it give more power than a 2.5" and why? thanks a lot

 

i'm not 100% sure on this topic, but from what i've gathered
4-1 gives better top range power (as opposed to 4-2-1)
4-2-1 gives better midrange power (as opposed to 4-1)
the larger collector 3", would have better top end over the 2.5"
i'd suppose it would have to be due to the size and flow characteristics of a larger piping which would be less restrictive

 

but do you know WHY?

 

anyone?

 

I don't know the exact answer to why a 4-1 header works better than a 4-2-1 but from what I understand of how a header works is that when the exhaust goes through the runners there's supposed to be a sort of vacuum which sucks all the exhaust gas out and sucks air in through the intake valves at the time of overlap (overlap: when the exhaust valves are almost closed and the intake valves are just opening). So I guess that 4-1 headers are less restrictive through the runners. Hope that helped out.

 

Slicedbread has the general theory correct. A 4-1 header will generally have more top end HP than a 4-2-1. The 4-2-1 will generally have more midrange power than the 4-1.

This is due to how the exhaust pulses are tuned for a specific engine and exhaust system. The gas flow pulses are waves and will bounce back with any change in diameter or restriction met and then bounce back once they hit the head itself. You want to tune/time these pulses to suck the next pulse out of the head which will also pull the next intake charge into the head during valve overlap and fill the cylinder better.

The larger tube diameter will flow more air which is generally good for top end but hurt midrange.

These are all generalizations and will depend on the exact system in the car- engine, header, and exhaust.