Replacing the OEM headers with aftermarket headers does not seem to significantly increase power based on what I have seen on here from others' results, notably: Les (velociabstract).

This has bothered me. Headers are so pretty. On V8 engines good headers can dramatically increase horsepower.

I have been trying to understand why this might be. The OEM headers have 1.75" primaries, which is great. The log style is not great, nor does the 1.75" outlet seem to be.

Here is what I have come up with. On V8 [and inline 4] engines, 4 cylinders feed into each header. For an even-firing engine, the exhaust pulses are separated by 180 degrees of crankshaft rotation. On a 90 degree V6 there are only 3 cylinders per exhaust bank, with 240 degrees of separation between pulses.

I think the scavenging effect in the collector may be largely lost on a V6 because of the increased separation between the exhaust pulses. I have tried to graph this below. With 4 cylinders per bank, the closer exhaust pulse spacing would overlap the low pressure part of the pulse with the opening of the next exhaust valve in the firing order.

v6exhaustpulses_zps3af40da5.jpg