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I have never seen the need in 2" primary tubes when the exhaust port in the head is only 1 3/4". The sudden jump in diameter from 1 3/4" to 2" will cause a drop in the velocity of the exhaust gases as they exit the head. Different story if you've ported the heads and the exh port is much bigger.
But this is exactly why I got the stepped Kooks headers. The first 6" of pipe is 1 3/4 just like the exhaust port. Then increases to 1 7/8" for the rest of the primary tube until it hits the collector and dia. changes to 3". This maintains higher velocity of the exhaust charge. Then the merge collector is designed to "pinch" the diameter so you don't lose velocity in that area.
And for those that don't know...Velocity = Torque This is also true in the intake track.
2" headers MAY provide a tiny bit more power at WOT on the very top end, But will result in less torque everywhere else. Especially idle to part throttle and mid range RPM.
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 Bryan 2018 ZL1 M6 |