icantthnkof1

I'm looking to fabricate my own headers, specififically in the mid power range. As of now, I'm looking towards a 4-2-1 setup. My question is, what inner diameter would be optimal for each section/transition? Understood there are many factors involved, just seeing if there is a rule of thumb for our engines, Or if I should follow the theoretical formula for scavaging?. Any insight would be greatly appreciated!!

Mort

I have no idea of how to do the calculations but 4 into 1 Long tubes ( collector after the final bend) are generally 1 7/8 " primaries on the M156. MBH is using 2" primaries on his Tri-Wye Long tubes. Some people say 2" primaries on the tri-wye are a little too large for NA but more suitable for FI but it may just be the design of the tri-wye vs the 4 into 1.

icantthnkof1

Thanks, that does help. Gives me something to chew on.

skratch77

I would not go 2in primaries unless you want to supercharge down the road.

I would also stay stock piping and just go catless.i know people will say I'm crazy but the car does lose throttle responce when you go full 3in piping and 2in primaries.

dynos show wot runs and dont exactly show how the car feels part throttle on the street

icantthnkof1

I wouldn't go 2" or wider. Just interesting how they work well without FI. Wondering what was intentional or prioritized (ie., bends, diameter, and fitment) as opposed to a by-product (ie., HP)