Exhaust port matching

[eventhorizon66]

How important is the match between the exhaust port exit shape and header flange shape? This seems to be largely ignored by the aftermarket. On a SBC most performance heads feature a square shaped port exit, while most headers use an oval shape (presumably to accommodate round, square, and d-port shapes). Aside from occasionally causing sealing issues, this causes a significant mismatch at the top and bottom of the port. I understand that a mismatch at the bottom (header being larger) can be beneficial by aiding in anti-reversion. But I would think the top of the port should match as closely as possible. When RHS flow tests their heads with exhaust tubes, do they use square flange tubes or oval?

[amick]

We use an oval/round tube when doing our flow bench testing to represent, as close as possible, having a normal header on the motor.

[eventhorizon66]

Thank you.