1963 340 hp exhaust manifold vs 63 425 hp

El Rat

Well Known Member
Gents what might you all think is the hp gain when going from 340 hp exhaust manifold to 425 hp manifolds or is it depending on the heads I.e 690 vs 817?

1958 delivery

Well Known Member
Supporting Member 2
All big engines pick up a lot of horsepower with good exhaust. The "bigger" the combo. cu in, cam, heads, compression etc the bigger the gains. A 340 hp is relatively tame, I would guess maybe 10-15hp. 425 horse 454 rats pick up 100hp going from closed to open headers.