[My427stang]

All the information above is essentially correct, however, to be totally 427 anal LOL the "riser" determination comes from the intake port location, however, the carb location ends up to do the same, so chicken versus egg kind of stuff

However, one bit of advice concerns me above. Most hardcore FE guys wont match the intake port to the low riser intake.

Matter of fact, most of us will do one of two things.

1 - Ignore the mismatch, because we know a low riser intake port angle isnt ideal, but we like the intake and want to run it
2 - Weld (or not quite as good, epoxy) the floor of the intake manifold to match the medium riser port and them port match for width

The reason is the short side radius (SSR) basically the floor of the intake port as it curves around to make the corner of the valve, is an area you want as gradual as you can for air flow

By opening the Edelbrock head to LR size, you essentially ask the intake charge to take more, and possibly a sharper corner by the time it gets its way to the valve bowl. At high RPM, this can even move so fast that the charge cant make the corner, and you lose airflow due to turbulence

By leaving it alone and ignoring the mismatch, you actually get a bubble of turbulent air in the intake, where the mismatch is, that acts like a ramp and really effects the airflow less because its so far away from the valve, and the head has time to bring it to the valve in a gradual corner. Seems illogical, but it is 100% true, that run from intake gasket to valve is infinitely more important than the intake gasket to carb.

The best bet is to fill the bottom of the intake manifold ports, and blend them as deep toward the carbs as you can. However, epoxy can loosen up, and welding 8 holes and porting them all aint cheap if you cant do it yourself

To make you feel better about ignoring the mismatch (even though it seems backwards), even Ford showed a 30 hp gain in the 70's with an SCJ 429 intake on standard heads. This is the worst mismatch you'll ever see (intake bigger too like this, but significantly bigger) , but B/L with the way air behaves, its better than hogging out the intake port floor on the head and affecting a nice clean flow to the valve