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One of the local Texas Pontiac racers runs a 455 SD heads with a small Ross dome to get closer to 9:1, Performer RPM, small tube Hedmans, Comp 242@ 0.050 cam and 3.55 gears in a 64 GTO and runs 11.80s in the 95 degree Texas heat. Look for the car in a future HPP.

I've flowed a bunch of different heads on my home meade flow bench some ported some stock Ds and HOs and RAIVS. You can get ported Ds to flow what stock round ports flow no problem. But just put a pocket porting on some round ports and you leave the Ds in the dust. EVEN in the low lift range. Friends stock 197s flow better than his ported 72 HOs(ported by a knowledgeable Pontiac porter). RAIVs leave them both esp if you put a flat intake valve in them. That stock 71 HO head also flowed as good as some SD Performance 6Xs we flowed the same day.

Done correctly you don't kill all the low lift flow with porting and the rounds have the same as a ported D.

When it comes to port size and velocity I disagree with Pete. Shoot the new Fast Burn small block Chevy has a 215cc intake port, Vortec almost that big for a 350 ci motor. Tell me a long stroke fast piston speed 455 is not going to make more "suck" on the port and pick up velocity! These SBC motors peak at 6000 rpm so not high rpm motors either.
These SBC are put on their crate motor.

11.80s and drive it to the track, sorry I'll vote for round ports any day. Throw some good rods in it and it can turn to 6000 without worry(my old worked cast ones did in a RAIV 455 too.)