Comp XE268 - O/T, Brand X!

JFYI: Pontiac lowered their compression in 71 like everybody else did but they did it differently than Mopars. They used the same pistons, deck height, gaskets, etc... but they opened up the head chambers to much higher cc's. I used to know the numbers but it's been so long It's hard to remember but I believe the older heads were around 70 cc's and the newer ones were as high as 100 cc's. Even though you read the compression was rated at 10.75 to 1 it's quite possible their ratings were flawed like Mopars were due to machining tollerances. According to my compression calculator using a stock bore 400 with the piston at zero deck, a .040 thick gasket, 10 cc valve reliefs, and a 70 cc head that only figures up to 10.21 to 1.

BTW: Just for the heck of it I ran a sim on Desktop dyno using flow numbers for the 6X heads (that's the only #'s I have for pontiacs) but used 70 cc chambers with the pistons at zero deck which came out to 10.21 to 1 compression, an Air-gap intake, 750 Holley, big tube headers, and the 274XE cam. It showed 433 hp at 5,000 rpm and 483 ft.lbs of torque at 4000 rpm. Of course Desktop dyno is not all knowing and assumes everything is perfect so unless you spend mega bucks on machining and have it dyno tuned by somebody that knows what their doing it probably won't be that much but it still should have no trouble getting the 400 hp you want.