400 sbc compression ratio

i have a 1976 400 sbc bored 40 over and decked with flat top pistons (4 small valve reliefs).cam is advertised duration of 268 intake 280 exhaust 480 lift and 110 degree lobe separation. i think it might have longer than stock rods as well but not sure so i put a pic to show where my piston comes up to. what would my compression ratio be if u used my ported305 heads with 202's that had a 58cc chamber but have been machined down, not sure how much. could i run 92 octane? to help with rod size i remember the shop saying they stroked it and made the motor more like a 409 or 411 or something. first time poster long time reader. Thanks, Ken


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Need more details. The info you give is sketchy at best. However, anyway you slice it, it won`t run on 92 octane with that high of a compression ratio.
There are a lot more heads on the after market with a larger chamber and far better breathing port wise.

Man,your looking at least a 12:1 CR, a no go for pump gas.

2.02's in a 305 head?

thanks guys i thought so. trying to save some cash on heads so i can install. by the way, 305 heads on an old smogger 350 works well done it 3 times and liked the results. but i get my machine work done real cheap so i guess it might not be worth it for everyone

Can you re-post the picture of the heads? In the other one, it looked like the intake and exhaust valves were ground flat where they came together............looked rather tight. Or ??????????

no touching just real close

how about some 72 or 76cc heads? would that lower compression enough to run pump gas? thanks again

Having done these heads many times I put the chamber volume at 60-63cc.
Installing larger valves in these heads makes the chamber larger than stock.
The #450 heads are typically 60cc in stock form. Based on the appearance of the spark plug boss in the chamber they do not appear to have been milled a lot.
The only way to tell for sure is to actually CC them. (its not that hard to do)
That being said, even at a max 63cc volume with a flat top piston a .025" deck clearance, the cr is going to be too high. Just over 11:1. If the block is decked and the pistons are flush with the deck at TDC the cr is more like 11.9 with a 63cc head. If the head is in fact 58cc or smaller the cr is over 12:1.
These heads are worth $$$'s to some one with a 327-350 project . Sell them and get some 72 to 76cc aftermarket high performance heads.
From the pic the guide boss is still too thick. (needs critical streamlining) They need further porting.
The bowl looks fine. So the remaining port work to finish the job could be done from the intake flange side without messing up the new valve job, using long 6" shaft carbide porting burrs. But looks like a real good start.

Unless you're going to run a huge racey cam and 100+ octane fuel, or change the pistons you;d be best to get a different head for this one.
The cr is going to be just too high for 92 octane no matter how you cut it.

You can buy good dished pistons and do this right for about $270.

KB 168 +.040 pistons

76cc it is then. i appreciate every ones advise

wow! those valves look like they are touching!

i think it's my cell phone camera, there is a gap

Wow, have you had those heads flow tested? Looking at them makes me wonder if you wouldnt be just as well of with 1.94s because of shrouding. ??