can anyone answer this 4 me or a round about? I have 2 455's, a '74 and a '70 complete motor. Now, my build up is the '74 block .30 over with 10:1 hyperutectic piston from Poston, '70 heads, '70 intake and '74 carb. I know all of the specs on both these motors stock. '74 is like 8.9:1 and the '70 is 10:1 comp 370 hp and 510 lbs. My main concern is will i come close to 10:1? The other stuff would be great to know too but i love surprises! But im also impaticent,mayby ya might know the round about hp and torque. Shes going in my '79 Trans Am and I plan to make her dance. Thanks guys! P.S.- If anyone is interested, i'm selling my left overs, only cuse i really have no room. Any offers concidered. Message me or even better call me #5165239247 ~Vinny -'70 complete block -'70 carb -'74 intake -'74 heads - exhaust manifolds
Vinny, The only way you are going to know the true compression is to measure everything before you put it together. Advertised compression means nothing due to manufacturing tolerances. Just because you put 10:1 pistons in an engine doesn't mean the compression will be 10:1 It could be more, but more likely will be less. Have the shop cc everything and measure it for you before they put it together. You need to know combustion chamber size, head gasket thickness, amount of overbore, piston dish volume, and how far down in the cylinders the pistons are(deck) Then it's just a calculation. Unless you have the block machined for 0 deck, don't be surprised if your compression is more like 9.5:1
Vinny, I have those exact pistons in my 71 Stage 1. Tony at Poston told me in my 71 motor with 71 cc heads they will be 9.3:1 comp. even though advertised at 10:1. I run steel shim head gasgets so that ups my comp another .5:1 so in all I would have 9.8:1 but I have two other things that work to up compression. .030" overbore and Poston stainless steel valves which supposedly take up 1 more cc in the heads. Here is how it shakes out but remember as Larry said you have to actually cc the set up to know for sure. 70cc heads + 21cc piston relief + 10cc for .040: deck clearance + 5cc steel shim gasket + 944cc .030" overbore on 455 cylinder = (70+21+10+5+944) / (70+21+10+5) = 1050 / 106 = 9.905 so after all that I estimate I have 9.91:1 compression and after checking it's pretty darn close but that's my castings. :Smarty:
It's been posted before..... ....but, here is a LINK to a compression calculator from Ross pistons. Handy little tool.
