Forgive me. I am an idiot when it comes to the mechanics of an engine. can anyone explain to me why the compression ratios are different between a 2bbl 350 and a 4bbl 350. Is it the block that determins the extra squeeze, or the crankshaft, or the pistons? If i have my 2bbl block decked will i see a great difference fot the effort if I am not going the performance route? Reason i am asking is i am in the begining stages of a rebuild on my block and it had to be bored out .020 over. Should i deck or not deck. It will probably nevesee the track so i am going for reliability. I am open for any opinions at this point, so if you got one, please shoot.
there is 2 set of pistions (and thats where the diffrence is between the compression ratios.) I don't know what they are off the top of my head but there about 9 to 1 for 2bbl and some 4bbls and 10.5 to 1 for gs 350's
Don, Compression ratio is determined by pistons(dished or flat top), deck height, head gasket thickness, and bore size. If you tell your machine shop that you want a certain compression ratio, they will calculate how much to deck the block in order to achieve that ratio. You can buy pistons that are advertized as 10.2:1, but they frequently will give you lower compression if you don't deck the block some. If you want the engine to run on pump premium, you need 9.5- 10:1 compression ratio. I would shoot for 10:1 exactly if I was you. A cam shaft also figures into the mix also. You can run more compression with a large duration cam because it bleeds off compression at low RPM. In 1970 there were two compression ratios available. There was a 2 bbl, and a 4bbl with 9:1, and a 4bbl with 10.25:1. In 71, 2 & 4 bbl had the same 9:1. In 72 and up all the motors had 8.5:1. Hope that helps.
