Anyone know what the stock compression ration is for a L31 Vortec 350? I ask because I'm getting one for practically nothing next week. So far I'm ordering a Performer RPM Air-Gap intake and the XR276HR cam from comp along with the beehive springs. However I have no clue what the stock dish pistons are in this motor. I would like to have a motor close to 400hp running off of 92-93 octane. So yeah if anyone could fill me in on what compression ratio I should shoot for to achieve this it would be a great help.

9.1-1

Thanks. Do you know what stock cc dish is on the pistons?

Also I think I want to increase the compression ratio to 10:1. Would a flap top piston achieve this or what size dished piston should I use?

Please forgive my ignorance I don't know much but am trying to learn.

It will be a roughly 16cc dish (I assumed a 4.00 bore, .015 shim head gasket, and .025 deck clearance, which is typical of GM)

A flat top piston will get it to almost exactly 10:1, depending on what head gasket you use, with those Vortec heads (which have 64cc chambers).

Sraight forwarded no-bs answers. I like this place already.

I'm looking at summit, and it has flat top pistons with 2 and 4 valve reliefs. Which is better and why?

Also I assume then I should run the .015 shim head gasket to achieve the 10:1?

Shim gaskets are fine, but the block and heads MUST BE ABSOLUTELY STRAIGHT if you are going to use them. If you have the block decked and the heads checked/shaved for straightness then they will be fine. If they aren't absolutely straight, you'll have problems with a shim gasket.

I'd recommend going with one of the thinner composite head gaskets (GM has one for the L98 Corvette engine thats quite good and still only .028 think). Most generic gaskets are between .036-.041 thick, which is also fine. Head gaskets can change the CR, but not a whole lot. It will only alter it a fraction of a point in either direction.

As for pistons, stick with the 4-relief sets, they are the 'generic' style. Keep it simple.