Hi,

Dropped off my 317 heads at the machine shop the other day and need to figure out how much to mill asap. The block is an LQ4, the cam I'm running is the Summit 8713r1 (specs below). I have flat top pistons, stock bore, block was decked about 7 thousands or so. Originally, I was going to have the machine shop measure ptv and everything, but they can't take on any more work right now other than heads. I just want to bump up compression a little bit, nothing crazy, and do not want any clearance issues of course. Using stock gm gaskets, btr beehive springs, and 7.4 pushrods (not sure if this is the correct length).

How much can I safely mill these?

 

Is your bottom end assembled? If so, how far out of the cylinder do the pistons come out?

 

Yes, bottom end is assembled. I don't have the engine with me at the moment, so I can't measure now. I remember reading that on a stock LS, pistons are above the deck by .008 or something like that. The machine shop said he never decks blocks more that .010, and he said mine was probably in the neighborhood of 6-7 thousands.

 

With flat top pistons you are already around 10:1 depending on where the piston ends up in relation to the deck.

That cam is small enough you won't have clearance issues.

After about .030 you start to make the decks thin. which starts to become an issue in boosted applications. I think around .050 or .060 you start running into fitment issue with the intake manifold.

 

take .03 off and run with it

 

Ok, thanks, appreciate the info! I am in California and will be running 91 octane (highest they have here). At what compression ratio does 91 become an issue? Also, the vehicle is a 62 K10 on 37's, so I'm not sure if there would be an ideal compression ratio for that?

 

You wouldn't worry about the fact that it was decked and taking that much off the heads? I'm completely new to this stuff so go easy on me hahaha

 

That's kind of a loaded question... There are many factors including the camshaft that play a role in that. 10.5:1 is probably a good all around number but you could probably stretch it to 11.0:1

Mazda's Sky Active use's like a 14:1 compression ratio but they control the cam timing to bleed off cylinder pressure as well as the Direct injection timing and length

 

Gotcha, so if I was targeting somewhere around 10.5:1, what does that equate to as far as milling the heads? .020, .030? And what length pushrod would be used? I bought 7.4's because that's what it seemed like I needed initially, but now I'm not sure if I'd need shorted ones. I'm running LS7 lifters.

 

You will have to measure when your done with everything. It depends on the combustion chamber shape. .025 took my 61cc to 58CC on my 706. With the 317 being so much larger.

With a stock bore, and .005 out of the hole you would need to be about 68cc with a stock gasket. That would be about 10.5:1