LQ4 Piston Specs
LQ4 Piston Specs I am looking for the specifications on stock LQ4 pistons. How much of a dish? Compression Height? How much do they sit down in the bores when stock? I need to know this info so I can figure out my compression ratio, please help me.
I usually use an 8cc dish volume and figure the piston to be .004" out of the hole. My CR calculator gives me 9.20:1 with these numbers which is GM's advertised compression ratio. Also if you calculate compression height with the .004" positive deck clearance that is 1.335".
Correction:
The stock compression is 9.41:1 Stock heads are 71.06 cc, stock pistons are 6.5453 cc, stock the pistons are out of the bores at top dead center, and with the stock head gaskets, the stock quench volume is 11.0984 cc
The stock compression is 9.41:1 Stock heads are 71.06 cc, stock pistons are 6.5453 cc, stock the pistons are out of the bores at top dead center, and with the stock head gaskets, the stock quench volume is 11.0984 cc
Stock gaskets have an 0.052-0.056" compressed thickness. The deck clearance is positive .004-.008. I did some searching and it appears that the 99 models with the steel heads are 9.2 and the later with aluminum heads are 9.4:1(I was using an old GM service manual). I came up with 9.4:1 using a 7cc dish which after some searching appears to be more accurate. There is alot of conflicting info out there and the truth is hard to come by.
If I enter the specifications from post #3 my calculator gives me 9.50:1.
If I enter the specifications from post #3 my calculator gives me 9.50:1.
OP: One block to the next, one head to the next, etc the measurements will be a bit different. For aluminum headed LQ4's I always use 9.4:1 as the static compression, but that will vary slightly. Use numbers that get you close, and you'll be fine. I've never used the tool that would measure piston volume to a tenth of a thousandth of a cc accurately, although I am sure such a tool likely exists.
Get in the right area on your calculations, and call it good. They are decent tools for engine combo design, but there is no end-all-be-all calculation for what compression (static, dynamic, real, etc) to use. It's all about the entire package, including fuel supply, tune, altitude and fuel quality, which those DCR calculators don't take into account 
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COMP - FAST - PACESETTER - DIAMOND RACING - EAGLE SPECIALTY PRODUCTS - CALLIES - COMETIC GASKETS
RAM CLUTCHES - MOSER ENGINEERING - KOOK'S HEADERS - ARP - GM BOLTS AND GASKETS - MSD - NGK
POWERBOND - ASP - AND MORE!
Look, this is really simple. Static compression is swept volume plus un-swept volume, divided by un-swept volume.
To find swept, 1/2 of bore times half of bore times 51.49 times stroke. So for the LQ4 you put in 2x2x51.49x3.6220492.
That gives the swept as 745.9972532.
Now, un-swept: First, ignore the piston-to-bore because most of it goes away as the piston warms up, and the rest of it
ends up getting filled with carbon depostis.
Now, that leaves the head at 71.06, according to GM. If you try a few un-swept volumes through the formula, you'll find
that it takes 88.7037cc to get exactly 9.41:1. Subtracting 71.06 from that is easy, but then you have to calculate how
much of that is the piston. That's done by switching to the LQ9, which is identical except pure flat top pistons. The
difference between the 2 is 6.5453, because the LQ9 is 10.08:1 at 82.1584 cc.
So, whomever was getting 9.50:1 was doing it wrong.
To find swept, 1/2 of bore times half of bore times 51.49 times stroke. So for the LQ4 you put in 2x2x51.49x3.6220492.
That gives the swept as 745.9972532.
Now, un-swept: First, ignore the piston-to-bore because most of it goes away as the piston warms up, and the rest of it
ends up getting filled with carbon depostis.
Now, that leaves the head at 71.06, according to GM. If you try a few un-swept volumes through the formula, you'll find
that it takes 88.7037cc to get exactly 9.41:1. Subtracting 71.06 from that is easy, but then you have to calculate how
much of that is the piston. That's done by switching to the LQ9, which is identical except pure flat top pistons. The
difference between the 2 is 6.5453, because the LQ9 is 10.08:1 at 82.1584 cc.
So, whomever was getting 9.50:1 was doing it wrong.
