I am currently looking at a rotating assembly from the WS6Store for a LS3 I am going to build. It is stock crank and +3 pistons to increase the compression. Not making a stroker keeping it factory cubes. I was looking at a Cosmetic head gasket with 4.07 bore and .04 compressed thickness. They cautioned me on using that and to measure the clearances which I had assumed would have to do regardless. My question is do you think it would work or should I increase the compressed thickness? I am trying to be around 11.5:1 compression. Any insight would be greatly appreciated.

 

To get the most compression possible and accurately calculate it, you will need to pre-assemble at least one hole (but preferably cylinders 1,2 7 and 8 all together) to measure where the piston sits in the block (above or below deck and by how much).

Then measure the head cc's and piston cc's. From there you can calculate an accurate compression ratio and will tell you what it would be with different head gasket thicknesses.

 

you dont use the headgasket to change compression, you use the headgasket to set quench.

 

I understand the head gasket doesn't set the compression but from what I understood it helped. I have been reading up on quench but I still do not have a firm grasp on it. Could you elaborate more on quench and how to determine what it should be?

 

The gasket does indeed "set" the compression, but ideally should not be used as a "tool" to do it. You should use chamber and piston volumes for that.

Quench is the close approach of the piston to the flat part of the head deck. It causes the cylinder charge to be violently squeezed into the chamber from the outside edges just as flame travel reaches all the way through the cylinder. It promotes complete and even combustion, without leaving large "pockets" of unburned mixture somewhere that can suddenly ignite as the piston reaches TDC. (detonate) Ideally it should be at about .035" or so; too little, and the piston might hit the head; too much, and the effect is lost. If it's much above .040" it doesn't work. The effect is greater the higher the compression is. It can make a potential difference of several tenths of a point of compression before causing detonation, or conversely, the ability to run considerably greater ignition advance. Either or both of these things may provide more power out of the same fuel with less danger of damage.

In this kind of motor the piston comes slightly out of the bore at TDC, making it very easy to put the quench distance into the sweet range by gasket selection. You should measure how proud of the deck the pistons are at TDC and choose a gasket about .035" thicker than that when compressed. Ideally you would only select gaskets after you have the short block in your possession. Much like you should measure your push rod length only after all of the stuff that affects it is bolted together, rather than trying to outguess it beforehand. As much as we like to just order everything and have it on hand and slap it all together in one uninterrupted flow, unless you're willing to have an assortment on hand, it's best to measure before buying.

 

In both a stock bore Lq9 with milled 15 0821s, and later with bored 60 over +4 dome pistons and milled 35 0821s, I have run Cometic 4.085 40 gaskets without issue.

I had seen several guys run that gasket in similar builds to improve quench and raise compression, and I just went with that.

I presently run about 11.7 cr with a 231-237, 114+2 cam and no detonation.