milled 243's
 

I am building as mild ls1 for my Nova. I have a set of 243's that have been milled .030, will that raise my compression ratio too high for 93 octane? Should I use a thicker head gasket? I have a set of stock 799's, would I be better off using them. I know the cam choice will make a big difference. I am running an M6 with 3.73 gears and A/C.

I run 93 with around 11.5 : 1 cr

243's milled .030"
Head Gasket .040"

Using summit compression calculator and assuming you have around 59cc chamber heads now (I used 59) you'll have 11.4-11.5 static compression with a .052 gasket (stock MLS head gasket).

You'll be okay on 93. Keep in mind if you do a decent size cam your dynamic compression will go down, and dynamic compression is what you focus on to know if the fuel you plan to use will be okay or not.

milled 243's
 

The pistons are stock flat tops. 11.5 seems a little high for a cruiser. Any recommendations on a gasket to drop it to around 10.75.

Head gaskets should be used to set the quench, not adjust compression - and from what I've seen any head gasket thick enough to drop three quarters of a point compression will probably be more likely to cause issues with pre detonation than just running the GM MLS gaskets as it is. I'm sure most competent cylinder head shops would also be able to remove a little material from the combustion chamber to make them a touch bigger if you want piece of mind.

If you have stock 799's that might be the way to go for now

Do not set compression with a head gasket. As said above you'll just FUBAR the quench and you'll have even more trouble with ping which is what you usually worry about with high compression. So you'll actually make the problem worse rather than better.

As I said, you're focusing on static compression. That does not determine what fuel you can get away with.

Dynamic compression does.

For an example on static compression.. there are cars straight from the factory with more than that these days. Mazda sky active-G engines are 13:1 and my wife has one in her Mazda 6, it can and does run on as low as 87 octane, my stock 5.3 in my truck is 11.0:1 and can and does run on as low as 87 also. The 6.2 is 11.5 and states premium recommended.

Pick your cam and then ask if 93 will work. Someone will be able to plug cam info into a dynamic calculator and tell you where you will be and if it'll be okay.

If you're concerned, use the stock 799's and a stock gasket or a .045 gasket from WS6store

Not trying to hijack OPs thread, just trying to get my head around the DCR compression threshold for 93 fuel here..
I hear 8.75:1 is ideal, 9:1 is about the upper limit but it depends on things like cylinder fill, cooling, quench etc
I also hear you don't want cylinder pressures exceeding ~210-215 psi.
Lets say OPs engine with an .045 head gasket comes out at 9:1 DCR, 210 psi cylinder cranking pressure and 11.6:1 SCR - do you guys think this would take full timing without pinging or would it depend on the finer details of the build?

As 00PooterSS said it's your dynamic compression that determines the fuel and detonation. The cam selection will play a big part in determining your DCR. for instance if you chose a cam with any amount of valve overlap (exhaust valve and intake valve open at the same time) you will bleed off a bit of compression thus reducing your DCR. that being said I wouldn't think a SCR of 11.5 to 1 would have any trouble what so ever with 93. I believe a factory LS6 had a 11.5/1 SCR and 0 degrees of overlap in the stock cam. As always with a milled head make sure to check your piston to valve clearance.

Keep in mind, truck engines have been 9.66-9.99:1 since 2007 and are good for 87 octane regular unleaded.

What is the thickness of stock GM gasket?

.051 - .053

this is what i could never understand. there both sides to this. i have talked to so many people its always 50/50 on this subject

i have milled 243/799 heads .030 as well. when i needed to get head gaskets half the the people told me to get the .045 ws6store ones and half told me to get .040. not sure which route to go i played it safe and got the thicker .045 ones from the ws6store but its always in the back of my head if i should have gone with the thinner ones. i didnt care about cost.

Good point, and I suppose the LS2 is about 10.9:1 and probably runs 91? (forgive me if I'm off, I'm used to RON rating)

I always thought ideal quench was between .030" and .035" so taking into account the piston protruding maybe .007" or .008" above the deck the .040" gaskets seem to put you right where you want to be with a couple of thousandths wiggle room either way in case its more or less.