Compression Ratio and target boost.

[Dan_Gyoba]

Okay, so I'm almost at the stage where it's time to bolt the head on, and I'm getting cold feet about the static compression ratio.

The block and head have been milled 0.005" each, so this is about 0.25mm shaved off. I have a 1.4mm MHG on the bench, but I'm thinking that it may not be such a great idea.

Assuming that the original gasket would have compressed to 1.2mm, and given that the pistons have a rating of 9:1CR, this would give me a static compression ratio of almost 9.2:1, which is very close to the 7M-GE's.

I'm planning on running 15-18 PSI boost. While I feel that this should be possible on pump (94 octane) gas, I'm second-guessing myself.

If I go with the 1.9mm or 2.0mm gasket, it gets closer to stock compression (Though I haven't worked out exactly HOW close to stock yet, since I'm trying NOT to convince myself to buy the additional gasket... But maybe it's a good idea?)

I'd like to hear some thoughts and opinions.

 

[hvyman]

I say run it. Just have a nice tune and your set.

 

[mk3-4-me]

thats pretty much what i have done to my 7m. had .004" taken off the block, the head was milled, but idk how much was taken off. Never got around to measuring the head.

But i have a 1.4mm MHG and im sure ill have higher compression. I also have JE pistons, on the site it says theyre 9.0:1 wtih the .040 over, i have .020 over so idk what they are.

Im going standaone, and am going down to SP to get mine tuned.

I hope this Higher compression helps low end and spool up!

plan on running around 25 psi through my 6766

 

[Devin LeBlanc]

You should be fine. I ran JE 9.0:1 pistons and had the block decked .010 and the head has been decked twice .010 twice so a total of .030 taken off. I run a Cometic 1.3mm Gasket with 28lbs of boost. Granted I run higher octane fuel the compression really isnt a problem, Just watch how much ignition timing you run.

 

[Dan_Gyoba]

I do realise from the head gasket thickness selection thread that a 1.6mm gasket is about what I need in order to maintain the stock combustion chamber parameters. Wish I'd gone through that first.

Still, it's not too late yet, and I could still go to the thicker gasket if it's really going to cause trouble.

Having the forged pistons, I think that I'll have a bit more leeway, and I like the idea of more off-boost power and faster spool to spend less time in that off-boost area. I'm just getting cold feet on if it's ENOUGH more leeway.

I suppose that at this point, the safest route is to get the 1.9 or 2.0mm gasket, which I calculate gives me 8.6:1 compression. This is still increased from stock, but with the squish and quench back into a safer zone. I just hate to spend the money if it's not a significant factor. Wish I understood the engineering well enough to know what 0.2mm of cylinder wall really means to quenching factors, when combined with the increase in static compression due to the pistons.

 

[Devin LeBlanc]

Oh if your forged pistons I would not worry at all. Bump that compression up!!