Pretty hard to say without knowing for sure all the parts you're using and what the pistons were designed to be used with. If you're using stock 07 sti heads (assumed from your description) with pistons developed to be 8.5:1 with the 07sti engine then with the .7mm gasket you'd be looking at 8.4ish and with the 1.1mm gasket you'd be looking at 8.1-8.2ish. The oem gasket is supposedly .6mm from what I've read and would give you 8.5:1. Thats also the gasket I would use unless you're running larger than 100mm pistons. If the pistons were developed to be 8.5:1 with a 2.5L wrx set of heads, then using them with your sti heads it will be different because they have different heads. In that case the oem .6mm gasket gives you barely over 8:1, the .7mm gasket gives you barely under 8:1 and a 1.1mm gasket gives you basically 7.8:1. I would recommend using the stock gasket to keep the cam timing correct. If you use a thicker gasket it changes the cam timing to be more positive on one side and more negative on the other side.

If you happen to know the volume of the piston dish then I would be able to know for sure assuming unmodified heads. If you modified the heads then it will change also.

 

Awesome,

Thanks so much for the detailed response.

I was told by my engine builder it is about 9.1-9.2 CR..

mods:

07 STI (Euro spec)

Engine:
CP 8.5 CR pistons 99.75mm some machine work was done to increase the valve clearance (refer to pic below), Darton sleeves, Thrust Tech forged con rods, STI Heat treated crank
Heads - Mild port and polish, GSC stg 2 cams, upgraded BC springs (single).

 

No problem. Just basically had to plug some numbers into my spreadsheet. I don't know the head volumes for the Eurospec sti's unless they are the same as the US models. If they are the same then I really don't know how your builder came up with 9.1 or 9.2 especially if you machined the valve reliefs in the pistons more. This would increase the volume of the piston dish and decrease the compression ratio a little bit. Looks like a nice build though I've heard good things about gsc cams and I like the bc springs. Never heard of the thrust tech rods. I'll probably look these up later, off to bed now lol. Might be a good idea to ask your builder how he came up with 9.1. Maybe he actually did some measurements. I don't think it matters much at this point being as you already ordered parts and stuff. But if you wanted to know then measuring the head volumes and piston dish is the most accurate way.

 

Thanks again

Sorry sorry I checked my sms history.. he said it is 8.7-8.8 CR but that was before they increased the valve relief area on the piston which you are right would reduce CR..

I had the GSC stg 2 cams in the last engine and everything u hear about them is tru - they are fantastic... increased spool and bottom end in a big way!!! I was running BC single springs but we had a valve connect with a piston (after we pulled engine apart we found it.. very minimal but still not good). The rev limit was set to 8000rpm may have over rev'd once before.. new engine limit will be 7800rpm

No need to measure, only asking because I'm interested

Tune is on hold as my tuner gets new ECUTEK racerom later in the week Can't wat!!

 

I know this is a fully-loaded question and this debate could prolly go for months, but what's the highest CR these EJ's can take, while still maintaining reliability?

Cuz I know honda's could boost 10.1-10.5 CR on stock motors, and as long as the tune was good they could last for years. I know its apples to oranges, but I'm just wondering if you bump the compression to say 9.2-9.5, if the increase in power would dramatically reduce the life of the engine, if built properly.

 

I know this is a fully-loaded question and this debate could prolly go for months, but what's the highest CR these EJ's can take, while still maintaining reliability?

Cuz I know honda's could boost 10.1-10.5 CR on stock motors, and as long as the tune was good they could last for years. I know its apples to oranges, but I'm just wondering if you bump the compression to say 9.2-9.5, if the increase in power would dramatically reduce the life of the engine, if built properly.

I don't know enough to try and anser that one lol. I personally wouldn't go over 9.5 but thats just me. I also don't have the experience to say it's a bad idea, just a lot of money I don't have to gamble on that. If you can tune it safely I don't see why it wouldn't last. Might be difficult or even impossible to tune safely on the crap gas we get over here though. I've never tried so I really don't know. I do know that 9:1 is pefectly fine even on the crappy california nevada arizona gas we get.