My machine shop told me I will have around an 8.0-1 CR with my cobb block and the headgasket he had on my 98 2.5rs DOHC heads

But I wanted to run AT LEAST 8.5-1... hopefully 9.0-1 CR....

anyone know which cometic gasket to get? and how much to shave my heads SAFELY ?

what head gasket does he have? if its not an STi HG then maybe using one will raise compression a tad.

do you already have the cobb block? if not go with higher compression pistons in it.

what about picking up some cheap WRX heads to up compression?

just some ideas.

was going to go with cometic HG...
I just gota find out what size and how much I can shave my heads down safely

also my 98 DOHC heads flow better than(earlier)wrx heads AND this car will still pass my mass state inspection...with basicly NO wiring..

why cometic? oem gaskets are great... and i've only heard of thicker than oem cometic gaskets, which would lower compression, not raise it.
but lets get back to the important question... do you already have the cobb block? changing compression via pistons is the correct way to do it.
shaving the heads is less than ideal, to say the least.

Milling the heads is probably not the best way to go. Without adjustable cam gears your cam timing will go way off. Plus you will loose piston to valve clearance just from the milling and the cam timing will make it worse. As for the head gasket the STI gaskets are pretty thin but it is important to maintain adequate piston to head clearance so that may limit how thin a head gasket you can run. Gasket thickness will also affect cam timing and P-V clearance.

What are the specs of the motor:

Bore:
Stroke:
Piston deck clearance:
Piston dish volume.

I can tell you what thickness HG will net you what CR if you have that info.

its a phase 2 EJ25 Cobb block. Everything is the same way it left thier shop...I'm still waiting on an email from them with all the specs on it...

Im doing 11.3:1 in my new motor. What is the reason you are milling your heads?

well the block is already done being built, its a cobb block that had a spun rod bearing... I have them rebuild it with new cleavite bearings, and sleeved with darton iron sleeves to the previous specs so I can use the cobb pistons...

I'm not sure what CR the block was to begin with....phase 2 EJ25. So I am waiting to hear back from cobb.

I had them deck the head a small amount to straighten it out a little bit. I now just have to find the smallest head gasket I can use to get around an 8.5-1 CR.

Im doing 11.3:1 in my new motor. What is the reason you are milling your heads?
Wow! Will you be boosting that?

I have custom made 9.2 CR pistons for my 04 STI.

Im sorry but seems really high.......

AR Fab made 675whp on 10.5:1 motor, soon to be 1000whp....


the FIA WRC motors are in the 10-11:1 range. that is how the make 500ftq and 300whp @ 2200rpms with a restrictor....

lets be fair and also explain that it is highly unlikely that any of these cars will ever use pump gas...

the FIA WRC motors are in the 10-11:1 range. that is how the make 500ftq and 300whp @ 2200rpms with a restrictor....

and they do this on what kind of gas?

some turbos require that you run at certain PRs in order to get the best out of them. I am not sure how running such a high CR will allow you to get anywhere close to the sweetspot of some of these larger turbos without being dependent on leaded race gas :confused:

I think for cars that will be run primarily on race gas, running higher CR's might work just fine. But for a car that will be run on pump gas most of the time and race gas for special occasions, a lower CR might be in order..

The higher you go in CR, the more likely you will never be able to run pump gas. Even without any boost, a CR of 11 is too high for most "premium blends" of pump.

That said botht he Solstice and the new Mini have turbos and are running CR's of 9.5, but with direct injection which is supposed to reduce the likelihood of det.

Another thing to consider is that depending on the cams one uses, the dynamic compression can be lower and a higher static CR might be in order.

11.3 with boost is the highest I've ever heard of for a private car owner. I don't even know Honda's that are in that realm.

9:1 at 20PSI boost on 93 octane here.

A few things I've learned:

1- Going from 8:1 to 9:1 did not make a very noticeable difference in how the car felt. I am sure it shows up in the dyno but it is not super noticeable when I drive the car. I.E. A 9:1 compression boosted 2liter engine is still a 2liter engine. I was expecting EJ25 like low end and it definitely didn't deliver that. Compression ratio is somewhat over rated in my opinion.

2- Stock STI head gaskets are 3 layers of stainless steel. PLENTY good. There is no reason to use anything else. ARP head studs, however, are a must in my opinion. I intend on using them in every engine I build from now on. (From this forum, and also from Quirt Crawford)

3- Do not mess with deck height, head gasket thickness, etc. Messing with the squish area of your combustion chamber is BAD news. The only proper way to alter compression ratio is through piston design or rod length. (from "Maximum Boost", by Corky Bell, also seconded by Crawford)

hhmm with all this info I think I'll stick with the 8-1 if it Really doesnt matter all the much only goin up 1/2 a point.... I would rather keep a more acurate cam timing etc etc.... going to call my machine shop tomoro.. thanks guys !

the heads are 98 DOHC with a half waybetween mild and wild P&P job & 3 angle valve job. He already ordered the cometic gasket...so I'm goin with that and the ARP head studs.