Ok I am about a week out from my build im waiting on main bearings and ordering head gaskets. I am starting to worry about my compression ratio, I have read that cams can affect CR is this true? I am going to be using drop in ej257 replacements made by MAHLE, and helix 264 cams, I will probably have the heads redecked, and use stock STi headgaskets because I have heard that thicker gaskets is not the way to go to raise CR. What am I looking at for my CR. Thanks.

My setup will be....

FP red turbo
Cobb rod bearings
Cosworth main bearings
Mahle pistons
stock sti rods

Maybe ill just have to use water injection to run more boost on pump if my cr is too high.

Should be fun ill start another post when the build starts taking place. Car will be tuned by Jon @ TDC mid april.

someone correct me if im wrong but i don't see anyway that cams can change your compression ratio. also milling the heads and block usually won't do much as it only takes off a few thousands. really the piston, head gasket thickness, rod lenth, and head CC will determine the compression ratio. if you can find all the specs then you can find the actual compression ratio.

The cams don't change your ratio. The heads can change your ratio as well as your pistons. Some people get pistons to make up for the higher compression from the WRX heads so they can run more boost.

they can change static compression . i read a post on it . do a search and it will pop up :)

cams with longer duration will alter the CR - they lower it. it depends on the opening point of the intake valve in degrees. the closer it is to 0 degrees the closer it is to "static" - static meaning the ratio if the valves would open @ 0 degress. the farther away from "0" (positively) the lower the ratio.

I have heard that thicker gaskets is not the way to go to raise CR. What am I looking at for my CR.
Thicker HG lowers CR not raises it. If you look at my thread (few below this one), I'm running the same setup. The CR was fine. I run less timing than 8.0:1 motors but the tq is awesome.

Also, I would suggest you NOT just drop in the pistons. Take it to a machine shop and have the bores honed to fit. Hell, it only cost me $50 to get it done.

Cams have a great affect on compression ratio... depending on your duration.
With my project I calculated my cams, they changed my dynamic compression ratio by 1 point. So basically my target compression was 9.0:1(dynamic) so I had to order pistons with a static compression ratio of 10.0:1 to make up for the difference.
And yes im still running 28+ PSI.

Cams can affect the dynamic compression ratio. You can think of it as loosing a little of your intake air becuase the valve is still open as the piston starts to compress. I don't know of anyway to calculate a dynamic CR, but obviously people have observed the effect. Calculating the static CR is pretty easy. I'm pretty sure with stock STI block and WRX heads (compression volume =51 ml (cc)) it's 8.7.

A thicker head gasket will lower your compression ratio, but it also affects the 'squish' or 'quench' area that introduces turbulence into the gas-air mixture. The 'quench effect' quenchs detonation. Knowing how much you can increase the head clearance with a thicker head gasket and still not affect the squish adversely is unknown and therefore risky.

If you were going for huge power, you would not be using the WRX heads. Personally, I'd rather live with the 8.7 CR than have the possibility of ruining my squish. It's not that high a CR--you can still run a lot of boost, maybe not as much as if you had a CR of 8.2 but probably enough for you application.

My $0.02.

Opps I just saw you are using Mahle pistons.

They are not 8.5 CR pistons as you have probably seen advertised. Their dish volume is identical to the stock STI pistons, head space is the same too. They are CR=8.2 just like the stockers.

Now use the stock head gasket and call it a day. Or if you are going to use the Cometic look for RiftsWRX thread on the 3rd head gasket he blew in the 2.5 forum.

Ok I am about a week out from my build im waiting on main bearings and ordering head gaskets. I am starting to worry about my compression ratio, I have read that cams can affect CR is this true? I am going to be using drop in ej257 replacements made by MAHLE, and helix 264 cams, I will probably have the heads redecked, and use stock STi headgaskets because I have heard that thicker gaskets is not the way to go to raise CR. What am I looking at for my CR. Thanks.

My setup will be....

FP red turbo
Cobb rod bearings
Cosworth main bearings
Mahle pistons
stock sti rods

Maybe ill just have to use water injection to run more boost on pump if my cr is too high.

Should be fun ill start another post when the build starts taking place. Car will be tuned by Jon @ TDC mid april.

The compression ratio of your setup will be 8.7 to 8.8 depending on how much is surfaced from your heads :)

Quirt Crawford
www.crawfordperformance.com

Thicker HG lowers CR not raises it. If you look at my thread (few below this one), I'm running the same setup. The CR was fine. I run less timing than 8.0:1 motors but the tq is awesome.

Also, I would suggest you NOT just drop in the pistons. Take it to a machine shop and have the bores honed to fit. Hell, it only cost me $50 to get it done.


Thats what I meant thicker gaskets is not the way to go to lower CR.

I will have each cylinder honed to each piston. What I meant by drop in replacment was factory 99.5mm pistons. Not oversized.

Are you running aftermarket cams? If so what kind? Ill go take a look at your thread now!

Cams can affect the dynamic compression ratio. You can think of it as loosing a little of your intake air becuase the valve is still open as the piston starts to compress. I don't know of anyway to calculate a dynamic CR, but obviously people have observed the effect. Calculating the static CR is pretty easy. I'm pretty sure with stock STI block and WRX heads (compression volume =51 ml (cc)) it's 8.7.

A thicker head gasket will lower your compression ratio, but it also affects the 'squish' or 'quench' area that introduces turbulence into the gas-air mixture. The 'quench effect' quenchs detonation. Knowing how much you can increase the head clearance with a thicker head gasket and still not affect the squish adversely is unknown and therefore risky.

If you were going for huge power, you would not be using the WRX heads. Personally, I'd rather live with the 8.7 CR than have the possibility of ruining my squish. It's not that high a CR--you can still run a lot of boost, maybe not as much as if you had a CR of 8.2 but probably enough for you application.

My $0.02.


Thanks for everyones help. What is the difference between static and dynamic CR? Sorry im a noob when it comes to this important stuff.

Also your right im not looking for huge numbers I probably bought a turbo that is a little too big, but I didnt want to grow out of it and I dont think I will. The red gives me room to grow into more power if I choose. I couldnt justify spending the same amount of money for a green and regreting it a year down the road when I wanted to make more power and kicking myself for not buying the red. Will it be more work to make it fit and will it have more lag? yes, but it should be fun regardless.

Thanks for everyones help. What is the difference between static and dynamic CR? Sorry im a noob when it comes to this important stuff.


i had no clue about dynamic CR so i googled it and i found this site that helped with alot and it also has the formulas and a calculator program on the bottem, http://www.empirenet.com/pkelley2/DynamicCR.html

Static CR is a calculation based on swept volume of the piston as it moves from the bottom of it's stroke to the top, AND the volume of the combustion chamber at TDC.

Dynamic CR is more of actual measurement based on the same things as above, but also including some more intangible variables like leaky valves, open valves (cams) ring seal. Cams can change the amount of air compressed if the intake and exhaust valves are open at the same time and the piston is moving up in it's compression stroke.

I'm not sure about Quirt's statement on your CR, beause I don't know the combustion chamber volume of the '06 WRX 2.5L engine. If it's less than the STI's the CR will be higher.

This is an excellent thread to learn about the EJ engine's CR and the mixing and matching of heads:

http://forums.nasioc.com/forums/showthread.php?t=630797