I've made a conscious decision to not go bigger on my turbo than the one I have now (db20g). That said, my motor build is simply a USDM STI longblock with forged pistons.

Everyone likes to do the "If you raise your compression, your motor will blow up!" quote if you talk about doing it. However, I believe it's directly related the amount of boost your turbo is supplying. I remember Phil at Element Tuning talking about slightly raising the compression for one of the car's he tuned (with a rotated setup!) to get slightly more power.

The question I have is this. If my turbo isn't that big, can't raising the compression slightly be a safe way to get more power? Wouldn't this additional power be an increase across the whole power band? I figured since I'm getting pistons, it's something I should consider.

You should contact the guy at element tuning and ask him more detailed questions. Also I noticed that he did this on a rotated turbo setup. That might be the crucial info that changes some things. I don't know excactly but I figured it's a thing to look into. Last, if you are looking to raise compression buy buying new pistons why not just get a bigger turbo? It would be less time spent on the build and you can prolly sell your current turbo. You probably couldn't sell the pistons as easy.

The only reason I would want a higher compression piston would be for better off boost response an quicker spool up, but if your sticking w/ a 20g IMO you might as well stick w/ a stock short block. Many people are running them relibably on the oem 2.5 w/ no issues. Now if you were going to throw a big turbo on there it would be a different story but I dont think your setup warrents the cost or the extra hassels that will come w/ tuning a high comp. setup.

IIRC element had the heads decked and used a thin headgasket to slightly raise comp. no more than .25 I would think
MAv

I think you can raise the compression somewhat and still run boost. The EJ207 Spec C heads have been used successfully with the EJ257 blocks and this raises compression to 8.7 (stock STI is CR=8.2). There are some people on this forum that have gone as high as CR=9 (some unintentionally).

The only risk you take with a higher CR is that you may not be able to run as much boost and thus less power up top. On the other hand you'll have better low end performance. If you throw race fuel (maybe water injection as well) into the mix then you have less to worry about. The main limit of CR is on pump gas.

Another thing to take into consideration is the cams. If you have cams with greater overlap, the dynamic CR is lowered. Thus a car with more aggressive cams can probably sustain a higher static CR than one with the stock cams. I thought of building an engine with higher compression and aggressive cams in the thought that the two will complement one another (better low end and boost onset with higher CR, and more top end with overlapping cams that can be sustained by a lowered dynamic CR).

If you have very good tuning capabilities, then it should be worth it. But make sure you have the tuning capabilities!

I roasted two motors that were 9.5-1 by running no EM. Now I have somewhat less compression ratio, maybe as low as 9.0-1 but still had knock until I got a Perfect Power SMT6. Then the knock went away but I am only running 8 psi. I suspect I could get as high as 13-14 psi now with the SMT6 AND the FMIC I just installed, but that would be pushing my luck.

So....

If you are thinking of raising your compression out of the 8.9-1 level into the 9-1 level, I would say no, don't do it.

If you have very good tuning capabilities, then it should be worth it. But make sure you have the tuning capabilities!

I roasted two motors that were 9.5-1 by running no EM. Now I have somewhat less compression ratio, maybe as low as 9.0-1 but still had knock until I got a Perfect Power SMT6. Then the knock went away but I am only running 8 psi. I suspect I could get as high as 13-14 psi now with the SMT6 AND the FMIC I just installed, but that would be pushing my luck.

So....

If you are thinking of raising your compression out of the 8.9-1 level into the 9-1 level, I would say no, don't do it.


Great info guys. Thanks.

I'm installing a STI long block, and am willing to get pistons even though my stock STI shortblocks can handle 20g's all day long. I want to raise the compression in the loss of boost because it is a daily driver. Pre-boost power and faster spool are desirable for my setup. I figured I'd do it with pistons to raise reliability over stock pistons and give me the option of running higher boost on water/alchy injection. The combination of higher compression, faster spool satisfy my daily driver needs while the pistons can help me get my desireable compression as well as add reliability for my alchy/water injection and hit higher boost numbers when it's running. Tell me if there is an error in this thinkin.g

As for tuning, I should have availability. I get all my stuff from World One Perfomance, and they have Phil from Element come down every few months to do tunes for people. I'm planning on using this to my advantage.

I have V7 big port heads with V8 cams going on my ej257 with wiseco pistons and oliver rods. I had the pistons custom made to 8.7:1. Hopefully in 3 weeks or so it should be up and running and then a few weeks after that getting tuned. I expect to run a little less up top in exchange for some good low end that I was looking for. I'll definitely post some dyno charts up to show how the compression affect the curves...

I have V7 big port heads with V8 cams going on my ej257 with wiseco pistons and oliver rods. I had the pistons custom made to 8.7:1. Hopefully in 3 weeks or so it should be up and running and then a few weeks after that getting tuned. I expect to run a little less up top in exchange for some good low end that I was looking for. I'll definitely post some dyno charts up to show how the compression affect the curves...


That would be awesome. Great setup btw!