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Discussion Starter #1
Assuming you had already done the required supporting mods and had a good tune, how much additional power could you expect to gain from increasing the compression ratio in an NA EJ to 11 or 12 to 1? Anyone have idea or actually done this?

I've looked around on this and other sites and haven't found and answer; probably b/c it isn't worth the effort or money and no one really does it. But I'm curious just the same...
 

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Very good question....i think the reason why sites never release dyno numbers on compression increases is that they are so heavily dependent on the supporting mods and a good tune, so hp/tq numbers can vary widely...

do i have a ballpark amount of horsepower/torque you'd get by going 11:1 or 12:1....? not really, not many people do it (you're correct) and I haven't heard much...I can't image it'd be more than 20 or so horsepower.
 

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Discussion Starter #3
Well, even if it was only 20, that's not too shabby. If only money grew on trees...

Thanks for your thoughts!
 

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iv heard a general rule of thumb is about a 3% increase for every full point of compression the engine is raised. so its really a minimal gain. im assuming the gain inrease gets larger with a big cam/p&p, etc. . .
 

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Yeah, when you start building an engine like that, all the parts work together and you'll sometimes get more or less power depending on part combos.
yuup! You can get a stronger lighter valvetrain, higher lift and longer duration cam shafts.... but even those parts can vary among manufacturers.
 

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Discussion Starter #7
So here's a related question: What effect would a change in compression ratio have on the forces/pressure operating in your engine? My thought [well, guess actually] is that it wouldn't be as drastic as stepping up to forced induction. But you'd probably want to fortify your engine anyway. Head studs, rods - or is that overkill?

Clearly, I've never had experience with this; sorry if its a stoopid question.

Thanks for the posts to date!
 

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First off, not a stupid question :)

NA subie engines are not as strong as the forced induction ones.

The NA subie engine is an open deck. The WRX and STI blocks are semi-closed decks (I believe this describes the way that the heads connect to the block and the way that the coolant flows around them....or something like that...i'm not a turbo block expert)

Basically from least strong to strongest....the order is the open deck (2.5i), semi-closed deck (08 WRX, 08 STI), closed deck (older wrx engines (some EJ20s in JDM legacy/WRX, EJ22 found in the 22B impreza)

For us, this means that the NA engine is just not as strong for holding boost (or increased compression) without modification.

Subie NA engines with turbo kits can only somewhat reliably hold around 5 PSI of boost with a 10:1 stock compression ratio...more boost is possible with lower compression turbo pistons.

Depending on what you take your compression ratio to 11:1 or 12:1, you may need to upgrade some internals, but you may not.

The best way to find out is to ask the piston manufacturer, they'd know for sure.

In my opinion, if you are breaking your engine apart for higher compression pistons, you might as well swap in some stronger parts (better headgaskets, stronger head studs, rods, maybe a stronger crank shaft.) It's not going to hurt anything and if you swap in high quality, stronger parts, it should definately not break.

I'd rather do it right the first time and not have the engine blow apart or start leaking and have to redo or replace things

***Replacing the internals of a subie NA engines will give you decreased engine longevity and you'll definitely require a tune then...NA subie engine building can (but not always) be a money pit, beware! :) ***
 

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Discussion Starter #9
Thanks Shark - lots of good info there! I thought this would be a good topic for those of us interested in NA tuning since it's not often discussed; leave no stone unturned so to speak.

Anyway, thanks again for your help!
 
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