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Old 11-21-2001, 02:31 PM   #1
Graham
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Default HighER Compression Pistons

After having done a search, whith limited results, I have some thoughts and questions about upping the Compression Ratio of an EJ25 block from the stock 9.7:1 to something in the range of 12.1:1. This seems like a feasible alternative to turbocharging, and the risk does not seem as great. However, I'm curious to know why more of us NA Subaru guys have not considered this more seriously. All that seems to dominate this section is upgraded cams, intakes, heads, pulleys, etc... With a compression ratio of something in the neighborhood 12.0:1, it does not seem unreasonable to see figures for horsepower leaping into the upper 200 range, right? So my first question is whether or not anyone knows of somebody performing this type of upgrade on an EJ25? What kind of work would be necessary on the heads, cams to be able to withstand this type of pressure? Sorry if I missed something in my search.

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Old 11-21-2001, 02:45 PM   #2
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The reason you dont see that is becuse for teh cost of building your bottom end/heads is alot more the going the FI route.
Trey Cobb will mill the heads to increase the compression to 10.5:1 IIRC. He was getting around 220at the fly with all his NA goodies.
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Old 11-21-2001, 03:14 PM   #3
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Maybe, if you add up the cost of the heads, $2500, then the cost of the high compression pistons depending on who you go with, in this case COBB, which custom makes them for $800. Here you are at a slightly high price of $3300, which is roughly the price of any turbo kit on the market today. The power figures with these kind of modifications are going to be nearly just as much as a, lets say 5 psi turbo kit. This of course is considering your running pistons with a C/R of around 12.0:1. This doesn't seem like to much more than a turbo kit. Get some engine management if you like, such as a Split Second or ITC, then your around $3800. Last time I checked its not considered "safe" to turbo your car unless you have about $4000. Interesting.

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Old 11-21-2001, 03:21 PM   #4
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Personally I'd love to see sombody go that route, It's starting to look more attractive now that you can use a Unichip to raise the redline somewhat. Im sure once you start looking at getting the best NA performance, things like the Intake manifold, TB exhausrt manifold etc will need to be changed. neither option would be cheap and when i comes down to sombody's hard earned $$ most people would rater have a torque'y 250 - 280hp engine than a peaky 230 - 250hp engine.
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Old 11-21-2001, 03:47 PM   #5
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Quote:
I have some thoughts and questions about upping the Compression Ratio of an EJ25 block from the stock 9.7:1 to something in the range of 12.1:1
Your car is 10:1, not 9.7:1.

This has been discussed over and over. The cost to get 250hp out of your car NA is going to far exceed the cost of a turbo package to get 250hp. With a bolt on turbo kit, the only real cost is the kit itself, but with higher compression NA motors you have to rebuild the whole motor, get the heads worked, new cams, etc. 280hp is really optimistic unless you seriously raise the redline.

Sure, it'd be cool, but it is very expensive and much more limited in power output.
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Old 11-21-2001, 04:12 PM   #6
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okay guys while NA is very cool on Small block chevy's and small block fords i have yet to see a very impressive NA 4 cylinder. THe amount you are going to spend trying to get the NA horsepower out of the motor of choice will usually end up eclipsing a turbo or FI kit by about 1.5 times. I would take most likely damn near 5-6 grand to build an NA motor. I have watched NA honda after NA honda get beat by a cheaper done FI car. Not to mention that those honda's are No where NEAR emissions legal. A properly done FI car even w/ engine management would still fall shy of the amount required to do a NA motor.

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Old 11-21-2001, 04:13 PM   #7
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all I gotta say is BTCC
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Old 11-21-2001, 04:13 PM   #8
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12:1 compression won't seem as sweet when you have to fill the gas tank with race gas every fillup @ $5 per gallon. You won't be able to run 12:1 on pump gas.

...among the other reasons this isn't really feasible.
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Old 11-21-2001, 04:24 PM   #9
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you could run 93 octance on 12.0:1 compression...with the proper timing and everything....but if you want the full gains you'd be needing to push 100 all day long.

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Old 11-21-2001, 04:49 PM   #10
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The JTCC Imprezas are pushing 280hp from 2.0L on race gas at 8250rpm, so it _can_ be done, but it wouldn't be much fun to drive anywhere but a track.
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Old 11-21-2001, 06:58 PM   #11
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"...with the proper timing..." meaning backed off enough so it doesn't detonate. Not much point in all that if you don't have an aftermarket ignition to take advantage of the higher compression and run appreciable advanced timing. 12:1 and BTDC timing don't really compliment each other.
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Old 11-21-2001, 07:01 PM   #12
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For raising compression, why can't the head just be milled? Or is there an interference problem with valves? Usually with a stock cam there isn't but this is from experience on V8 and Honda stuff. Also concerning octane requirements, if the car can run all day with the stock 10:1 on 87 octane, I see no problem with 12:1 on 93 octane. I think that is because cooling is much more uniform on these engines than the typical old American V8.
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Old 11-21-2001, 07:13 PM   #13
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Once again, it CAN be done, it is just not cost effective to do it. $3000 in head work and cams, total engine rebuild with forged pistons (and hopefully forged rods) is another $1500 minimum. Now add the cost of the fuel system you will need to support it (just like going to a turbo) and the cost of the upgraded ignition to get it to burn efficiently. Now add some electronics.

Compare to a really nice, complete, BOLT ON turbo kit for $6k that will put out 25% more power without ever opening the motor.


Ben
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Old 11-21-2001, 10:38 PM   #14
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Actually, you can run 12.0:1 or more with pump gas, and no, you don't have to kill the power with retarded timing to do it. My (may it rest in peace) old Yamaha ran 13.3:1 on pump gas, and it would run resonably well on 87 octane in a pinch (when I didn't have the money for 93). I still made around 120 crankshaft horsepower with 629 cc. That's not to say that any motor with that kind of compression and tuned for pump gas would pass an emissions test though. All I'm saying is that it can be done. (read, stand-alone engine management)
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Old 11-21-2001, 10:58 PM   #15
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With all due respect, if the only thing you want to do is raise the compresion, why not just mill the head? Higher compression raises power across the complete engine operating RPM, regardless of cam or head work. I don't think that it would stress the engine to the point that heavy duty rods and pistons would be needed because we are not raising the RPM limit of the engine. It certainly could have adverse emisions consequences. I don't think it would stress the engine any more than forced induction would. It is the converse of a forced induction engine, instead of cramming more air and fuel into the engine to produce more work, you take the normal amount of air and fuel and compress it more to do more work, and it happens whether you do heads and cams and pistons and balancing or anything else.
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Old 11-21-2001, 11:10 PM   #16
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Wouldn't milling the heads also decrease the volume of the cylinder chamber as well? 2.5 with a milled head becomes 2.3something?

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Old 11-21-2001, 11:19 PM   #17
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Nope. The displacement of an engine is determined by the bore and stroke of the engine, not the volume of the combustion chamber.
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Old 11-22-2001, 03:34 AM   #18
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you can only mill a head so much before you have gone out of tolerance and ended up with a problem then you have nothing but garbage. remeber when you are milling a head you have to think about the fact that the intake still has to mount up and so does the exhaust...so don't be shocked if the head can only be milled .005".

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Old 11-22-2001, 04:09 AM   #19
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Quote:
Wouldn't milling the heads also decrease the volume of the cylinder chamber as well? 2.5 with a milled head becomes 2.3something?
That'd be an awfully large change in displacement. Maybe you are thinking of destroking the EJ25 using the EJ20 crank which gets you 2.34L?


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Old 11-22-2001, 04:44 AM   #20
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Quote:
Originally posted by bsquare


Your car is 10:1, not 9.7:1.

no, 2000-2001, IIRC (and Im pretty sure I am) is 9.7:1
not like thats much of a difference (cus' its not)
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Old 11-22-2001, 04:53 AM   #21
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98 and 99 are 9.7:1, 00+ are 10:1, no question.
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Old 11-22-2001, 05:01 AM   #22
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Quote:
Originally posted by bsquare
98 and 99 are 9.7:1, 00+ are 10:1, no question.
whatever I wont fight over it since its only 3/10ths difference and Im not 100% sure, but until I see hard proof, I believe it was Subaru themselves that state low 9.x:1 98-99 and 9.7:1 00-01.
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Old 11-22-2001, 05:03 AM   #23
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kian ben is right...MY98-99 were 9.7:1...MY00-02 are 10.0:1

Jeremy
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Old 11-22-2001, 09:05 AM   #24
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Hard proof.

It can probably be found in your manual. Not positive on that though. There are other interesting bits in there as well.......

Greg
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Old 11-22-2001, 12:55 PM   #25
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once again my00+ ej25's are 10.1:1
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