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MY99 GF4 EJ227 JDM 6spd
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Discussion Starter #1
I had this in mind when I was thinking about how much more reliable it is to run a lower CR when turboing the EJ25. Also which CR would be just right as in reliable and just right for boost.

I was thinking of maybe 9.0:1 CR pistons. This would still be a good CR right? It's obviously lower than OEM CR, which is 9.8:1 and 10.0:1. The reason I think 9.0:1 is that it'll build boost sooner and have a better torque curve and sooner torque curve. It'll be forged too! Would this be a suitable CR to be able to put out 400bhp (crank) and still be reliable?

Fue
 

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its hard to say what a proper CR is for a boosted engine. it depends on how much boost, how much lag you're willing to have, combustion chamber design, valve timing, octane, you name it. there is no magic number. the best way to find out is engine dyno time and lots of money :) that being said, boost is a more effiecent (read gives more power) way of compressing air then the piston. this is why people lower the CR to run more boost.
 

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Conebasher uses 9.0:1 pistons in a closed deck EJ25 running 19-ish psi. Compression = low end torque and if you auto-x, rally-x, ralley you will want the highest compression possible... if you drag race screw the compression ratio, you need big hp numbers (9 sec Hondas have squat for torque but make some damn impressive hp numbers).
 

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keep in mind conebasher, being an auto-x car, has a torque curve similar to that of the WRC car. its all low-end torque and drops off quite heavily on the top end. obviously not what you want for drag racing.
 

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MY99 GF4 EJ227 JDM 6spd
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Discussion Starter #7
I was researching some info on "ConeBasher" but couldn't find any specs on it. I saw some a while ago on COBB's site. I hear that it shoots from 0-60 in 1.9 seconds. It's posted on COBB's forum.

Say now with 9.0:1 CR pistons on our EJ25 motors, the power wouldn't be high up there in the RPM but rather low as in probably 7-8k RPM Redline. Maybe not that high but a guess.

If the EJ25 motor had 8.5:1 CR pistons, the peak hp/tq would be at a higher RPM than that of a 9.0:1 CR, right? Assuming the motor having the same stroke/bore.

Fue
 

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the peak hp/tq would be at a higher RPM than that of a 9.0:1 CR, right?
well, you could run more boost. boost is generally directly proportionate to rpm, if the engine can take it. look at the US-spec EVO, its 8.8:1 and makes 19psi of boost @3500rpm but at 6500rpm its making only 16.24psi. this isn't because of the turbocharger but because the engine would exploded b/c the CR is too high to handle that much boost at such a high rpm.
 

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Oh so Cobb built the motor? Too expensive for me. I'll skip having the spare motors deck closed in and just get the heads ringed. I was thinking of using some forged pistons with a lower compression then 9 to 1.
 

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There are other companies that can close the deck, its not hard... just labor intensive. If you can get a stripped block you could even do some reinforcing i.e. semi-closed. I've seen blocks that just have "pins" (cylindrical aluminum/steel) bits placed (welded) at strategic places to help with cylinder wall integrity.
 

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MY99 GF4 EJ227 JDM 6spd
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Discussion Starter #11
10th Warrior said:
look at the US-spec EVO, its 8.8:1 and makes 19psi of boost @3500rpm but at 6500rpm its making only 16.24psi. this isn't because of the turbocharger but because the engine would exploded b/c the CR is too high to handle that much boost at such a high rpm.
Semi-closed block would keep it from cracking the cylinder, but the USDM EVO isn't a semi-closed.

If 9.0:1 CR were used, and 400hp is the goal, semi-closed or closed block would have to be kepted in mind. Right?

How would using 9.0:1 fair on track days as in road course with 400hp in mind. Would this be optimum for 30 minutes of track time? Turbo engines do get hot when run over a period of time. Supra owners who do track time turn down the boost to keep the engine cooler.
 

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With an adequate I/C, radiator you should be fine. There is also I/C water spray, N2O/CO2 I/C sprayers, water injection, etc to keep intake air temps down.
 

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i tried lower compression pistons. i was at 8.5 to 1 and raised this a quarter by using a thinner gasket. now i am going back up to 9+ to 1 to take advantage of my ceiling of ten - twelve lbs of boost after the intercooler. my sds ecu has boost retard as well as knock sensor retard. which wasn't even needed at the lower conmpressions. i also have a bridged deck block.for those of you keeping score, engine out five times, trans out five times.
 

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Discussion Starter #15
TTT

Assuming fine-tuned, would pushing 400hp with 9.0:1 would be less relieable than say 8.0:1 - 8.5:1?

At least trying to stick with pump gas i.e. 91 octain...

FuJi
 

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Let's think about it this way. My Ty is running 8:1 pistons and puts out around 350 hp and 450 ft/lbs of torque, and hauls 4,800 pounds of sheetmetal and crap around. I get a few degrees of knock with 91. Make the 9:1 and I bet it would be problems!
 

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Wait, wait, wait, someone said something about a Subie running 0-60 in 1.9 seconds? That's ridiculous if it's true, what the hell could give that kind of acceleration?
 

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Conebasher, Cobb Tuning's auto-x monster runs 0-60 around there. Thing is the car is geared completely differently, and is tuned for auto-x... not ultimate power. The engine is a 2.5 closed deck block running 9.0:1 compression and ~19 psi... it only runs on race gas and gets ~2-4 mpg.
 

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Discussion Starter #20
Cobb's Conebasher has a dog box and 1st-2nd shift is just flat out shift. Quite an amazing machine!

okies so now in conclusion, running 8.0:1 - 8.5:1 would be the best CR for being streetability (bring able to use pump gas) and reliable. This was a great discussion IMO! Thanks to al who contributed!:)

FuJi
 
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