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Old 08-30-2007, 12:04 AM   #1
qhatherly
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Default Best compression ratio for EJ257, GT30R .82, 94 pump

Hi all, I tried searching but it's tough to find specific answers to this question.

My block is out and apart, and it's time to make a decision on pistons. The motor is from a 06 STi. Heads will have a light pnp, and hopefully won't have to be decked, using OEM head gasket and ARP studs. Turbo is a GT30R .82. The car will run on 94 octane (low percentage ethanol blend) pump gas with no meth, alky or water injection, and no race gas.

My question is, what is the magic balance between compression ratio and boost for peak, daily driven power on this setup? Keep in mind, I'd rather have a power band with more up top instead of a little more off boost pep. If anyone has any first hand experience I'd really appreciate it.

Thanks a lot.
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Old 08-30-2007, 12:22 AM   #2
flycaster
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I'm not sure there are any "magic numbers" out there since so much depends on the aggressiveness of tune, but it seems like most guys are running between 8.2:1 and 8.5:1, and between 21 - 23 psi on pump only.
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Old 08-30-2007, 09:23 AM   #3
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Flycaster is correct. There is no "magic number" as there are so many other variables.

From reading your signature, I see you have a WRX. Are you planning on running WRX heads on the EJ257 or did you get STI heads also?? You say you have an 06 motor, but i'm just confirming you have the whole motor, not just the shortblock. If you throw WRX heads on the EJ257, the CR gets bumped way up due to the cc size of the WRX heads.

Anyways, I had an 8.2:1 compression motor pushing a 30R .63ar and it wasn't bad. It was still a little laggier than I was expecting.

We built a motor for a local guy with Wiseco 8.9:1 compression running a 35R and surprisingly this car had quite a lot of off throttle pep! Then it went on to make 425whp on a Mustang Dyno at 20psi on 94 octane.

I now have the same Wiseco's in my car with my 35R setup, p&p heads and big cams. My car feels pretty good off boost. Only have ~60 miles on the motor, so no power numbers yet.
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Old 08-30-2007, 02:33 PM   #4
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Quote:
Originally Posted by SloRice View Post
We built a motor for a local guy with Wiseco 8.9:1 compression running a 35R and surprisingly this car had quite a lot of off throttle pep! Then it went on to make 425whp on a Mustang Dyno at 20psi on 94 octane.
Tim, did you find that the higher compression led to a little less timing advance, not to mention a slight reduction in boost? I guess what I'm trying to understand is why Bailey has always said, "they'll make virtually the same power, but they just get there a little differently."
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Old 08-30-2007, 02:49 PM   #5
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Quote:
Originally Posted by flycaster View Post
Tim, did you find that the higher compression led to a little less timing advance, not to mention a slight reduction in boost? I guess what I'm trying to understand is why Bailey has always said, "they'll make virtually the same power, but they just get there a little differently."
yeah, those "symptons" sounds like the benefits of a higher CR
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Old 08-30-2007, 03:02 PM   #6
SloRice
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Originally Posted by flycaster View Post
Tim, did you find that the higher compression led to a little less timing advance, not to mention a slight reduction in boost? I guess what I'm trying to understand is why Bailey has always said, "they'll make virtually the same power, but they just get there a little differently."
Honestly, I asked Tim to not push this motor when he tuned it. The kid we built the motor for had been given the run around by a few shops and had gone through 2 motors prior to this one. I wanted it to make over 400whp on pump gas and that was it. We could easily turned the boost up and made more power. I did notice though that we definately didn't use as much timing though. My car on pump gas was about 24-25 degrees at redline. This car was about 19 degrees at redline. After that, no increase in horsepower.
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Old 08-30-2007, 03:28 PM   #7
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Originally Posted by Homemade WRX View Post
yeah, those "symptons" sounds like the benefits of a higher CR
Heh, with the motor you're building, all of us are going to stand back, listen, and learn. Your tune is going to be unique.
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Old 08-30-2007, 03:29 PM   #8
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...I did notice though that we definately didn't use as much timing though. My car on pump gas was about 24-25 degrees at redline. This car was about 19 degrees at redline. After that, no increase in horsepower.
What about at peak torque? Same type of decrease?
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Old 08-30-2007, 03:42 PM   #9
SloRice
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yea, I think it was down a couple degrees at peak torque also, but I can't remember.
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Old 08-30-2007, 10:38 PM   #10
qhatherly
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I'll be using a the complete 06 STi motor, I guess what I'd like to know is, as I'm dropping CR, is there a point where the allowed boost and timing that it yields starts to drop off more drastically.

Say I'd much rather run more boost and timing than have off boost response, so a lower CR will be beneficial. But what is the point where the benefits aren't worth the sacrifice (ie. how low is too low)?
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Old 08-31-2007, 09:03 AM   #11
SloRice
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I guess I'm just trying to figure out why you would want to lower the CR. You will make less off boost power, but make the same on boost power as you would with a higher compression motor. The only difference is it will take a lot more boost and timing to make the same power.
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Old 08-31-2007, 11:16 AM   #12
qhatherly
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I was hoping to stretch the power band out, but if it isn't worth it maybe I'll just stick with 8.5:1.
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Old 08-31-2007, 01:02 PM   #13
SloRice
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by dropping compression, you are essentially lowering the VE of the engine.

In a perfect world, we want high compression and lots of boost, but detonation doesn't allow that unless you can suppress it using race gas all the time.

Go with something in the mid to high 8's for compression ratio.
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