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Old 02-24-2007, 03:36 PM   #1
Drink
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Question Big boost, big cams, high compression?

I'm looking for some insight from those running similar setups.

Essentially I'm doing 272i/e w/ +1mm valve train. I am debating on what route to go compression-wise. I like my high-compression insta-spool, but I'm worried I will have a too low of a threshold for detonation on 93oct if I run another STi HG. Traditionally, I don't trust changing comp ratios by using a different sized gasket, but I am still concerned OEM won't be sufficient.

I'd like to run about 23-24ish psi on pump.

Anyone have any experience with this?
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Old 02-24-2007, 05:07 PM   #2
PeteDucati
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No experience, but you can have material removed from the combustion chamber to increase volume.

BUT - with better flowing heads and cams, can't you safely run higher compression? I thought it had something to do with better volumetric efficiency or something which makes detonation less likely.
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Old 02-24-2007, 05:12 PM   #3
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Quote:
Originally Posted by PeteDucati View Post
BUT - with better flowing heads and cams, can't you safely run higher compression? I thought it had something to do with better volumetric efficiency or something which makes detonation less likely.
I thought so too, I wouldn't be as concerned if I was running 100 or c16. I just wanted to confirm this can be done in a relatively safe way.

I know the chamber can be shaved down, I was wondering if that could be avoided though.
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Old 02-25-2007, 09:06 PM   #4
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anyone?
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Old 02-25-2007, 09:12 PM   #5
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Interesting........

I may have to rethink my motor options since the motor i had built for some reason the guy keeps stalling on shipping it.

Im still thinking of a jdm v7-8 shortblock + cosmetic gasket though instead of a 2.5l.
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Old 02-25-2007, 10:41 PM   #6
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DrinkAV8, define "high compression"
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Old 02-25-2007, 10:50 PM   #7
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Not sure if your looking to have a motor built for you or whatnot, but if I were you I'd definitely talk to Howard at AR Fab, he does a lot of work with high compression motors and may be able to give you some good insight. His stroked motor had a 10.5:1 compression ratio and he does build a stroked wrx motor with a 9.8:1 compression ratio.
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Old 02-25-2007, 11:14 PM   #8
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I am looking to run 9.2-9.5 on my motor...still working out my last bit on the decision before getting the pistons made.
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Old 02-26-2007, 12:45 AM   #9
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Quote:
Originally Posted by SloRice View Post
DrinkAV8, define "high compression"
Sorry should have clarified ... 8.7:1

OEM STi pistons for now.

Should have stated "Higher" compression I had no idea people we doing setups upwards of 10:1
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Old 02-26-2007, 05:40 AM   #10
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Quote:
Originally Posted by DrinkAV8 View Post
Sorry should have clarified ... 8.7:1

OEM STi pistons for now.

Should have stated "Higher" compression I had no idea people we doing setups upwards of 10:1
Isn't 8.7 just about stock on the USDM Sti?

TMS
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Old 02-26-2007, 08:06 AM   #11
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Are you using 2.0 or 2.5 heads? When you had the heads machined for the valves, did you have the chamber volume measured? Also, at that time, were the combustion chambers polished?
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Old 02-26-2007, 08:53 AM   #12
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8.7:1 is not high compression. You will be fine with your setup.
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Old 02-26-2007, 09:20 AM   #13
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8.7 is plenty fine so long as you have a good quench pad....
my buddy's evo was seeing 32 psi with meth injection on 93 pump...this was on the factory 16g
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Old 02-26-2007, 12:53 PM   #14
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Quote:
Originally Posted by PeteDucati View Post

BUT - with better flowing heads and cams, can't you safely run higher compression? I thought it had something to do with better volumetric efficiency or something which makes detonation less likely.
Yeah, the cams bleed off some of the pressure due to the overlap so you can run higher compression, i believe that's what you're referring to.
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Old 02-26-2007, 03:16 PM   #15
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9.2 custom pistons on the EJ257. I will likely go with Cosworth cams, which will effectively lower the dynamic compression somewhat. Cams at 8.2 are a dog at low RPM even with AVCS.
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Old 03-09-2007, 04:56 PM   #16
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I am debating on which cr to go with on my build. I like running pump gas but there is a gas station 1 mile away with 110.??? Ill either go 8.7:1 or 9.0:1.Leaning towards 8.7 though.
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Old 03-09-2007, 05:39 PM   #17
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We are currently running 10.5:1 static compression on our L-time attack car (clock car) This engine always sees race gas.
On a car that sees pump gas I would stay with 9.5:1 static compression or lower.. higher compression can be done depending on application and tune.
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Old 03-10-2007, 08:06 AM   #18
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Jeremy what about a 83 mm stroked block. is a 8.7 static CR or higher usable with cams on 91 Calif. pis gas with a smaller turbo.
I'm looking at 102mm bore and 83mm stroke. I want long rods but my BASIC math says problems with keeping the piston below the deck and with a reasonable CR even if I go with smaller pins raised in the piston.
Jeremy we need to talk about a stroker block. I have shot your company e-mails but I have had little luck. I'm going for gobs of low end on a small turbo
(deadbolt sz55 or bigger) but need a street-able Cr to decide on................... Need to know if the 83 mm crank will work or should I go 81? All depends on the rod angle right?
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Old 03-13-2007, 06:02 PM   #19
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I was wondering about this as well. I live in Wyoming and we get 91 octane at best. I am building a 2.2 block with 25 heads from a 98 legacy. I am using a vf 22 and lloking for better response off the bottom. What compression could i safely go with and still push 15-19psi (should be the sweet spot for this turbo correct)? Car will be run off a 95 Sti-ra ecu for a short time with a move to an autronic later.
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