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Old 08-04-2012, 11:00 AM   #1
rs_runner
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Join Date: Mar 2011
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2002 Impreza 2.5rs
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Default 2.1stroker on e85, target comp ratio?

Im looking at a full e85 build on a 2.1l. Will be using ej207 heads... looking to spin to 8500. What would be a good target compression ratio?

I was wondering if the avcs and bumped ratio would give more torque throughout.
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Old 08-04-2012, 07:38 PM   #2
LBell05STI
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Call your tuner and builder and ask them. If they can't give you a suggestion they shouldn't be touching your car. Lower compression allows for more timing and boost. The only time I've bumped compression on a vehicle is if I used it for a single purpose where a compression bump would beneficial.
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Old 08-04-2012, 08:09 PM   #3
Nuke209
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NF Performance

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what turbo
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Old 08-05-2012, 03:16 AM   #4
Concillian
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CR is a tricky thing. It will gain you power when you can't gain more boost (small turbo) and you're at MBT (the point at which more timing advance gives no additional power)... However it reduces margin from knocking more than boost or timing does. For ultimate power, it's a tuning tool of last resort, because it gives the least bang (additional power) for the buck (knock margin).

CR gains power at a (ballpark estimate) ~1/3 of the % gain in CR as long as you can maintain MBT. So going 8.5:1 --> 10.5:1 yields something on the order of 8% gain at a given boost level. This is not really a huge gain, and this is why most tuners advocate pretty conservative CR bumps.

I've been considering the exact same route as you, and I haven't talked to a tuner who would recommend anything higher than 10.5:1, even when my goals are not lofty (350 HP range on E85) and I tell them I don't plan to run more than 22psi in order to keep the stock MAP sensor fuel cut. Most even balk at 10.5.

I think that says something. I think 10.5 is pretty conservative in my case, but people are scared of high CR.

3MI Racing has had higher compression auto-x builds, and I'd highly recommend talking to them before making a final decision. Micah is a much bigger advocate of high CR than anyone else I've talked to in the Subaru community. Anyone else I mention ~11.5:1 CR to as a motor focused on maximizing response, economy and off-boost torque on a ~350 HP motor would give me a deer in headlights look back. Literally stare back at me wide-eyed. Meanwhile, Micah tries to talk me up higher than that...
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Old 10-03-2012, 09:13 PM   #5
sonic_wrx189
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Quote:
Originally Posted by rs_runner
Im looking at a full e85 build on a 2.1l. Will be using ej207 heads... looking to spin to 8500. What would be a good target compression ratio?

I was wondering if the avcs and bumped ratio would give more torque throughout.
Nice build. Same build and heads siting in my garage... Contemplating pistons and cr's myself...
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