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Old 03-23-2005, 11:46 AM   #1
Scooby Newbie
Member#: 83650
Join Date: Mar 2005
Default Phase I and Phase II compression differences

Is the compression raised from the pistons or raised from the difference in head design? I'm getting my engine in my 97 outback replaced with a Phase II block and DOHC heads. Will I get phase II compression or Phase I?

I'm having the heads milled too, so that will increase it that much more.
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Old 03-23-2005, 11:11 PM   #2
Scooby Guru
Member#: 5218
Join Date: Mar 2001
Chapter/Region: MWSOC
Location: SAUL'S Motorsports
96L Most Over-
Developed Beater


I did this very thing....first with 96DOHC heads and then 98DOHC heads. Both sets were milled slightly and with the addition of a thin Cometic headgasket, I'm up very close to 12:1 CR.

The ratio is changed by both piston shape and combustion chamber shape. The DOHC has slightly smaller chambers while the phaseII pistons are 10:1 vs 9.7:1.

If you wish to run less than 92-93 octane gas, I would either forgo the milling unless needed and/or use stock thickness or custom extra thick gaskets to keep the CR in check.

Jay Storm
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