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Old 07-01-2001, 06:23 PM   #1
Scooby Specialist
Member#: 6964
Join Date: May 2001
Location: levittown ny us
2001 impreza
pearl ridge blue

Default piston

i am planning to intall piston heads, valves, valves springs
camshalfs in my01
i want to go n/a then turbo
i want to prepare my engine for what the turbo is going to do to my engine
i going to put a nos kit in too
so what i am get to is that , if i develop such a hig compression ratio , i can 't go turbo, right?
anyone with advice or suggestions
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Old 07-01-2001, 09:11 PM   #2
Subie Blues
Scooby Newbie
Member#: 6438
Join Date: May 2001
Location: Lakewood, Oh USA
01 RS


That's right,, big compression a no-no with forced induction. On production cars adding turbos/SC, the limit is about 8-10psi safely with lots of control on ignition. Forced induction effectively raises the compression ratio. The big drag motors run 30-40 psi with CR's of 6 or 7. Since you are going into the motor, lower the CR to about 7 or 8 and you should get plenty of room for about 20 psi. Look for a knowledgeable shop to do the work and they will keep it straight. If on your own then find a book or two to calculate the effective ratio. Wish I had the money to go at it right now.
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Old 07-01-2001, 09:30 PM   #3
Worst Mod Ever
Member#: 369
Join Date: Oct 1999
Chapter/Region: SWIC
Location: Phoenix, AZ
12 Vagon


I wouldn't go as low as 7:1 on compression ratio if the car is street driven. It would be very slow when not in boost. I am switching to 8.5:1 which should allow for pretty high boost pressures street driven on pump gas. I would guess a well managed 12-15psi wouldn't be impossible on pump gas with an 8.5:1 compression ratio.

When I finally get my internals and the car running again I can let you know how much off boost response is lowered by switching from 10:1 to 8.5:1


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Old 07-01-2001, 10:14 PM   #4
Scooby Specialist
Member#: 6935
Join Date: May 2001
1999 Impreza 2.5RS
Rally Blue


8.5 or 8.6 to one is supposed to be the ideal compression ratio for a supergharged or turboed motor.
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Old 07-02-2001, 09:38 AM   #5
Dirty Underwear Gang

An 8.5:1 compression ratio running 15psi on a 2.5L engine would get you somewhere close to 300hp & a broken tranny... Be careful.
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