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Old 18th September 2009, 14:03   #1
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Default How Do I Lower the Compression Ratio?

Hi All,

I have a 2008 Subaru Impreza 2.5s (hatchback). I'd like to put in a turbo charger and have been reading the manual on turbo charging.
I am trying to find an inexpensive way to reduce my compression ratio (currently 10:1 and I'd like to get it lower than or around 9:1) although it really should be around 8.5:1 ; is there any cost effective way to do this? thanks for your all your help

cheers!
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Old 18th September 2009, 15:09   #2
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Thicker head gasket should do the trick I guess.
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Old 18th September 2009, 15:22   #3
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Thicker headgasket is a way, but it is not something that is advisable. It will also not lower CR by the margins the OP wants.

Methods are :

1) Pistons with less stroke
2) Crankshaft with less stroke
3) Head work to increase compression chamber volume.

An 8.5CR is required only if you are turbocharging it to insane levels and want to run dirty fuel.

Find an enthusiast forum for your car (being a Scooby, should be no trouble at all) and talk to people who have done what you want to do to your car. A generic guide won't be of much help at all if you don't know what your engine likes and dislikes and what parts are available off-the-shelf for it.
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Old 18th September 2009, 16:14   #4
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Well, stock the car runs at 170 HP with a T3 turbo in it, it will probably do around 250. You think I can run that without blowing out my pistons and crankshaft? any head work I do cant be a DIY and in the US it will cost me a bomb.
Another option for me is to remap my ECU, probably will be able to push it up to 200 with a decent remap. What do you think"
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Old 18th September 2009, 17:33   #5
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What about getting a piston having a hollow type crown ,like a depression.Though the CR lowered will need to be calculated ,but along with a thicker gasket ,
Subaru Forester Head Gasket | Always In Stock - Replacement, Felpro, Ishino, 00-05
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Old 18th September 2009, 18:41   #6
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Quote:
Originally Posted by 1489910
Methods are :

1) Pistons with less stroke
2) Crankshaft with less stroke
3) Head work to increase compression chamber volume.
I would not alter stroke or rod length to reduce C.R, doing that changes the entire characteristic/geometry of engine, A simple more straight forward would be to get aftermarket low comp pistons for the application.
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Old 18th September 2009, 19:13   #7
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Quote:
Originally Posted by ImmortalZ View Post
Thicker headgasket is a way, but it is not something that is advisable. It will also not lower CR by the margins the OP wants.

1) Pistons with less stroke
2) Crankshaft with less stroke
3) Head work to increase compression chamber volume.
Piston does not have a stroke.
reducind crank stroke will reduce displacement
Trying to machine a combustion chamber is pretty suicidal.


Turbo specific (Cu?) thicker head gasket willnot only reduce CR but also hold boost better. Dished pistons if you want more.

Last edited by Mpower : 18th September 2009 at 19:20.
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Old 19th September 2009, 19:46   #8
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Sorry guys was busy with some work.

So it appears as though the thing to do would be to put in a thicker head gasket ( should be cheap?) which, even if it doesn't lower the CR down to 8.5 should be sufficient for what I am after?
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Old 20th September 2009, 01:22   #9
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For peanuts (comparatively), yeah. Low comp pistons will still cost a bomb.

Last edited by pranavt : 20th September 2009 at 01:30.
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Old 20th September 2009, 02:08   #10
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Is it not possible to stick to the same CR and maybe use a lower boost pressure to achieve similar power and torque?

Shan2nu
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Old 21st September 2009, 11:29   #11
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Quote:
Originally Posted by Shan2nu View Post
Is it not possible to stick to the same CR and maybe use a lower boost pressure to achieve similar power and torque?

Shan2nu
I don't know. I am open to suggestions. Will the head gaskets lower the CR by enough for me to put in a turbo?

There is a company that sells a diagnostic tool with a bunch of pre-programmed ECU maps ( can be configured) for the impreza. That may be my most cost effective option. Especially considering I'm planning on selling the car and moving back to India permanently in another 3 years or so.
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