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Old 05-20-2023, 12:19 PM   #1
euroP
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Default Rcm high compression pistons compression test help

Ok so I have a 2011 sti with stock heads, but with the rcm high compression pistons ans the rcm head gaskets. I wanted to perform a compression test, do.i do it when it's cold or hot? When doin the compression test, should press my throttle all the way when doing the test? How.many cranks should.i give it per cylinder?
I know there is ALOT OF forums about doin compression tests, but I see nothing on here with the pistons or head gaskets I have in. Also what isa good compression test result with my build?
Compression ratio.currently should be around 9.2:1
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Old 05-20-2023, 12:26 PM   #2
19std
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Have you considerd calling or emailing rcm? They seem fairly responsive.
*or the rcm distributor you got them from.
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Old 05-20-2023, 12:47 PM   #3
euroP
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Originally Posted by 19std View Post
Have you considerd calling or emailing rcm? They seem fairly responsive.
*or the rcm distributor you got them from.
I will try thanks. I have emailed them. But if anyone has an idea please lemi kno

Last edited by euroP; 05-20-2023 at 01:09 PM.
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Old 05-26-2023, 04:02 AM   #4
Subie_
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Quote:
Originally Posted by euroP View Post
Ok so I have a 2011 sti with stock heads, but with the rcm high compression pistons ans the rcm head gaskets. I wanted to perform a compression test, do.i do it when it's cold or hot? When doin the compression test, should press my throttle all the way when doing the test? How.many cranks should.i give it per cylinder?
I know there is ALOT OF forums about doin compression tests, but I see nothing on here with the pistons or head gaskets I have in. Also what isa good compression test result with my build?
Compression ratio.currently should be around 9.2:1
The static compression ratio has little to no impact on a compression test. Cylinder-to-cylinder deviation is what you really care about.

Per the Subaru factory service manual, the cranking compression for a 10:1 engine is 145-185 psi @ 350 rpm, whereas the range for an 8.5:1 engine is 142-171 psi @ 350 rpm. So, a 9.2:1 is basically the same range.

Do it when the engine is hot if possible.
No need to press the pedal.

Warm up engine, & turn off.
Disconnect fuel pump harness in passenger side foot well.
Remove fuel cap & bleed off fuel pressure (optional).
Remove all 4 plugs.
Thread compression guage into any cylinder. I start with the closest to the windshield & prop the gauge up so I can see it.
Crank engine ~4-5 revs continuously.
Go check guage & record peak pressure.
Reset guage & repeat for remaining cylinders.
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Old 05-30-2023, 01:01 AM   #5
euroP
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Ok. So far compression test results. Done on a cold engine
Cyl1-145
Cyl2-145
Cyl3-145
Cyl4-125.
Wat u all think?

I'll try do leak down tomorrow
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Old 05-30-2023, 02:19 AM   #6
Alistairh
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If you put a teaspoon of oil in cyl 4 and test again, does the compression come up?
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Old 05-30-2023, 10:17 AM   #7
euroP
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Will try that today and let you knwo. Thanks

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If you put a teaspoon of oil in cyl 4 and test again, does the compression come up?
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Old 05-31-2023, 09:25 AM   #8
euroP
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Quote:
Originally Posted by Alistairh View Post
If you put a teaspoon of oil in cyl 4 and test again, does the compression come up?
Update on this. Thecomprrssion test did not change when I put I oil.in it.
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Old 05-31-2023, 03:47 PM   #9
benflynn
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Next leak put some air in there and see where it is going
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Old 06-02-2023, 01:41 PM   #10
euroP
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Quote:
Originally Posted by benflynn View Post
Next leak put some air in there and see where it is going
So jus did a complete leak down. Cyl 4 seems to be jus a rings issue, so ill leave it as it is. It's only a 10% leak. Cyl 1-3 has 6% leak. So I guess that's y difference I'm compression test. Does that make sense??
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Old 06-03-2023, 11:03 AM   #11
benflynn
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It will run like that and use oil, like a ring land issue would. But I will say it’s easy enough to fix, 300$ gasket set and set of rings, and I have killed one I let burn oil cause I eventually ran it low.
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Old 06-05-2023, 11:50 AM   #12
euroP
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thanks i think im just gona leave it like this for now as the car still makes power...and deal with it when the cash is a little better than now..jus hope she doesnt fail until then lol
thanks for all the help and assistance everyone who posted!! much appreciated.

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It will run like that and use oil, like a ring land issue would. But I will say it’s easy enough to fix, 300$ gasket set and set of rings, and I have killed one I let burn oil cause I eventually ran it low.
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