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Old 01-16-2013, 01:45 PM   #1 (permalink)
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Default Compression test results

My car is an 05 Forester XT, with 140,000 miles. My mechanic did a compression & leak-down test while he was changing out spark plugs. My problem is that I have no idea what to make of the results. He says the compression, by cylinder, was 132,132, 135,139. Does this sound low? The spread between high and low is right at 5%. The car seems to run ok... should I be concerned?
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Old 01-16-2013, 02:09 PM   #2 (permalink)
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Those look pretty good for 140,000 mes. They are pretty consistent across all 4 . What should they be? 140+
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Old 01-16-2013, 02:21 PM   #3 (permalink)
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You should be good there...the difference between the cylinders is more important than the compression numbers themselves in my opinion...different equipment might read different, also how quick you do the test after the car has been shut off changes the readings too..

What were the leakdown results? They are just as important or even more important than the compression test imo.
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Old 01-16-2013, 05:51 PM   #4 (permalink)
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Full Results:
Cylinder 1 - 139# & 4% LD
Cylinder 2- 132# & 5% LD
Cylinder 3- 135# & 4% LD
Cylinder 4- 132# & 5% LD

So, what do you think? Any cause for alarm?
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Old 01-16-2013, 06:03 PM   #5 (permalink)
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You aren't looking for PSI as much as you are looking for consistency. I believe the "actual" PSI is somewhere up in the 170s, but depending on the gauge, they usually read low. My compression gauge (even on m newly rebuilt engine) only read around 140/cylinder.

I believe the alarm would raise if a single piston was off by around 7-10% more than the others, but overall it looks great for the mileage.
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Old 01-16-2013, 06:55 PM   #6 (permalink)
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Yea you are good, leakdown numbers look great, especially for that mileage...
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Old 01-16-2013, 06:57 PM   #7 (permalink)
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Quote:
Originally Posted by SkateandDestroy View Post
Yea you are good, leakdown numbers look great, especially for that mileage...
Awesome.... thanks for your insight.
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Old 01-16-2013, 07:39 PM   #8 (permalink)
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Quote:
Originally Posted by Blue Fox View Post
You aren't looking for PSI as much as you are looking for consistency. I believe the "actual" PSI is somewhere up in the 170s, but depending on the gauge, they usually read low. My compression gauge (even on m newly rebuilt engine) only read around 140/cylinder.

I believe the alarm would raise if a single piston was off by around 7-10% more than the others, but overall it looks great for the mileage.
This is very true.

I see to many people saying "your numbers are too high/low, you have a problem," while in reality, all the numbers are close to eachother, there is no problem.

The actual number varies depending on temperature, pressure, and the gauge you are using.
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Old 01-16-2013, 07:46 PM   #9 (permalink)
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(P1 x V1) / T1 = (P2 x V2) / T2
Compression Ratio (volume ratio) = 10:1 on a N/A Subaru (if that is what it is) ignoring the closing position of the inlet valve.

P1 is atmospheric or 14.7PSI (absolute).
Ignoring heat of compression (i.e. T2 = T1) then P2 at top of compression will be 147PSI absolute or 132.3 PSI Gauge.

So with this approximation compresion pressure arround 130 to 135PSI are good!
(Testing a hot engine will elevate these theoretical numbers)
Cheers!

Just noticed you said it was an XT. Redo the math for the turbo CR and it is still okay.

Last edited by BillyCorgi; 01-16-2013 at 08:00 PM. Reason: added data
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