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Old 05-17-2009, 09:37 PM   #1
williaty
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Default What are Correct Compression Numbers with Stock and Wild Cams?

So while hanging around with some of the local guys, my compression test numbers came up again. What came out of the discussion was that none of us had a really firm handle on what the numbers "should" be. Consulting the factory service manual, it says, on page ME(H4SO)-2, that the compression pressure should be 148 to 185 psi. However, on page ME(H4SO)-21, it goes into greater detail and says that the cylinders must be within 148-185PSI with less than 7PSI variance between cylinders. Now, a couple of things about that jumped out at us.

First, none of us have ever heard of compression test results coming in anywhere near 185psi. Have any of you? The Subaru manual doesn't specify how many times to let the engine crank over. I've heard that differing numbers of revolutions can make different results.

Second, 7PSI variation between cylinders is WAY, WAY more strict than the commonly accepted 10% variation. Is Subaru being overly cautious, or are most of us being overly relaxed?

So, what compression numbers are actually "right" in the real world for a car with stock cams and a car with wild (Delta 2000, Cobb Spicy, etc) cams?
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Old 05-17-2009, 09:42 PM   #2
Qman
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Quote:
Originally Posted by williaty View Post
So while hanging around with some of the local guys, my compression test numbers came up again. What came out of the discussion was that none of us had a really firm handle on what the numbers "should" be. Consulting the factory service manual, it says, on page ME(H4SO)-2, that the compression pressure should be 148 to 185 psi. However, on page ME(H4SO)-21, it goes into greater detail and says that the cylinders must be within 148-185PSI with less than 7PSI variance between cylinders.

Now, a couple of things about that jumped out at us. First, none of us have ever heard of compression test results coming in anywhere near 185psi. Have any of you? Second, 7PSI variation between cylinders is WAY, WAY more strict than the commonly accepted 10% variation. Is Subaru being overly cautious, or are most of us being overly relaxed?

So, what compression numbers are actually "right" in the real world for a car with stock cams and a car with wild (Delta 2000, Cobb Spicy, etc) cams?
Service manual numbers are for perfect, brand new engines. A 7psi variance seems a bit loose for a "new" engine. The "industry" standard is 10% between highest and lowest cylinders for a used, working condition engine. Engine compression isn't that critical as long as it is over 100psi and within 10%. You really do not have to over think this. For a vehicle that is making slightly over stock compression does not require it. For full race engines yes, for 200hp engines, no.

Using cams is not like re-inventing the wheel. Subaru engines do not have adjustable timing gears like a Honda, Ford or GM product. There is no variance for cam timing. It goes in at a set position. It stays at that point until the belt stretches or breaks.
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Old 05-17-2009, 11:42 PM   #3
PhilSine
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When I did a compression test on my 98 OBS a few years back with 135k mi on the clock the results came back 183 psi with no more than a 2 psi variance across all 4 cylinders.

Not sure what you can take from my experience but my only point is that 185 psi is not too far fetched. Even my professional mechanic friends found it hard to believe what my results were and one of them repeated the test himself with nearly the same results.

98 2.2l engine 9.7:1 CR
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Old 05-18-2009, 01:37 PM   #4
yarrgh
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My current engine is about 10 months old, maybe 8K on it. It has Cobb mild cams. I check compression when the engine is warm and let it hit compression 5-6 times when cranking and get an average # over 3 runs. It's right around 180 psi with maybe 5 psi difference. My old short block(with same heads and cams) would get around 172-176 psi.
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Old 05-18-2009, 01:41 PM   #5
Zac86
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proper way is to let it turn over 3 times.
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Old 05-18-2009, 02:54 PM   #6
Odyknuck
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The compresson (using two seperate gauges) on my 2000 legacy motor w/ 100K miles is 210, 205, 210, 205. I pulled the heads on Saterday and there is very little carbon on Pistons so its not from Carbon build yup.
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