Compression Test Results?.?
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Compression Test Results?.?
I took the Christmas Eve day off to complete a compression test on my '04 996 motor. Let me know if these numbers are good, bad, or indifferent.
To clarify, I removed all 6 spark plugs and then starting with the number 1 cylinder I screwed in the compression test tool. I then turned the motor over 7 revolutions using the starter motor. After the 7th revolution I stopped and read the gauge. I did this three times per cylinder then averaged the three number for a "Compression Test AVG" result in PSI.
Cylinder
1 = 247 psi
2 = 250
3 = 249
4 = 249
5 = 247
6 = 245
These numbers are down about 5-10 points compared to my compression test from 12-2009.
What should these numbers be on a strong engine vs. an engine that's wore out?
Thanks
To clarify, I removed all 6 spark plugs and then starting with the number 1 cylinder I screwed in the compression test tool. I then turned the motor over 7 revolutions using the starter motor. After the 7th revolution I stopped and read the gauge. I did this three times per cylinder then averaged the three number for a "Compression Test AVG" result in PSI.
Cylinder
1 = 247 psi
2 = 250
3 = 249
4 = 249
5 = 247
6 = 245
These numbers are down about 5-10 points compared to my compression test from 12-2009.
What should these numbers be on a strong engine vs. an engine that's wore out?
Thanks
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I'd be interested to hear what the experts have to say. I was of the impression that compression testing was old technology, and that leakdown was a much better measure of the wear on your motor.
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2016 Ford Transit Connect Titanium LWB
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Retired Club Racer & National PCA Instructor
Past Flames:
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2004 GT3 Track Car
1984 911 Carrera Club Racer
1974 914/4 2.0 Track Car
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From: Land of the Old People
I don't have a frame of reference of what factory compression is for your car. Compression tells one story... You are simply cranking the motor over to see how much pressure it is generating. In a compression test, you would want all the cylinders to be within 10% of each other which it looks like yours are. It sounds like normal wear as each cylinder is going down at the same rate. If one was significantly lower than the other, it would tell you that you have an issue in the low cylinder(s). The normal decrease you are seeing is likely general ring wear.
A leak test is when you put pressure into cylinder and measure how much stays in and how much leaks past. Test done with the cylinder top dead center, valves closed.
Doesn't sound like you have any issues but a leak test would give you a more accurate read of power loss due to the leak. I wouldn't be too alarmed unless you start getting more than 10-12%. When that happens you're starting to lose a decent amount of power. As a frame of reference, a newly-refreshed motor is still going to have 3-5% of leak. It's simply impossible to get metal directly to metal without some air loss between them.
A leak test is when you put pressure into cylinder and measure how much stays in and how much leaks past. Test done with the cylinder top dead center, valves closed.
Doesn't sound like you have any issues but a leak test would give you a more accurate read of power loss due to the leak. I wouldn't be too alarmed unless you start getting more than 10-12%. When that happens you're starting to lose a decent amount of power. As a frame of reference, a newly-refreshed motor is still going to have 3-5% of leak. It's simply impossible to get metal directly to metal without some air loss between them.
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I have a leak down tester rig and compressed air but I took the lazy approach this morning. I've done the leak test in the past and having to get to the crank shaft pulley to turn the motor manually (to get to TDC) for each cylinder tested is a pain.
Additionally, the mechanics I have talked with recommend performing that test on a Hot to Warm engine (to insure parts have expanded snug to each other). And getting mine warmed up and then pulling the coils and plugs would further burn me fingers.
Furthermore, if I then determined I have an intake/exhaust valve seat problem or major ring problem it would bring me down because as many have said, these engines are not designed to be rebuild. I'd need to start looking for a new one.
I just wanted a basic check up not the full diagnosis of future demise. Here is a data log of oil pressure during a typical lap. I have the x51 baffle but oil starvation will surely be the problem in the eventual end. Low pressure is 20psi under 1.5 G's and 70psi at 0 G's.
Additionally, the mechanics I have talked with recommend performing that test on a Hot to Warm engine (to insure parts have expanded snug to each other). And getting mine warmed up and then pulling the coils and plugs would further burn me fingers.
Furthermore, if I then determined I have an intake/exhaust valve seat problem or major ring problem it would bring me down because as many have said, these engines are not designed to be rebuild. I'd need to start looking for a new one.
I just wanted a basic check up not the full diagnosis of future demise. Here is a data log of oil pressure during a typical lap. I have the x51 baffle but oil starvation will surely be the problem in the eventual end. Low pressure is 20psi under 1.5 G's and 70psi at 0 G's.
Last edited by Brinkley; Jan 26, 2013 at 10:47 AM.
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From: KC
You can't really compare raw compression from a year ago to what you get now. That is a function of how warm the engine is, how fast it is spinning (which can be battery strength and/or wear), etc. All you can really look at is whether the values look reasonable (they do) and whether they are close to each other)they are.
Your Oil Pressure vs. G's is interesting, but given all the high pressure and high g data points, I suspect you are seeing more of pressure vs. RPM. If you really lost pressure due to cornering G's, I would imagine that you would not see the large number of high pressure and high G readings.
Your Oil Pressure vs. G's is interesting, but given all the high pressure and high g data points, I suspect you are seeing more of pressure vs. RPM. If you really lost pressure due to cornering G's, I would imagine that you would not see the large number of high pressure and high G readings.
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PS: Put it on a rack and leak it from below. Way easier....
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+1 to what has been said, compression wi vary considerably depending on the barometric pressure, cranking speed throttle plate opening etc... Your mainly interested in the variance between cylinders which at les than 1-1/2% looks ok in my book. The leakdown is always a bit of a crapshoot in the boxer horizontal layout, everything from oil in the cylinder to carbon on the exhaust seat can give you a false positive or mask a problem. Anymore I think the best metrics are oil consumption and a dyno pull.
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The oil pressure is all over the place on these cars, even with full x51 versions - I figure if it doesn't hit 0 psi, then oil is still moving keeping it alive!
