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Join Date: Feb 2007
Posts: 20
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compression for 1981 911 SC 3.0 ?
Hi, I am considering buying a 1981 911 SC 3.0 and want to know what range the compression test results should be on the cylinders ?
thanks, Levon |
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01-08-2008, 01:28 PM
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anybody ?
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01-08-2008, 03:31 PM
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Senior Member
Join Date: Feb 2000
Location: Lacey, WA. USA
Posts: 25,308
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I've seen plenty of early SC (compression ratio of 8.6:1) numbers in the 160-170 range. Later SC's (compression ratio of 9.3:1) can be higher. I think my cylinders have seen 185. But the thing you're really looking for is sameness. 170-171-168-173-172-135 is bad.
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Man of Carbon Fiber (stronger than steel) Mocha 1978 911SC. "Coco" |
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01-08-2008, 03:42 PM
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if the compression is in the high 150s, is that bad ?
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01-08-2008, 03:55 PM
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Location: San Carlos, CA US
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Compression in the high 150'x are good. It is even better if you have all the numbers within 5% of each other.
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01-08-2008, 03:57 PM
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The maximum compression reading for a car with 9.3 compression ratio is about 150 PSI. This is the theoretical maximum with no leakage from the cylinder. Anyone getting readings above that has a bad guage or some inertial effect of the needle on the guage.
As has been said here if they are all even within 5 PSI or so then the rings and valve sealing are good. -Andy
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01-08-2008, 09:17 PM
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Join Date: Jun 2004
Location: Rio Rancho, New Mexico
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Andy,
I must respectfully disagree with your assertion that the maximum pressure from a 9.3:1 compression engine is 150 psi. Your calculations do not include the "heat of compression". The factor for air is 1.4. This in actual practice will run a bit less due to heat absorption by the engine itself, (the cylinder walls, piston and head). It is not bad gauges reporting compression numbers for new engines at 180 psi and higher, it is the effect of the heat of compression. Notice the head of an air compressor runs quite hot, yet there is no combustion taking place. So where does this heat come from? What happens to air when it is compressed? The calculations are rather involved. I have yet to see a simple formula. But it is there.
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DOUG '76 911S 2.7, webers, solex cams, JE pistons, '74 exhaust, 23 & 28 torsion bars, 930 calipers & rotors, Hoosiers on 8's & 9's. '85 911 Carrera, stock, just painted, Orient Red |
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01-08-2008, 10:41 PM
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