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Engine compression

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SargeVT

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I am still chasing the back fire through my rear car on my '67 3000 MKIII. Checked the compression and found it to range from 85 to 100 dry and 100 to 125 wet. Number 4 was the only one that reached 125, the rest were either 100 or 105. Want to know where the compression should be. Tried to do a search but didn't have any luck. Computor skills I have not! Should I arrive at right number if multiply 13.6 (atm.press.)X 8.25? javascript:void(0)
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I'm not sure what the correct Healey compression should be, but your readings sound low. I would think on a warmed engine it should be around 140-150.
How you performed your compression test may also have affected your readings. The most common method is on a cold engine, all plugs removed , wide open throttle, 3- 4 cranks. Record your readings. Warm the engine & repeat. Your warm readings should be higher.
A simple test but not conclusive to test for botom end problem or top , would be to squirt some oil in to the cylinder & do the test. If the reading go up dramaticaly it's possible you have a ring sealing problem if they are not affected it could be valve related.
Remember to do all test with WOT.
 
HI Sarge, /ubbthreads/images/graemlins/iagree.gif Trboost's response,140-to-150, is correct for the Healey engine in good condition. Additionally, the differential pressures,Cylinder-to Cylinder, you quoted are to large while the measured pressures are quite low.???---Keoke
 
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Should I arrive at right number if multiply 13.6 (atm.press.)X 8.25?

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Not exactly. To begin with, the rated compression ratio is not acually achieved.Especially at low rpm. Compression cannot begin until the intake valve is closed. This usually happens somewhere around 30 to 50 degrees after bottom dead center when the piston has already started it's up stroke. So a good deal of the 8.25 to 1 is taken up before the intake valve closes & compression begins.

Offsetting this is the heating effect of compression which raises the pressure. The result should be somewhere around 140 to 150 as mentioned. Altitude can markedly affect the numbers. They would be lower at any altitude above sea level where the 150 psi is rated.

If you put more than a couple of cc's of oil in the cylinders the decreased chamber volume will give an artifically high reading.

The definitive test would be a cylinder leak down test. This requires an air compressor, calibrated gage etc. This test will tell you exactly how much & where the cylinder is leaking, rings, valves, head gasket?

If anyone is interested, I did a fairly complete write up in the June 2004 "Austin Healey Magazine" published by the AHCUSA.
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