Compression Testing

I plan to do a compression test this weekend on my 70-911E 2.2 as follows:

1. Warm up the engine.
2. Pull all the plugs
3. Crank the engine several times for each cylinder.
4. Write down the results.

Questions:

Am I missing anything here?
What range of numbers should I expect for an engine of this kind in good shape?

I have a leak down test from about 2 years ago. The numbers are all in the 94 to 97 range. What does this mean, what exactly is a leak down test?

Thanks

Allan,

You didn't miss much! Just be sure to hold the throttle wide open while cranking. Sometimes you need an assistant, as it usually requires three or four hands to do everything at once, and watch the gauge, too! I would expect compression numbers in the 150 to 200 range for a 9.1:1 engine.

As far as the leakdown numbers go, those are just the raw numbers off the gauge. But, quite good , I should say. The 'normal' practice is to subtract them from 100 psi, the regulated input pressure, which would yield values from 3 to 6%. A leakdown test doesn't tell you much by itself. Combined with compression numbers, they can be useful to troubleshoot problems with rings, intake, or exhaust valves prior to making a rebuild/refresh decision. You must listen at the tailpipe, intake air horn, breather tube, and underneath the engine ... to decide whether leakage is occuring past an exhaust valve, intake valve, rings, or cylinder/head junction.


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Warren Hall
1973 911S Targa

Out of curiosity, how / where is the input pressure actually input? I am assuming the reading is taken from a gauge in place of the sparkplug.

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Rob Fix
'78 SC Targa

The leakdown tester has an inlet fitting, an outlet hose going to a fitting or adapter into the sparkplug hole. One or two gauges are mounted on the device. One gauge on the 'more deluxe' models monitors the regulated input pressure. There is a calibrated orfice across which the second gauge (or only gauge on 'simplified' models) registers the differential pressure. If there is no leakage, and therefore no airflow, the gauge reads 100 psi, for 0% leakage. For a reading of 95 psi, leakage is 5% ... etc., etc.

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Warren Hall
1973 911S Targa