Acceptable Cylinder Pressures

I recently (1 hour ago) did a compression check. The numbers I got are ...
#1 : 110 psi #4 : 85 psi
#2 : 130 psi #5 : 85 psi
#3 : 105 psi #6 : 100 psi

I would think +/- 10 psi would be acceptable, but those 85s and 100&105 seem way too low.

Frequenters might remember me as the guy who was chasing poping/backfiring a couple months ago (2.4L upgraded to 2.7L with Webers). Nothing yet, but it looks like a top end rebuild is in order!

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Brian Scotti
'73 S Coupe

A couple of questions about your procedure/method of doing the test ...

1. Was the engine warmed up, or cold when you did the test?

2. Did you have all of the sparkplugs out when you did the test?

3. Was your starter cranking at full speed and was the battery fully-charged? Any noticeable, known starter problems?

Those numbers do look bad ... about as bad as I have ever seen, without a 'dead' cylinder!


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Warren Hall
1973 911S Targa

Warren,

1) The engine was cold, so I guess the pressures could be lower than expected.

2) All of the plugs were removed.

3) Starter was at full cranking speed, with fully charged batteries. I cranked the engine over for a few seconds (5 or so) to build pressure.

I understand why you asked about the engine temp, but why did you ask about the spark plugs being removed? Just curious. If my procedure was wrong, please let me know how to do it better next time. I wouldn't doubt the cylinders with the 85 psi readings are dead or dying ... the plugs were black!

Thanks

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Brian Scotti
'73 S Coupe

Brian,

The reason for having all of the plugs out is so the engine will spin over at the maximum possible speed.

Was, or is, the 'black' sparkplug dry and sooty, or a 'wet-looking' oily black?

If you have an old set of plugs that are still usable, but not fouled, try warming the engine up for 10-12 minutes, then repeat the compression test.

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Warren Hall
1973 911S Targa

Warren,

The plugs from the low pressure cylinders were a shiny black, but not wet with oil. What would good cylinder pressures look like? I don't know what compression ratio I was running.

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Brian Scotti
'73 S Coupe

Most of the 2.7 p & c sets had 8.5:1 compression ratio, so an average between 130 psi and 180 psi could be expected, with no individual cylinders varying more than 10% from the average. That would be 'good' or acceptable.

If that wet-looking sparkplug still looks that way, 24 hours later, then it must be oil ... broken or worn piston rings seems likely. Other problems seem likely, too, such as pulled head studs.

I suggest that you check all 24 head studs for tightness at the soonest opportunity. If the engine sat up for a while, over a period of months, carbon could have absorbed moisture and been shed from the combustion chamber, and some particles may have gotten stuck between valves and seats.

There are really too many possibilities to be specific in diagnosing a cause for your low compression!

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Warren Hall
1973 911S Targa