3.2 acceptable compression psi?
 

Hi all.
I just got the compression leakdown test back on an '88 Carrera I am looking to buy.

Cylinders 1 thru 6 all read at an even 150 psi.
Leakdown ranges from 4% to 7%.

The mechanic says that these are good readings.
I called another and he said it should be between 160 and 175 psi.

My manual doesn't give a poor/good/excellent scale in psi to go by.
Same with the leak down.
Any body have the specs on these items?

Thanks

I don't think you'll ever find specs on the psi because it varies directly with cranking speed (ie battery strength). Temperature would also affect it. Looking at the variances is the useful measure.

Having said that, I run a compression test with every valve adjust on my '88 Targa. PSI values have been between 165-176 over the last 6 years.

Compression Readings
 

Compression readings will also vary depending on whether the throttle was blocked open or not. If the throttle plate is closed, the added restriction will reduce the guage readings. For that reason I prefer the following procedure:

Dry test
1. pull the coil wire and fuel pump fuse or relay
2. remove the spark plugs
3. block the throttle wide open
4. install the compression gauge in cylinder #1 and crank the
engine until max pressure is reached (should be only 3-4
revolutions)
5. record the reading, release the pressure and repeat the
procedure. If there is more that a couple pounds difference
in the readings, repeat a third time.
6. go to the next cylinder and repeat the process.

Wet test
If you have a cylinder that is reading "low", remove the gauge,
spray some oil into the cylinder (helps seal the rings) and retest.
If the compression goes up several pounds, then you have bad rings. If it doesn't change much or any. the valves are probably bad.

Good luck!

Fred Cook
'80 911SC coupe