So many people have asked what's involved with the differential compression check and why is it so expensive? As I was going to do it on a aircraft, I might as well explain the steps. I will be doing it on a aircraft engine but the procedure is the same.
This is the tool you need. The end that goes in the spark plug hole for a Porsche is connected to a hose with a O ring on the ent to seal it. The rest is the same.
The adapter is screwed in the spark plug hole (lower plug in place)
the gage is attached to the adapter and the other end connected to a air source.
With the cylinder at TDC (holding the prop but putting a Porsche in gear with the parking brake on) The air is turned on to 80 psi. there is a small orifice thats lets a metered amount of air to enter the cylinder. If any leaks, the cylinder gage will read low and you can hear air coming from the intake, exhaust, or crank case. The amount of air loss is the differential expressed in % (like 80/72=8 psi=10%). If the air was coming from the exhaust, it would be likely be an exhaust valve that was leaking or burned. The rest of the cylinders are checked in the same way. This check gives you a good indication on the health of the engine. Hope that helps. BTW, it takes about a hour to perform this check.