The leakdown test has been specified by almost all Porsche specialists for a PPI for a long, long time. It is well recognized as being in some ways more valuable to have than a conventional compression test. I like having both. A leakdown test (that the OP may be familiar with as an Aircraft Mechanic) allows the inspecting tech to pinpoint the likely cause of an engine performance deficit - and allows the Purchaser to be able to negotiate on price, based on knowing the costs typically to put right a given fault. On an aircooled 911, up to at least the end of the 964 era (1994) each cylinder barrel is about $500 to replace. They cannot be bored and replated with any degree of success. Even "Scotchbriting" them and fitting new std size rings is tricky... They are Nikasil plated (or Alusil plated). That is $3000 for parts, right there. The leakdown test will discern, generally, whether it is piston rings / barrel which is causing lower compression, or whether it is an intake valve/valve-guide, or an exhaust valve/valve-guide, or whether it is (likely) a broken cylinder stud (causing a head gasket leak). Note that an aircooled Porsche has 6 individual cylinder heads... all are separate. A borescope investigation is also warranted. An easy test if you are already doing a Leakdown or a Compression test. Leakdown tests are useful for both water cooled and aircooled Porsche's. In having done leakdown tests, and comparing it to standard compression tests...as indicated, they are both valuable. I tested one car that had perfect compression by conventional compression test. When I did its leakdown test it was evident I had a dead cylinder as I had 25% leakdown in that one cylinder. Compression tests often test for camshaft lobe profile general integrity, along with a visual of same to the degree that is possible. If a camshaft lobe is worn down, cylinder compression when cranking will be poor... See attached FAA inspired Teledyne-Continental document about "Cylinder Differential" testing on aircooled aircraft engines. Final point: the automotive trade cannot seem to get it's mind wrapped around the orifice size necessary to be standardized in leakdown testers... for any of this to be meaningful. Meanwhile, the FAA has it prescribed down to the gnat's eyebrow. Worthwhile looking at the FAA docs to really know how to evaluate engine health condition... at least this aspect of engine health.
Originally Posted by atikovi
Unless it's got a miss or is running poorly, why? And if so, a relative compression test with a scope would be much simpler, or you could check through the OBD for misfire counts. This isn't the 50's anymore.
Originally Posted by Cdn17Sport6MT
And that PPI includes a cylinder leak down test, along with (or perhaps instead of) a conventional compression test.
Originally Posted by wings&wheels
Oh, to the OP one final thought; Pre Purchase Inspection! Have fun.