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Walt Fricke Walt Fricke is offline
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Join Date: May 2004
Location: Boulder, Colorado
Posts: 5,051
Porsche Crest

Some elaboration on the pithy "nuts" might be in order, as it surprised me also to hear what Longacre had to say.

TDC works fine to see if the valves are sealing well (alas, I've had more personal experience with that than I would have liked) or perhaps head leaks (I suppose, though I have no experience with that) or burned pistons or their equivalent (sigh). If your rings are bad, it should tell you that too. Where I saw worse values for some cylinders on my SC when it seemed down on power, these mapped to the broken rings I found when I tore it down. These have all been TDC readings. Maybe I have been lucky, but I just turn the crank to the spark plug firing TDC in the usual way (the Z1 mark) and add air. The valve springs on the other cylinders usually conspire to keep the crank from turning when pushing at pretty close to, if not right at, a sort of zero crank angle where the torque on the crank is very low.

TDC readings may not give you an indication of overall cylinder wear. I think the wisdom is that wear is least at TDC and BDC because things are moving slowest there. If a person were worried about this, he could take readings at various places in the compression stroke after the intake closes right up to TDC. You'd have to fix the crank at about the position you wanted (putting in 5th gear with brakes locked might do). But I can't imagine taking the time to diasble all the valves just for that, far less for wanting to measure BDC (where I would suppose wear might be the least, as cylinder pressures and temperatures must be lowest there). Better to spend the money on a chassis dyno run if you are getting into that kind of detail on overall engine condition.

Longacre must be thinking of pushrod V-8s, where disabling the rockers can be a snap. Why BDC there I can't guess.

Walt Fricke (carries his leakdown tester in the tool box to the track)
Old 07-19-2005, 07:10 PM
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