Originally Posted by ptuomov

I think about it in a slightly different way. Not saying that your description is incorrect or anything, just that I think of it differently. My way of thinking about is the following:

The orifice between the pressure gauges flows a certain amount of air per unit of time, given the pressure on both sides of the orifice. If the pressure drop over the orifice is from 100 (absolute) psi to 95 psi, the flow thru a 1mm diameter orifice is about 0.70 SCFM. If the pressure drop is from 100 psi to 90 psi, it's about 0.96 SCFM. If the pressure drop is from 100 psi to 85 psi, it's about 1.15 SCFM. It's probably not too far from a square root relation in the pressure differential for given downstream pressure. Now, from the pressure drop you can tell whether more or less air is leaking thru the piston rings. In fact, you can compute it if you know things like the restrictor orifice diameter.

As a side note, I think that for comparable measurements one should really set the downstream pressure measurement to be a constant constant at some number. A drop from 200 psi upstream to 180 psi downstream means 1.92 SCFM flow. A drop from 100 psi upstream to 90 psi downstream means 0.96 SCFM flow. It's the same pressure ratio drop but double the pressure drop. It's not square root in the pressure drop because air compresses. But one should also take into account that the same cylinder and rings will "leak" a lot more when the downstream pressure is 180 psi than when it is 90 psi. So I think one should always test leakdown with the same downstream pressure (that the cylinder sees) or at least report the downstream test pressure along the pressure drop ratio if one has a fetish for precision. That said, if the flow thru the rings is about proportional to the pressure drop, it's not necessarily a bad approximation to just report the pressure drop ratio.

Now, what does the leakdown number tell us? I think that when the engine is not running, the ring end gap is the main cause of the leak down number. The leakdown in absolute is not very useful, it just tells you what the piston ring end gap is. But you already know that if you set it!

It is, however, very useful if one measures the leakdown with new rings and then measures it regularly as the engine has been run for more hours. The change in the leakdown number will indicate how much the cylinder and the rings have worn. As the bore wears or the rings wear, the effective diameter grows. Suppose the new engine bore diameter is 100mm, the piston to bore clearance is 0.04mm, and top ring end gap is 0.3mm. The ring gap area is about 0.012 mm^2. Suppose that this will give a "leak down" in a leakdown tester from 100 psi absolute to 90 psi absolute, and this will give some SCFM number. After engine has been used, suppose that the SCFM from the leak-down test has increased by 1/3, then the the ring gap has also increased such that the gap area has increased by about 1/3 (ball park, government work, etc.). If one knows the proportion of wear coming from bore vs. the rings, one can even compute an approximation of the new piston to bore clearance from the leakdown number.

Originally Posted by AO

Back on topic.... here are a few pics of one of the pistons. You can see the slight carbon build up in the dish. Not a big deal. The rest looks fine to my eyes, but I do wear glasses nowadays.

Originally Posted by Imo000

Thats a little more carbon then normal. The right side of the picture has a pretty good buildup. Use Permatex Gasket remover for the carbon. Spray it on and leave it for 5min.

Originally Posted by Imo000

Nothing works faster and easier on carbon than Permatex Gasket Remover. 5 min and the carbon turns into liquid. I came across this by accident when working on another Porsche engine and have used it on other engines since.

Originally Posted by AO

But then I saw what may be the beginning of an intermediate shaft failure. Need to get Greg to send me one of his new fangled versions. I think I dodged a bullet here. What you see here is metal starting to "gather" at the splines. Yikes!

Originally Posted by andy-gts

AO,
Check your TT bearings. That wear looks like rubbing which would mean the shaft flexing.
__________________
Colin J


shaft flexing from 500 rwhP????

Originally Posted by FBIII

The last piston seems to have been detonating more than the others. If the pistons are cleaned and left with a pock marked surface, will that create am environment for further detonation? Kind of a piston version of a hot spot in the combustion chamber.

Originally Posted by AO

The heads cleaned up very nicely. Next is to make a spring compressor so I can take the springs out and begin checking valve guides.

Originally Posted by Tony

Id have to dig it up, but i think i made one out of PVC pipe, then cut a slot in it that gave me access to the retaining clips ?

Originally Posted by 69gaugeman

I would replace the mains as it had not been done before. He just did the rods 6 years ago so I would leave them alone, as they can be done (relatively) easy while in the car.

Originally Posted by Leon Speed

I assume it is not as simple as just putting in new mains but that it would mean measuring and matching all bearings and crank? Maybe machining involved if fitting oversize bearings, stuff like that?

Originally Posted by AO

Yesterday I made my valve spring compressor tool and took the keepers out of one of the heads. Today I should finish the other head.
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