Supercharged '91 GT Refresh
#77
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Andrew,
Could 500ft-lbs + and "heavy boots2 have something to do with that wear pattern on the splines?
Do you intend to take a peek inside the gearbox while you are at it?- not that you are not taking on enough as it is.
As I think about it this could be a good opportunity to better analyse/understand the "ravages" of advanced wear patterns to "bolt on" boosted928's doing something not taken into account in the original design.
Your comment on the piston crown cleanliness and no vapour recycle is an interesting albeit not too surprising observation.
Regards
Fred
Could 500ft-lbs + and "heavy boots2 have something to do with that wear pattern on the splines?
Do you intend to take a peek inside the gearbox while you are at it?- not that you are not taking on enough as it is.
As I think about it this could be a good opportunity to better analyse/understand the "ravages" of advanced wear patterns to "bolt on" boosted928's doing something not taken into account in the original design.
Your comment on the piston crown cleanliness and no vapour recycle is an interesting albeit not too surprising observation.
Regards
Fred
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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.
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.
Thank you for expanding on the theory.
#80
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I did have a TT bearing failure a year and a half ago. I replaced the TT, so maybe it as from that.
Fred, sorry to disappoint, but I do not have any plans to take out or open the trans.
Fred, sorry to disappoint, but I do not have any plans to take out or open the trans.
#81
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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.
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But as to your question, the short answer is yes. Having a rough surface can cause hot spots which then leads to more detonation which lead to further damage to the piston and maybe the head.
Today I cleaned up the heads and started cleaning up all the nuts and bolts.
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.
Pics from today...
![](https://lh6.googleusercontent.com/-Z0p8cNg_mYQ/UsoRUKkUaVI/AAAAAAAAKP4/_157gpp6abE/w1704-h1278-no/IMAGE_335.jpg)
![](https://lh4.googleusercontent.com/-lUFuBBMscOg/UsoRWsF0afI/AAAAAAAAKO8/oCMa_nL0grA/w958-h1278-no/IMAGE_336.jpg)
![](https://lh3.googleusercontent.com/-6fBR80hN77Q/UsoRZcSwurI/AAAAAAAAKPE/BWQvZkeWSLo/w958-h1278-no/IMAGE_337.jpg)
![](https://lh5.googleusercontent.com/-k9dAhRzs8s0/UsoRcZRAt9I/AAAAAAAAKPM/hxc-6U3NZZw/w958-h1278-no/IMAGE_338.jpg)
![](https://lh3.googleusercontent.com/-td_7kBVx7dg/UsoRfBaRwPI/AAAAAAAAKPU/cSMqfTslW4Q/w958-h1278-no/IMAGE_339.jpg)
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#84
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Thanks Tony. Already looked it up. Will post my solution. Today is a snow day for the kids. So I might be able to get a few more things done if I have the pieces laying around.
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Andrew,
I bought this Valve spring compressor a long time ago. Best money I ever spent. Makes easy work of valve removal.
I bought this Valve spring compressor a long time ago. Best money I ever spent. Makes easy work of valve removal.
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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?
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The cost of the bearings is low compared the the amount of work to take the cradle off and remove the crank. The rod bearings can be done with the engine in the car. I wouldn't try the main bearings. Now that he has the engine out it is maybe an hour or two of extra work and he knows he will be good for the life of the engine (main bearings at least).
#88
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Thanks guys for all your commentary and input. I will be looking at the mains. I've never done that before, so I will be asking for guidance when I get there.
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.
I made mine by following this thread from 2006 (wow 8 years ago) where Konig928 and Tony talked about their home made too. Mine cost me a whopping $3.28 for a 4' length of 1" PVC and a PVC T piece. I had some scrap 2x4s and copper pipe laying around. The black pipe is from a pipe clamp I made many years ago. With about an hour of monkeying around to get the fit right... I had a pretty descent valve spring compressor.
![](https://lh3.googleusercontent.com/-Kg2JQQZCGQ0/UsxM3QwJI1I/AAAAAAAAKas/PEPXhipKZ1A/w1586-h1190-no/IMAGE_342.jpg)
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.
I made mine by following this thread from 2006 (wow 8 years ago) where Konig928 and Tony talked about their home made too. Mine cost me a whopping $3.28 for a 4' length of 1" PVC and a PVC T piece. I had some scrap 2x4s and copper pipe laying around. The black pipe is from a pipe clamp I made many years ago. With about an hour of monkeying around to get the fit right... I had a pretty descent valve spring compressor.
![](https://lh3.googleusercontent.com/-Kg2JQQZCGQ0/UsxM3QwJI1I/AAAAAAAAKas/PEPXhipKZ1A/w1586-h1190-no/IMAGE_342.jpg)
![](https://lh3.googleusercontent.com/-cYW3PO9q5SU/UsxM01McE5I/AAAAAAAAKbE/3tVGhFfD7HA/w1786-h1340-no/IMAGE_341.jpg)
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