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Although the electrode is bent, it's been just barely been touched. I seen plugs from exploded engines that looked like someone took a hammer to them and beat them flat!
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So I have been doing some math and need to find the right pistons to replace the ones I blew up.. the original ones were HYP and had .025 shaved off the top. Based on what I am reading I need a shorter compression distance to make them the right height correct?
Based on my numbers of 9.48 - 3.85/2 - 5.956 - .025 I am left with 1.574 for a compression distance for the piston to be installed with a .025 clearance to the deck. Does this seem right?? |
Sounds like thats to far down in the deck, you will loose out on 'quench'. I would suggest a deeper dish in the piston top while leaving a flat area to add quench.
Of course this would mean a 'custom' pistons and the cost could be a budget buster... |
If you had hypereutectic pistons shaved, that may be your problem. From the Keith Black website the article on the hyper pistons says they need the polished tops to reflect the heat back into the combustion chamber and reduce the heat going past the top of the piston to the top rings. This is the reason they are recommended with such a close cylinder fit. If you took off the polished tops without polishing again to factory specs, it may have caused heat to travel to the first ring and cause a ring bind or a ring groove failure.
With 393 strokers, be sure to use the right dish piston to keep your compression reasonable. With a "0" deck, you will need approximately 16-20 CC dish on your pistons with 58-60 CC head chambers to maintain around 10 or 10.5 to 1 compression ratio, which should handle pump gas if you're using aluminum heads. This is a little high on compression if you're running cast iron heads. As Ernie stated above, especially on stroked engines, you need to keep your quench distances as close as you can to get the full efficiency from your engine and to avoid detonation problems. Bob |
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Although this claim from Keith Black’s website may be technically correct, it is a little misleading. Hypereutectic has to do with the silicone (I hope that is the correct word) content in the aluminum. When above some percentage, the silicone will no longer dissolve into the aluminum and it forms crystalline structures in the metal. This alloy has a much lower thermal coefficient of expansion than aluminum. That is the reason hypereutectic pistons can be fitted tighter than cast aluminum pistons. If Keith Black is claiming the polish of their hypereutectic pistons allow a tighter fit than other companie’s hypereutectic pistons, then it would make some sense to me. However in my amateur, armchair General’s seat, after a few thousand miles the shinny piston tops will be covered with a nice black coat of carbon and I have my doubts that this will matter much. |
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Do you think it would be safer if I got some Forged ones and did the same shave.?? it would be great to find some with the right compression distance but I don't think that distance exists does it? |
I definitely feel you would be better off with forgings.
Detonation is functionally equivalent to hitting the piston crown with a sledgehammer. Sometimes it will break the piston crown down to the top ring land (as in your case) and it sometimes knocks the skirt completely off the piston. As noted, aluminum pistons are alloyed with silicon. The higher the silicon content the lower the rate of thermal expansion, but they become increasingly brittle as the silicon content goes up. In an engine that might experience non-progressive combustion, this brittleness is a risk. Alloys with high silicon content cannot be successfully forged since they are no longer malleable. That is why forged pistons require a looser cold fit and audible piston slap sometimes occurs until they warm up. The payback is the "toughness" and resilience of the lower-silicon-content forged pistons. I've seen them hammered so hard by detonation that the ring lands are bent down far enough to pinch the rings, but nothing "broke". Detonation and pre-ignition are evils to be avoided, but the forged pistons give you a little more room for error. |
Olddog, interesting analysis, sounds reasonable, good logic. But as you must have seen over the years as well as me, sometimes logic gets turned on it's head when you get a more complete picture of whats going on. As for me, I wouldn't run hyper pistons anyway.
"Apparently", from what I've picked up along the way, there not very forgiving of detonation, for one thing. A simple timing mistake could lead to breaking one like a china tea cup! :D If your looking to run on the edge of max allowable compression (and thinking quench) I'd go forged. |
Rather than having to shave the tops of any pistons after purchasing, may I suggest you determine the actual deck height of your particular block based on its year of manufacture (you may have already mentioned this) and have the wrist pin and ring pack put in a set of Forged Pistons by the manufacturer? Yes, forged would be the better choice.
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Maybe this caused it?
Have a real close look at the ends of the top rings to make sure that they have not been fitted with too tight a ring gap to the cylinders. If you get into a marginal detonation or even preignition at low engine rpm the sudden temp spike can cause a rapid cyl temp rise that will cause the top ring to expand rapidly and butt the ring ends. The ring then has no where to go and the drag of the ring in a bore which is now 'too small' will pull the top ring land. This is even more prevalent in 'stroker' motors with short pin height piston's ,Top ring gaps in these combos should be increased to around 0.025" or 0.028" , maybe even more if you know that it will be subject to this sort of thing. Second ring gap has to be increased slightly as well.
While some may be keen to blame an engine builder for this, it quite often happens due to a combo of things- stale fuel-cold motor/hard acceleration- too 'fast' an advance curve- 'lugging' the engine in high gears @ low rpm. etc etc. Jac Mac. |
so I have finished rebuilding my motor and its in the car minus the intake. I am debating on whether or not if I want to put the webers back on or go with my 4 barrell. The bottom end is all forged now. I suppose it is tougher than it used to be huh and I should just try the webers again.. ;>)
:LOL: :LOL: :LOL: |
Which pistons did you end up using?
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I got some SRP pistons from JE. These pistons had the correct height so I don't have to have them cut by .020 like the last set was.
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Those are nice its JE's house brand but of very good quality they will take some beatings and should be good for the long haul.We have used those exclusivly in our engines (JE) for years and never had a problem.Investment parts may cost more but ther worth it down the line...WB3
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