Polyurethane has excellent damping properties and the results of using it for this part are very good until...it fails. The heat will kill it sooner or later. I think I have tried everything in terms of composition, hardness, shape, but I underestimated the temperature. I don't know where it comes from, it may be engine or exhaust or both, I don't really care. Polyurethane is not good for this application, temperatures exceeding 100 Celsius are too high for this material. Special additives can push it up to 120 degrees but for a limited period of time.
Actually even the book says that polyurethane is an excellent elastomer for automotive use excepting the engine.
I hope this helps somebody. If anyone has a heat resistant recipe I didn't know, good for him.
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