Quote:
Originally Posted by Flieger
From what I have heard the reduction in journal size on the 70.4 mm crank and the corresponding reduction in torsional stiffness was what led to the torsional harmonic mentioned above moving into the range of engine speeds which were regularly seen by the racing engines. This caused the flywheels to be shed.
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If that were true then the 3.0 liter SC crank would have suffered the same fate as the 2.4/2.7 crank. Porsche went back to the 22mm wide journal and all was well.
Quote:
Originally Posted by Flieger
I thought I read that the 917 engine had a bad torsional vibration but that this was below the rev range normally used.
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917 had the same 24mm wide journal as the 2.4/2.7/2.5ST/2.8RSR cranks and indeed had a torsional issue. And that was with a Ti rod....
The harmonics did not rear up again until the 964. The larger piston and increased stroke was too much...They solved the problem for the street car by decreasing the journal width to 21mm. The downside was there is not enough bearing material to withstand the requirements on the competition engines and were forced to run the 22mm wide cranks in the 993 RSR's.