Quote:
Originally Posted by BURN-BROS
The weak area of the 911 crank is the thickness of the throws. 22mm rod width yields a 7mm throw thickness. This is acceptable for the 2.0/2.2/3.0 cranks utilized in high rpm applications. However,when Porsche changed to 24mm width, thy picked up a nasty harmonic above 7500 rpm and destructive @ 8000 rpms.
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Does this mean that the web is only 5mm thick for the 70.4mm cranks with the 24mm rod?
Do you know what type of vibration occusrs is it a torsional or a lateral? Shedding flywheels sounds like torsionals
Max,
Do you think reducing the big end journal diameter has much effect on the stiffness other than to increase the effective length of the web when its diameter s reduced? When you combine this with the stroke increase and the possible thinning of the web then there must be an influence on the mass-elastics.
All engine have the potential to suffer from torsional vibration and as long as it is well away from the running range it shouldn't be an issue.
If the crank is very flexible in this area then there could be a witness from the edges of the rod bearings which would possibly show increased wear as a side effect.