1) i doubt if a 7300RPM over-rev would cause a rod bolt failure by itself. the rod bolts in the cayman motor are pretty much the same ones in my 3.4L 996 motor, which has a 7300RPM redline. it works out to less than 6% more force (than 7100RMP), which will be inside the design limits for the bolt. risk from over-rev is generally to the TOP END of the motor, not the bottom. the pistons go faster than the valves can retract & a collision occurs.
2) reducing the rev limit will DEFINITELY reduce fatigue and stress inside the motor. force on the rods & rod bolts increases with the SQUARE of RPM's, so the effects become a lot more pronounced as the revs go up. at 10000 RPM, the accelration at the piston is around 250,000 fps/s. at 5000 RPM, you're under 65,000. you've halved the RPM's, but cut the force by 4.
if you drop your observed rev limit from 7100 to, say, 6700, the max forces reduce by about 11%. over time, this can make a difference. personally, i wouldn't bother. as i stated earlier, the functional rev limit is really dictated by the valvetrain.
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Originally Posted by MikenOH
One of the guys on the Cayman board had a connecting rod bolt failure--due to an over rev--on a motor that had about 40K. 7300 rpm for 1/2 second--boom.
After the engine was torn down, IIRC, he mentioned that that there is a consensus that these connecting rod bolts will strech over time and that this is really one of the big weak links in the motor for people that do track time and tend to run the motors in in the upper rpm band. His take away was to keep rpm's down under 7K--more in the 6500 range-- since the power drops off above that anyway.
Any thought on this?
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