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Originally Posted by JoeFromPA
JFP -
Thanks so much. That is exactly the type of information I was looking for...
I'm guessing that Porsche uses a metallurgy in their crankshaft that allows them to break under such stresses. This isn't a criticism; they designed the crank to be used with a specific flywheel and provide longevity.
Perhaps it is the specific application of a solid-mass aluminum+steel flywheel that's less than 1/2 the weight of the stock dual-mass flywheel on this particular crankshaft....I haven't seen/heard cranks breaking as even a concern when switching back and forth between single mass/dual-mass on an otherwise stock rotating assembly.
Reading Jake's comments, I understand now why a vendor hasn't chosen to develop a 25+ pound single-mass flywheel for these engines. It's just not a choice they want to entertain, given the available options.
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This isn't a problem that only affects Porsches. Any engine crankshaft is prone to failure from harmonics if those harmonics excite the crank at any of its resonant frequencies. Plenty of kids have learned this the hard way by removing the harmonically damped crank pulleys on cars and replacing it with an un-damped, underdrive pulley with disastrous results.
For that matter, it's not just internal combustion engine crankshafts that are prone to this. Many aircraft turbine engines have specific speeds (which are only seen during start up/ shut down on most engines) that absolutely have to avoided, as they are the resonant frequency speeds of the shaft. This can be a problem during a "hung start," when, if the pilot does not abort the start sequence, the engine could be left at one of these critical RPMs for longer than the time it takes to simply "spin through" it on the way up or down, with very severe consequences.
It is worth noting, however, that the cranks in these engines are NOT forged.
For whatever reason, Porsche chose to not utilize a harmonic damper on these engines, at least until the M97 3.8 came out, and thus the dual mass flywheel is of critical importance.
On most cars, this isn't a huge deal as the car already has a harmonic damper on the other end of the crank.
I'm of the opinion that you CAN run a single mass flywheel IF you put some damping back into the equation with a dampened front pulley. If the engine is internally balanced that's obviously a huge benefit which will lessen the loads that need to be damped in the first place.
As to making a "heavy" single mass flywheel, there would be absolutely no reason to bother. If you want a heavy flywheel, keep the stock dual mass. If you want a single mass flywheel, it may as well be lighter and enhance the free-revving capabilities of the car.