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Old 01-22-2013, 08:40 AM   #209
58mm of Bliss
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Join Date: Jul 2003
Location: PEI,Canada
Posts: 4,368
My Ride: 330i, 330Ci
Originally Posted by Twin_Turbo_S54 View Post

Does whp matter for the vibration damper or is it the frequency that happens at certain engine speeds which will destroy the crankshaft?

As far as I know, frequencies increase up to a point between x rpm and y rpm and cause the crankshaft and bearings to move, flex and destroy.

What matters whp or engine speed?

BTW, have you taken logs of your turbo speed sensor? I follow you with pleasure Adam.

It's a function of power and frequency (RPM).

If you ran the engine on a Spintron, which basically spins the crank at a desired RPM without combustion, you will find harmonics are present. This is a function of the inertial loads from the rotating assembly (pistons, rods) Looking at the piston/pin mass alone, you have approximately 500 grams X 4300g (@8000 RPM) of deceleration at TDC which is 4,700 lbs pulling up on the crank.

The combustion event can be 1800+ PSI peak for a gasoline turbo, and over 3000+ PSI for nitromethane for example. Imagine 1800 psi acting across a 3.3" diameter piston. That's approximately 16,000 lbs force onto the crank, which is then released very quickly.

So we have approximately 20,000 lbs of force change very quickly on the crank. (Again, limited to only the piston assembly mass)

By changing RPM, we vary the force change a little bit, but the main amplitude change comes from combustion. So you can see that by varying power levels, which changes the peak cylinder pressure, that the required harmonic damping effect changes a lot.

The harmonic signature of the crank, or the frequencies that it resonates at, don't really change much with RPM....but the amplitude does. Most of the time with modified engines, the harmonic problem shows up because of spending more time in a problem area....or with added power, the problem area is bigger.

Last edited by PEI330Ci; 01-22-2013 at 08:47 AM.
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