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Dave in MN · 08-07-2020, 02:12 PM

Brent,
We may have discussed this before so sorry if it is a repeat. For the rest, it may be of interest.

When using a lightened flywheel, I have had the experience of the "naturally occurring harmonic vibration" change rpm range.

A few years back, while doing some component testing for my touring car, I discovered the following: On an engine with a counter-weighted and balanced crankshaft, using an extremely lightened flywheel caused the vibration to present itself between 50 and 54 mph with a standard ratio rear end. Normally I see this vibration between 42 and 46 mph. Having the vibration show at 50 - 54 mph could not be worse for driving average roads with a touring engine.

The solution was to install a heavier flywheel, about 10#, and the harmonic vibration dropped back to between 46 - 49 mph which is much more drive-able. I believe if a standard weight flywheel were to be installed, the range would have dropped into the normal range of 42 - 46 mph. All the flywheels and matched pressure plates were balanced.

We even tried Ron Kelly's method of modifying a SB Chevy front harmonic balancer and mount it to the inside of a lightened flywheel. It actually worked the best in narrowing the vibration range. Voodoo or fuzzy logic I know but it was hard to argue with good results.
Good Day!

08-07-2020, 02:12 PM	#11
Dave in MN Senior Member Join Date: May 2010 Location: Jordan, MN Posts: 1,416	Re: Lightened Flywheel Brent, We may have discussed this before so sorry if it is a repeat. For the rest, it may be of interest. When using a lightened flywheel, I have had the experience of the "naturally occurring harmonic vibration" change rpm range. A few years back, while doing some component testing for my touring car, I discovered the following: On an engine with a counter-weighted and balanced crankshaft, using an extremely lightened flywheel caused the vibration to present itself between 50 and 54 mph with a standard ratio rear end. Normally I see this vibration between 42 and 46 mph. Having the vibration show at 50 - 54 mph could not be worse for driving average roads with a touring engine. The solution was to install a heavier flywheel, about 10#, and the harmonic vibration dropped back to between 46 - 49 mph which is much more drive-able. I believe if a standard weight flywheel were to be installed, the range would have dropped into the normal range of 42 - 46 mph. All the flywheels and matched pressure plates were balanced. We even tried Ron Kelly's method of modifying a SB Chevy front harmonic balancer and mount it to the inside of a lightened flywheel. It actually worked the best in narrowing the vibration range. Voodoo or fuzzy logic I know but it was hard to argue with good results. Good Day! Last edited by Dave in MN; 08-07-2020 at 05:21 PM.