alluminum flywheel?

[Hib Halverson]

It's not the aluminum flywheels that rattle.

It's the transmission. The correct nomenclature is "gear rattle". It happens at idle and at very low engine speeds under light load. It occurs because, at idle and low engine speeds and only four firing impulses per revolution, engine speed actually fluctuates enough at a low frequency that the main drive gear and the counter gear start "rattling" because of the clearance between the teeth.

In the manual transmission nomenclature, "center distance" is the distance between the centerline of the drive gears and the centerline of the countershaft. The greater the center distance, the more pronounced the gear rattle will be. During early development of the S6-40, it was quickly obvious that gear-rattle was a problem.

The dual-mass flywheel designed for the first LT5?one of the first applications of that technology in a light vehicle?was a partial solution to gear rattle. The flywheel had a spring damped secondary mass the purpose of which was to "smooth out" the torque fluctuations caused by firing impulses at idle RPM.

Install an aluminum flywheel, which is a standard or "single mass" wheel, and you eliminate the gear rattle mitigating the function of the dual mass wheel. With an early-style, 610-Nm transmission, the increase in rattle is very noticeable. With a late-style, 540-Nm transmission, the increase is noticeable but not so bad.

As I was not a drag racer and liked the quicker engine acceleration which comes with an aluminum flywheel, I installed a McLeod aluminum wheel in Barney, my dear-departed '95 SN 140. There was an increase in rattle, but I cut that about in half by installing higher output ignition coils, then opening up the plug gaps by .010 or so and going to Red Line Lightweight Shockproof Gear Oil which used a calcium-based EP additive package that has a cushioning effect between gear teeth.

Those solutions would still be valid, today.