Look up rv6ejguy's commentary on his EJ22 with a Marcotte.

System reliability is predicated upon the lowest natural frequency being sufficiently below idle firing frequency to keep transmitted vibration down and avoid operation in resonance. This is done by having high enough mass moment of inertia flywheel and low enough spring rate between flywheel and the input gears. This is called a "soft system". Put a low Inertia flywheel in, and you raise the natural frequency towards the firing frequency, which, at best, increases the firing pulse sizes passed through, and at worst, goes near resonance and amplifies firing pulses.

The other big inertia in the system is the prop, much larger than everything else. A low Inertia prop will require either softer springs or higher inertia flywheel or both. If you know the mass moment of inertia or make and model of your prop, the maker of your redrive should be able to specify flywheel and isolators. If they can not tell you what is needed to make their gadget run, maybe re-think whose gadget you use.

Mass moment of inertia is the sum of dm*r^2 where dm is mass at each radius and r is the radius. You can minimize mass at any given MMOI by preferentially putting mass toward the largest radius you can. Limits are placed by the radius and length of the housing the flywheel and isolator operate within.

Ross DI find he had quite a bit of vibration with his standard Marcotte system and his rubber isolators appeared to be life limited. He did some careful modeling, then added inertia to his flywheel with a substantial decrease in vibration and presumably increased isolator life. You can look up all of his posts with the Advanced Search tool. He has a LOT of input on how to do EJ engines and PSRU correctly.

Billski