The car is in the shop getting the NGR oil "upgrade" per the Mopar TSB. I heard that this will help, but is there a way to eliminate it?
It's Sunday.. it's snowing out.. don't let me be obnoxious because most are aware of this.. yet for any engine newby's..
Neutral gear rattle has been around for a long time in the combustion engine world, as you would know the crankshaft rotates due to a force applied from each piston in a sequence, basically it's a pulsating torque, not an evenly applied torque since the stroking piston force is off then on off then on... like pedalling a bike, only applying power with each pedal stroke. The worst case would be less number of cylinders and a higher compression ratio and bigger engine displacement (per cylinder). Porsche dealt with this in the 60's or so using a dual mass flywheel. A DMF is a 2 piece flywhweel with springs sandwiched between them, so one half of the flywheel acts as a vibration damper, absorbing the rotational pulse. LUK is a typical manufacturer of DMFs.
You do not want a DMF in a Viper to resolve NGR, it's heavier and they really have a poor history of reliability. Diesels typically utilize a DMF, but many owners opt to go with an aftermarket single / solid mass flywheel and live with a bit of gear rattle since a SMF is stronger and more reliable. It's a problem with diesels because of a higher compression ratio and large displacement, a stronger pulse with a 17.5:1 compression ratio, some are around 20:1 c/r, and like a Dodge Cummins, they are a 6 cylinder with large displacement and high compression which are the ingredients for NGR.
Once the car is in gear and the clutch is released the coil springs in the pressure plate will absorb the pulse since it's now connecting the crank to the driveline as an opposing force, but there still may a bit of rattle even at low rpm. Some manufacturers incorporate various sized springs in the pressure plate to absorb a pulse at different frequencies when in gear, regardless, once the clutch is depressed you don't have a vibration damper with a single mass flywheel other than the other end of the crank there is a damper (at the accessory pulley) which absorbs the pulse so the crankshaft doesn't break due to the pulsating rotational forces. Diesel guys increase the idle to increase the frequency which reduces the neutral gear rattle with a SMF and swap the front damper which they claim helps.
I wouldn't be surprised of there was a a bit more NGR with a lightweight flywheel, different clutch packs, etc.. Different oil can soften the vibration of the gear clearances, yet in the end of all this babbling there doesn't seem to be a solution other than a DMF, increase the idle, experiment with different front crank dampers or putting an electric motor in your car.
Best regards,
Mike
I forget which magazine it was.. but here's the car below.. hey the article mentions Canada too eh' 
