Gears & Axles - Bit of a whine with new 3.90's - need suggestions!?!?

Hi all,
Just finished putting in a new set of Motive Performance 3.90's in place of my stock 3.42's. Test drive showed no noise on throttle, no noise on deceleration, but a noticeable whine at very slight throttle when engine speed is matching ground speed (if that makes any sense). :confused: It's not bad (almost none) in 1-4 gears around town, but you really notice it in 5 and 6 above 45 mph.

The install went nice with pinion depth being right on the mark and solid pinion spacer being shimmed properly for exactly 15 in-lb of pre-load. The contact pattern matched the install sheet picture very well. It was centered between the root and top of the tooth with our only concern that the slack side may have been a tiny bit too close to the inside of the ring gear. But, it still matched the picture so I think we're OK there.

The only spec that we "cheated" a bit was the backlash. We put the stock side to side shims back in and ended up with .006" while the Motive kit called for .008" to .012". We had another set of gear instructions from a different mfg for a friends' 3rd gen, and those instructions said that .004" to .008" was OK. So, we stayed with the .006" and put the deal together.

So, my questions are
1. Could our lack of backlash be causing the whine?
2. Can I expect the gear to break in a bit to the point where the whine may get better?
3. Or should I just pull it appart and shim the carrier to get my .010" backlash?

Thanks for any help,
Scott

Anyone?

Thanks,

I'm no expert, but I would stick with what motive recommends, although I can't believe that .002-.004 would make that much of a difference. I am thinking of going with these also, I'm interested to hear how this turns out.

I just put a Moser 12 bolt in my car with 3.90 gears and the thing is loud as hell. I'm not sure if its supposed to be that loud or not. Same thing as you though, its usually only loud in 4th-6th gear when I'm not really on the gas. :confused:

Whine will never get better, it will only get worse (if anything).

Setting gears up with different backlash than recommended increases your chance of having a noisy rearend. While .006 is not excessive, it may be enough to cause noise. You mentioned that there was a touch of too much toe on the coast side, that probably isn't helping matters any either.

However, as long as the noise does not get any louder and the gears are set up correctly as you said, you may just consider living with the noise if it is not that loud. Or you could always give reinstalling the gears another whack...

I just put a Moser 12 bolt in my car with 3.90 gears and the thing is loud as hell. I'm not sure if its supposed to be that loud or not. Same thing as you though, its usually only loud in 4th-6th gear when I'm not really on the gas. :confused:

When you are not on the gas that is called the 'coast' pattern of the gears doing the work. If the noise bothers you a lot you can have someone reset the gears for you to help combat the noise. But yes...they can be pretty noisy especially after being raced. If you have an aftermarket torque arm it makes things 100 times worse also because of the noise that is transmitted.