My current OEM setup is really worn out (was installed badly) and needs to get replaced.

We don't have strips and slicks here. Roar driving and drags only.

1. Should i go 3.42, 3.73, 4.10? Is it dangerous for me to go higher? Is 3.73 a good compromise considering it's not for a race car, just occasionally drag race it.
2. I found motive sets for 155$ + master kit 99$. Is this ok? You guys think some other manufacturer is better?
3. Where can i find a manual including tolerances/specifications for swapping gears? I'm going to ask a local mechanic to do it and he'll need some specific information to do the job.
4. I have the OEM torsen differential. I read somewhere 4.10 need paddle modifications. Is it true? Will 3.73 be ok?
5. Does ratio change affect speedometer and odometer accuracy? Where is speed read?

Anything else you guys might want to add feel free.

Thanks!

P.S. Having about 30 degrees of play really means I need a new set doesn't it? :P

 

Are you saying that if you hold the driveshaft or pinion steady that you can rotate the ring by 30 degrees? This cannot be measured at the wheels btw. If this is true your rear needs some serious attention. May even need to start with a fresh housing.

 

If your mechanic needs a manual to do a gear swap, he's not the right person to do it.

Speed is read by a sensor on the tranny output shaft housing. The PCM needs to be reprogrammed with the correct rear axle ratio.

 

Well the 30 dregree thing is just my guess by the feel of things (maybe it's 15-20), and it's by rotating the driveshaft (pinion) against the ring, with the wheels locked by handbrake.

The mechanic said only to provide anything that may help him do the job as good as possible. They only do modern european cars at his shop. It's impossible to find a competent classical american car specialist.

Injuneer: Well ok then I'll run it with the offset for a while and when i get the OBD1 swap i'll have them adjust that.

Please give me some input about the ratio in my case (city drive mostly, drag race rarely, no slicks/track), the compatibility with the torsen and the company from which to get the set

 

Decide how much more grunt you want off the line... 3.73's will make it feel like you added 9% more rear wheel torque, 4.10's will make it feel like you added 20%. Then there's the traction issue.... do you have sticky tires and suspension upgrades to provide traction with a 20% increase in rear wheel torque. Finally, there's the fuel mileage issue. Nice thing about the M6 is that the higher numerical gears make 6th gear more usable, minimizing MPG loss. If you seldom drive at 80 MPH for long periods of time, it shouldn't make much of a difference.

To complicate things even more, Motive makes a 3.90 gear (14% increase) for the 7.5" 10-bolt. Motive gears are good, as far as I know. I run US/Strange, but I have a 12-bolt.

I don't know about the paddles on the Torsen. I've seen the paddles recommended for 4.10's, but I don't know if they are required for other ratios.

As far as the instructions, make sure you get specific Motive instructions, if those are the gears you buy. They changed their manufacturing and setup recommendations several years back. Most shops should know about it by now, but if they don't do domestic cars, they may not know about it.

If by "offset" you mean you will just run with the speedo/odo error until you get your PCM swapped out to an OBD-I unit, your speedo will be reading 9% to 19% higher than actual speed, and the odometer will be tacking on 9% to 19% extra miles That means you could be racking up 1190 miles on the odo, when you actually only drove 1000 miles.

 

Thanks for the comprehensive info.

I've decided and ordered "performance" (175$ instead of 155$) motive 3.73 gears and master bearing install set (99$). I figured I already can't hold grip, I'll still be able to reach 62mph in 2nd gear, I won't fuxx my highway MPG and it will definitely make me happy anyway (praying i get rid of the banging sound too).

I'll try mailing Motive for instructions BTW I live in Romania (Europe) that's why people aren't familiar with this type of install.

I'm ordering a madz28 PCM tomorrow anyway so I'll do them basically at the same time, or very soon at any rate.

Cheers!

 

Solid bearing spacer (for pinion) - useful or not? Worth getting?

 

Definitely, but it takes longer to set up.

 

That may be a problem... looking into the instructions I found on a site http://www.drivetrain.com/ringpinioninstal.html I'm not sure at all anyone here can do it properly... Let alone if I make their jobs even harder :P God help me ...

 

That is one job you need done right or else it will fail. If there's any hesitation from the mechanic about doing the rear end or installing the solid spacer & shims, then you don't want that shop working on your car. Just last summer, I had to fix one that a shop screwed up, and it cost the owner several hundred to buy the new parts to fix it and the mechanic didn't stand behind his work.