I want to switch to 3.91 final drive gears in my Z4MC. It seems there are two routes - buy the gears and have them installed in your diff, or buy a diff with the gears installed and swap for your entire stock diff. Which is better? Which is cheaper? Yes, I've decided against the 4.10 gears and no, I won't change my mind.

 

my e46 m3 had the OEM Motorsport 4.10 diff...i would say go with the diff. 4.10s are better tho than 3.91s.

 

Thanks for carefully reading my original post before responding so thoughtfully.

 

could you explain why they are made by BMW? is it like on their motorsports catalogue?

 

I did some research into this awhile back. The two places I found that sold complete 3.91 diffs were diffsonline.com and turnermotorsport.com. From what I found the best route to go was to just buy the entire diff. It might cost alittle more but in the end you have the OEM diff shall you decide to swap back to stock. Plus swaping out the diff is not that hard at all. Swaping the gears in the diff must be done by a professional and I have heard stories of grinding nosies if the gears are not installed right. If you were to have the gears installed in you oem diff I estimated the price to be around $2,000 vs $3,000 for the entire diff.

 

What gives me pause is, well, my car is NEW, and Dinan, TMS, and DiffsOnline all give you new gears in a rebuilt diff, so I'd be paying more to put an old part on my new car. And they get a new diff out of the deal.

 

Call my friend Dan at Diffsonline. They can do any BMW diff you need. In fact the just supplied one to some car for Fast & Furious 4. I;m actually going to the driving range with him tonigth and can ask if u want. Hes a really good dude, and the mechanics that do the diffs are awesome.

Oh, and Im sure if you dont mind the downtime, he can put new gears in your new diff.

 

I actually just spoke with Dan, the owner of Diffsonline and he does actually have the option of getting a new one done for you. Obviously it just costs a bit more. And also, you would be surprised at where alot of suppliers of modified diffs get them from *ahem ahem*

 

Just an update to this - I did get the 3.91 gears installed, going the "entire diff' route. I've written a couple of posts on the difference here. Thought I'd link it as you don't hear a lot about mods for the Z4M and this, imo, is a great one.