The MCoupe Gets a 6 Speed Part I

When it came to the original BMW M-Coupe, the options available for order were very limited.  It only came with one available transmission throughout its 4 year production run.  The awkwardly spaced ZF 5-speed.  It works fine for a stock application and normal street driving.  It shifts very precisely and accurately as long as the shift pins aren't on their way out. But it leaves something to be desired for the performance minded enthusiast.  With first gear being a little too tall for the rest of them and 5th gear being one to one, this limits your available options for swapping out just the final drive while maintaining the streetability of the car.  If you go too short with the final drive, you lose fuel economy and highway comfort.  That's where the ZF 6 speed out of a e46 330i comes in.  With a more evenly spaced gear set and a 6th gear that's 0.85 to one it allows for a shorter final drive ratio while maintaining highway comfort.

Unfortunately, it's not a direct swap and some parts have to be fabricated.  I was fortunate enough to find an almost complete "kit" that included a 3.91 Torsen diff put together by the owner of Clownshoe Motorsports.  He recently pulled it out of his MCoupe and he didn't put very many miles on the wear items.  It came with pretty much all of the necessary odds and ends and a few extra parts as well.

First up, it was removing the differential and swapping speedometer wheels and covers.  A big thanks to Jonathan Thayer of BimmerDiffs for running an informative site on BMW differentials and specifically the removal and replacement of the Z3 differential.  Since I'm replacing everything in between the flywheel and the differential, I went ahead and removed the driveshaft and exhaust from the headers back to make for a little bit more room under the car before removing the differential.

The MCoupe, exhaustless, wheelless and diffless.

The MCoupe, exhaustless, wheelless and diffless.

Removing and replacing the differential requires dropping the rear subframe a bit.  The differential came out with surprising ease but one of the subframe bushings did put up a fight.  For anyone who has worked on the rear end of a semi-trailing arm BMW, removing/replacing the subframe bushings can be the most aggravating part of the whole process.  Unbolting the rear sway bar from the frame of the car is not necessary for just removing/replacing the diff but it does help when lining everything back up for re-installation as it allows for more movement of the subframe.

The two differentials out of the case swapping speedometer wheels. On the left, the 3.91 Torsen unit and on the right the stock 3.23 clutch pack unit.

The two differentials out of the case swapping speedometer wheels. On the left, the 3.91 Torsen unit and on the right the stock 3.23 clutch pack unit.

After installing the 3.91 differential, it was time to remove the transmission, pressure plate, clutch and flywheel.  The e46 transmission has a shorter input shaft and different spline.  This requires a new flywheel, pilot bearing, clutch and pressure plate.  There is no need to remove the old pilot bearing that sits in the crankshaft because the new one sits in the flywheel retainer and doesn't interfere with the old one.

The different input shaft splines.  6-speed on the left.

The different input shaft splines.  6-speed on the left.

It is a fairly standard transmission removal.  The only "hard to find" thing I needed was an E11 Torx socket as most standard E-Torx sets don't come with one.  A few of the bolts are hard to get too but not unreachable with a common set of tools.  I had to improvise a little bit removing one of the bolts because I broke my 3/8" u-joint removing a rusted on e30 transmission a while ago and hadn't replaced it yet.

BMW Special Tool No. 486251

BMW Special Tool No. 486251

With the removal pile as big as it's going to get and the install pile getting smaller, it's a wonderful sight.  One thing I will not miss from the stock setup is the dual mass flywheel.  Rather than having a sprung clutch disk, BMW was using a solid clutch disk and a dual mass flywheel.  The dual mass flywheel is a great idea in concept and it works great when new but after it gets clogged up with clutch dust it just creates problems.

All of the old parts.

All of the old parts.

To be continued...