[Didster]

Hi guys,

My diff has started making some horrendous deep whirring noise with vibration at speed and is worse when braking. Mechanic has said it's definitely the diff, does this sound right?

I've found a decent use replacement diff online but it has the 3.08 MT ratio rather than the 3.46 in the AT. My car is an AT but I've read you can fit these directly with no issues will just lose a bit of acceleration?

Also my car is flashed by MHD Stage 1 and xHP Stage 3 gearbox. Will changing the final drive in the diff create problems with that and/or will my speedo be out?

Help appreciated!

[bmuum3]

For the vibration, do you feel it in your seat or steering wheel?

[Didster]

In the seat definitely.

[feuer]

Originally Posted by Didster

I've found a decent use replacement diff online but it has the 3.08 MT ratio rather than the 3.46 in the AT. My car is an AT but I've read you can fit these directly with no issues !

Who told you that? Only early 2006/2007 AT have larger casing diff. Newer AT have small casing diff. All MT have larger casing diff. Your is 2008 so it won't fit.

[e90TiAg335i]

I've actually had someone local replace a small diff with a large diff and used the small diff/driveshaft without issue. Everything bolted up. The large diff does use a shorted driveshaft but there was still, end to end, play in the small diff driveshaft.

[PR3CI5N]

A few people have ran the 3.08 in their AT. Terry being one of them in his 135i. The longer gears work great for the 1/4 mile and 1/2 mile track. Staying in 4th gear for longer and 5th gear longer has its advantages when it comes to ET and traps (depending on your power level of course).

[limitdown]

Here's a reference table of our various diffs.

I have an August 2007 335i and I bought a fully built large pumpkin from mFactory when I replaced my broken factory diff.

[Didster]

Well the MT diff is in and all went fine and fitted without a problem. Strangely the car "feels" like it pulls stronger now and the turn of speed seems quicker but that may be my butt dyno out of whack!

Annoyingly the fault remained and the mechanic had misdiagnosed it so if anyone wants a perfectly good AT diff with 3.46 ratio please let me know. The fault turned out to be a sticky caliper/brake shoes. ARGH! Oh well, live and learn.

[TheeGovernor]

Originally Posted by PR3CI5N

A few people have ran the 3.08 in their AT. Terry being one of them in his 135i. The longer gears work great for the 1/4 mile and 1/2 mile track. Staying in 4th gear for longer and 5th gear longer has its advantages when it comes to ET and traps (depending on your power level of course).

There are no issues with the transmission getting confused because of different final ratio?

[V8bait]

Originally Posted by TheeGovernor

There are no issues with the transmission getting confused because of different final ratio?

There's tables that can be adjusted if necessary, but final drive changes generally don't need them.

[feuer]

Originally Posted by e90TiAg335i

I've actually had someone local replace a small diff with a large diff and used the small diff/driveshaft without issue. Everything bolted up. The large diff does use a shorted driveshaft but there was still, end to end, play in the small diff driveshaft.

Of course. Small diff > longer shafts, large diff > shorter shafts. Thickness is the same.