3.15 rear diff in E90 330

In the lastest Autoweek which had an article about the E90, they mentioned that the 330's are going to have the same 6MT which come in the 04+ E46 330's except that they are going to have a 3.15 final drive ratio instead of the 2.93 we have currently. I find this interesting as alot of people with 330 MT's do diff swaps so it seems to add some credibility to the mod. I'm hoping that someone can answer some questions for me: Is the E90 engine going to have the same or a higher redline? Will the 3.15 diff from the E90 330 bolt right on to a 6MT E46 330 without having to swap the flanges or any of that? I realize that we might not be able to answer these questions untill the E90 makes it over here, but I was thinking about putting a 3.15 diff in my 330, but I'm a bit reluctant to do so because I've read some things about doing the flange swap that it must be done properly or it screws up the load on the gears or something like that, so I would prefer to swap in a diff where all of that isn't necessary. Anyway it's an interesting change they made to the E90 which is going to be the same gearing as in the current 330, but with a shorter final drive. You would think with the more powerful E90 engine the tall 2.93 rear drive would've been even less of an issue so I don't really understand the change. The 2.93 is great for 6th gear at 100 mph, but for most driving conditions I think the 3.15 would be a perfect compromise and I guess in light of this news BMW must think so also.

3.15 is a waste of money. Don't swap for anything less than a 3.38, and a 3.46 would be better.

The E90 330i has a 7,000 RPM redline, which is a ~7.7% increase over the old 6,500.

More than a coincidence IMO, the 3.15 ratio is roughly ~7.5% increase over the 2.93 ratio.

I think BMW is simply adjusting the final drive in the same proportions as they increase engine speeds, so that max speed in each gear remains roughly the same, but obviously allowing for great torque multiplication. Obviously, rear tire height has an impact on this, but I think they are trying to keep everything in proportion.

On a similar note, the M3's 8,000 RPM redline is ~23% greater than 6500. The 3.64 M-diff is also, ~24% shorter than the 2.93. The ZHP package cars went to 6800 (a ~4.6% increase), and have a 3.07 rear end.. which was a ~4.6% increase over the 2.93.

Originally Posted by ///M-Spec

The E90 330i has a 7,000 RPM redline, which is a ~7.7% increase over the old 6,500.

More than a coincidence IMO, the 3.15 ratio is roughly ~7.5% increase over the 2.93 ratio.

I think BMW is simply adjusting the final drive in the same proportions as they increase engine speeds, so that max speed in each gear remains roughly the same, but obviously allowing for great torque multiplication. Obviously, rear tire height has an impact on this, but I think they are trying to keep everything in proportion.

On a similar note, the M3's 8,000 RPM redline is ~23% greater than 6500. The 3.64 M-diff is also, ~24% shorter than the 2.93. The ZHP package cars went to 6800 (a ~4.6% increase), and have a 3.07 rear end.. which was a ~4.6% increase over the 2.93.

Ah....I'm not that surprised. The extra 500 rpms is probably where the new engine finds it's peak HP rating also. It goes to show that there is a lot of benefit to increasing the redline if you're thinking about installing a shorter rear diff on a 330.

Originally Posted by Moderato

..but I'm a bit reluctant to do so because I've read some things about doing the flange swap that it must be done properly or it screws up the load on the gears or something like that...

I can vouch for this, I put a 3.46 on my 330, and it had 323 flanges on it. The input flange is a real pain the , you have to torque the pinion nut exactly (like 125 lb ft) to preload the bearings, or else you get premature wear and rattling/whining. The output flanges are cake, you just pop them off and pop the new ones in. In retrospect I would have taken a diff from another 330 (but then it wouldn't have been a 3.46)

Originally Posted by ///M-Spec

The E90 330i has a 7,000 RPM redline, which is a ~7.7% increase over the old 6,500.

More than a coincidence IMO, the 3.15 ratio is roughly ~7.5% increase over the 2.93 ratio.

I think BMW is simply adjusting the final drive in the same proportions as they increase engine speeds, so that max speed in each gear remains roughly the same, but obviously allowing for great torque multiplication. Obviously, rear tire height has an impact on this, but I think they are trying to keep everything in proportion.

On a similar note, the M3's 8,000 RPM redline is ~23% greater than 6500. The 3.64 M-diff is also, ~24% shorter than the 2.93. The ZHP package cars went to 6800 (a ~4.6% increase), and have a 3.07 rear end.. which was a ~4.6% increase over the 2.93.

Very interesting. I never noticed the correlation between redline and BMW's choice of differential ratios. Makes sense.