e90 6mt rear differential rebuild going from 3.85 to 4.1

[srmast1]

I am just curious if anyone on this forum has done a rear differential rebuild for a 6mt, e9xm, going specifically from the stock final drive ratio of 3.85 to the rebuilt 4.1 final drive. I'm curious if it is really noticable, if it hurts milage on the highway badly, cost to value, etc.. Any feedback would be helpful!

[GRM Scott]

That seems like a mild ratio bump, not insane but you should certainly feel the difference. Obviously top speed will drop a bit but each gear should wind out with that much more verve, a final drive swap can be a really effective way to liven up a car.

Never been in an E9X M with a shorter drive, but I've got a similarly rev-happy Integra that I changed from its stock 4.4 final drive to a pretty extreme 4.929, and it was a riot afterwards, it really suits the nature of a rev-happy machine. Of course, it's also spinning around 5000 RPM at 80 in 5th gear, but the M3's 6th cog should keep things a bit less insane (although check the ratios on a calculator and make sure you'll be able to live with whatever RPMs you're going to end up with if you do long highway stints).

[GRM Scott]

Oh hey, Turner Motorsport had a nice chart on their differential page:

Gear RatioGearSpeedRPM
4th Gear @ 40 MPH
3.854th40 MPH 2,350 RPM
4.104th40 MPH 2,500 RPM
4.454th40 MPH 2,700 RPM

70 MPH Cruise
3.856th70 MPH 3,000 RPM
4.106th71 MPH 3,250 RPM
4.456th71 MPH 3,500 RPM

5th Gear Speed at Redline
3.855th168 MPH 8,250 RPM
4.105th157 MPH 8,250 RPM
4.455th145 MPH 8,250 RPM

[srmast1]

I do highway driving so I can't do the 4.4, bit the 4.1 seems like a reasonable option. It's just finding someone besides Turner Motorsports to do it. Apparantly it's tough to get a hold of the 4.1 gear set. I'll have to find someone in socal or around San Fran here in Cali. No one seems to have done this mod.

[Vybz]

I'm finishing up the break in on my 4.1. The 7+% torque gain is definitely noticeable. I haven't been able to fully stretch it's legs, but the car definitely pulls harder in each gear, and that's with a 6k rpm limit and not being able to heavily load the drive train. There are others on the forum who have been rocking it longer, I'd ask them for more insight. The increased drain on gas is noticeable too, but it's nothing too crazy. Highway cruising at 70mph I'm at about 3200 rpms now. Also you lose top speed if you weren't aware of that, but I doubt you spend much time in the 186 mph range.

So it's up to you. Is 7+% increased torque and quicker overall acceleration worth the loss in top speed, (it's only like 8-10 mph) and the decreased fuel economy.

[srmast1]

Originally Posted by Vybz

I'm finishing up the break in on my 4.1. The 7+% torque gain is definitely noticeable. I haven't been able to fully stretch it's legs, but the car definitely pulls harder in each gear, and that's with a 6k rpm limit and not being able to heavily load the drive train. There are others on the forum who have been rocking it longer, I'd ask them for more insight. The increased drain on gas is noticeable too, but it's nothing too crazy. Highway cruising at 70mph I'm at about 3200 rpms now. Also you lose top speed if you weren't aware of that, but I doubt you spend much time in the 186 mph range.

So it's up to you. Is 7+% increased torque and quicker overall acceleration worth the loss in top speed, (it's only like 8-10 mph) and the decreased fuel economy.

You make the swap to 4.1 sound very attractive indeed..where/how did you do it? Are you located in California?

[JJH]

What about diffsonline? > http://diffsonline.com/bmw-e9x-m3-di...l#.V9I-Eo9OIy8

They offer the option of just swapping out the ring gear and pinion set for a new ratio. They also have a very useful gearing calculator on the 'technical information' page.

[FogCityM3]

Get the car tuned to 8600 rpm rev limit and you really don't give up much in the way of top speed at each gear. 3rd gear is heavily relied upon on most US tracks and having more torque available in that gear would definitely be helpful in the 5500+ rpm range, perhaps reducing the need to downshift into 2nd at certain corners.

[dparm]

Originally Posted by FogCityM3

Get the car tuned to 8600 rpm rev limit and you really don't give up much in the way of top speed at each gear. 3rd gear is heavily relied upon on most US tracks and having more torque available in that gear would definitely be helpful in the 5500+ rpm range, perhaps reducing the need to downshift into 2nd at certain corners.

Yep, 3rd gear is where I spend 95% of the time at the track. I obviously get into 4th gear in the long straight sections, and occasionally short-shift to 4th depending on the corner setup.

It's quite rare for me to go down to 2nd gear. I find 3rd has more than enough torque.

[srmast1]

Originally Posted by dparm

Yep, 3rd gear is where I spend 95% of the time at the track. I obviously get into 4th gear in the long straight sections, and occasionally short-shift to 4th depending on the corner setup.

It's quite rare for me to go down to 2nd gear. I find 3rd has more than enough torque.

Dparm, have you done this swap to 4.1? Can you easily feel the difference in thrust if so?

[Move Over]

If you are interested, I have a differential for sale, stock gearing. Might be good for you to do a built diff so you can just do the swap. PM me if you are int. Thanks

[dparm]

Originally Posted by srmast1

Dparm, have you done this swap to 4.1? Can you easily feel the difference in thrust if so?

Nope, my car is bone-stock other than the BBK and slightly wider tires.

[srmast1]

Apparantly, it's the least popular mod for the e9x since nobody seems to have done it.

[lukester]

here in Germany we all run the 4.1 diff.
it's still more than ok for daily driving and if you talk about the Nurburgring Nordschleife.
With a stock setup you will more or less not need the 6th gear.
But with the 4.1 it just fit's perfect as you'll use 3/4/5/6th gear.

1st get's very short, but in case you want to run a 184mm race clutch like e.g. Tilton
the short 1st gear helps a lot to get the car rollin without having to use a lot of clutch.

[dparm]

Originally Posted by srmast1

Apparantly, it's the least popular mod for the e9x since nobody seems to have done it.

To be honest, the car is geared fairly aggressively already. My 7th gear isn't as low as it could be. (it would've been nice if it had been below 1:1 to make highway driving easier)