Would a gear swap from 2.93 open diff to 3.46 LSD worth it?

A930Rocket · 08-06-2021, 06:40 PM

In a previous thread, I thought about adding a supercharger to my 2005 BMW 330ci. It’s not completely off the table, but I’m thinking a gear swap in the differential might be the best bang for the buck. From what I’ve read, the spec E46 cars use a five speed with the 3.46 rear end. My question is, having a six speed, will I be rowing the gears a lot more with a 3.46 rear end? Gears 1 through 5 all seem similar, but I’m not a gear expert.

While at Roebling Road last weekend, I was behind a spec E46 and going down the front straight, while I was in fourth, I heard him shift to fifth, but he was not pulling away from me.

Edit: thinking about it just now, if he had a five speed with a 3.46, would he be able to go further down the front straight in third before shifting the fourth. Maybe this was the shift I heard? I was already in fourth, by five seconds or more.

What do you think about a change to the 3.46 with LSD from a 2.93 with an open diff. It will probably cost $2000-$2500 for a new rear end with LSD in material only. I would install it, to save labor.

This website lists all of the cars and their gear ratios.

https://www.bimmerboost.com/showthread.php?61350-E36-E46-E90-E91-E92-E93-E85-E86-E83-E82-E88-BMW-transmission-gear-ratios-final-drive-and-redline&

330ci 6spd
E46 Coupe.
4.35 - 1st
2.5 - 2nd
1.66 - 3rd
1.23 - 4th
1.00 - 5th
0.85 - 6th
3.93 Reverse
2.93 Final drive
6500 Redline

323cic 5 spd
E46 Convertible
4.23 - 1st
2.52 - 2nd
1.66 - 3rd
1.22 - 4th
1.00 - 5th
-
4.04 - Reverse
3.46 - Final drive
6500 - Redline

Shifter · 08-06-2021, 09:16 PM

Go here,

Gear Ratio Calculator

Punch in your numbers and it will calculate your rpms at speed. The site is geared (heh) towards trucks, but it will work for your purposes.

Why not grab a rear end from an auto car out of the junkyard and bolt it in to try the gears? It would be a bunch cheaper to try, especially if you don't like it.

pavulon · 08-07-2021, 06:56 AM

Maybe consider a torsen style diff.

masraum · 08-07-2021, 09:15 AM

I think you'd feel a difference going from 2.93 to 3.46, especially in the lower gears. Remember, the final drive gets multiplied by the various gears.

A good idea would be to work out your max speed in each gear with the current rear and old rear either using excel or a website. If it turns out that your top speed in 1st would change from 40 to 25mph, then you might want to rethink things (which seems unlikely).

I'd like to swap to a torsen-style diff in the Boxster myself. It's a more friendly option than a clutch style LSD for a street driven car. If you track the BMW, then you may prefer the clutch style LSD, but if it's not primarily a track car, then you may be happy with the Torsen/helical style diff. My wife's old miata and my Mazdaspeed miata both had Torsen style diffs.

A930Rocket · 08-07-2021, 01:50 PM

Good advice, guys. It’s a weekend car and used on the track. I’m going to look for a used 3.46 and give it a try. Then I can think of adding limited slip.

Speaking of adding it, since the gear won’t change, can I install the limited slip myself? A few YouTube videos make it look easy. 😂

masraum · 08-07-2021, 02:44 PM

Quote:

Originally Posted by A930Rocket

Good advice, guys. It’s a weekend car and used on the track. I’m going to look for a used 3.46 and give it a try. Then I can think of adding limited slip.

Speaking of adding it, since the gear won’t change, can I install the limited slip myself? A few YouTube videos make it look easy. 😂

Folks do, but my understanding is that to do it right there are a bunch of things that have to be set to certain specs. Maybe the design of the rear in that car means that's not the case.

A930Rocket · 08-07-2021, 06:13 PM

Quote:

Originally Posted by masraum

Folks do, but my understanding is that to do it right there are a bunch of things that have to be set to certain specs. Maybe the design of the rear in that car means that's not the case.

That’s my understanding, setting the backlash, wear pattern, etc. One video I watched, the guy had it apart and started by installing the ring on LSD, followed by the bearings on each side. Then he dropped it into the housing, added the races, snap rings and stub axles. No work on the pinion. That was it.