[cuco]

Originally Posted by mapleridge

Sorry. Im not the most knowledged in differential gearing. Can someone give me a quick breakdown of how it affects the driveability/power?

The gear ratio sets how many times the drive shaft turns per 1 turn of the tire/axle. 3.73:1 means the driveshaft will turn 3.73 times every 1 revolution of the axle/tire. The higher the gear ratio more times will the engine turn the drive shaft and more torque you will get at the wheels. The car will accelerate quicker with higher gears but will climb the revs faster and the shifts will be shorter. With a higher gear ratio diff you will be cruising at higher rpms than with a lower gear diff.

[driverman]

Adding to cuco's post....

If you increase the axle ratio by 10%, say from 3.23 to 3.55, you will increase the torque applied to the rear wheels by 10% in all gears at all RPM's. It's a great way to mod a car for better performance, BUT there are down sides.

All the gears won't take you to the same speed as they did before, there will be more engine noise on the highway and gas mileage will drop.

[mapleridge]

Thanks guys.

[jamie323i]

And I am cool with any of these down-sides. AT diff is going on the car tomorrow, should be interesting.

[cuco]

Take pics!

[jamie323i]

Ok so I've done the swap, and had it on the car now for a couple weeks.
Everything was a direct swap, going from a 3.23 manual transmission diff to a 3.73 out of an AT.
Works great! noticeable increase in torque, and makes 6th much more useful. I really think this is the diff the car should have had in the first place.
I didn't get many pictures the day of the swap, as I wasn't feeling well and just wanted to get it done. However the pictures of the actual swap aren't all that interesting and seeing as it was a direct bolt up, there's not much to tell.
As for the old diff, I decided to take it apart to see the damage, and there was lots!
The pinion bearings were slowly disintegrating and sending bits of mashed-up metal all through the housing and internals, most all of the bearings were starting to go the same path and I found a large amount of metal filings ground to a paste in the output shaft bearings, seen in one of the photos below. Also, the ring and pinion look rather unhealthy.
Still kinda unsure as to why the thing failed in the first place, but it's fun to look at the carnage!

[Xpander]

Thanks for sharing.
I actually have been thinking of doing this swap eventually to gain some extra torque. Seems like best bang for the buck.

[jamie323i]

Originally Posted by Xpander

Thanks for sharing.
I actually have been thinking of doing this swap eventually to gain some extra torque. Seems like best bang for the buck.

It's worth it if you can find the diff for a decent price, hard to explain how the car feels now, not like it has much more power, but just uses it better... also pulls all the way up to redline, where as before there was hardly any useful power above 6k rpm.

cruises on the highway at 120 km/h at 3000 rpm, fuel mileage loss seems negligible

[driverman]

Thanks for sharing.

Going from a 3.23 diff to a 3.73 diff should result in 15% more torque at all engine speeds in all gears AND 15% lower max speed in each gear. Is that how it feels?

[jamie323i]

pretty much haha

[driverman]

Jamie:

Here are the gear ratios in my E90.

1st 2nd 3rd 4th 5th 6th
4.320 2.460 1.660 1.230 1.000 0.850

If your 323 has the same ratios, going from a 3.23 diff to a 3.73 diff should make 6th gear feel the way 5th gear used to feel.

What did the mod cost you - parts and labor?

Thanks.

[jamie323i]

I believe my 323i has the same ratios, and yes, 6th is more like 5th used to be.
I bought the diff used for just under $900 and had it shipped for another $150 or so. I installed it myself so the labour was free, but it took a friend and I about 4 hours while having beers. We didn't remove the exhaust as advised by the workshop manual though, which meant a little ingenuity was needed at times to get some bolts loose and then torqued. The only issue we really had was that the driveshaft was quite well fused to the input flange of the diff, took a lot of pounding and wedging to get it loose.
I spent an additional $65 at BMW for new bolts for everything that came apart, except for the 3 holding the diff to the sub-frame. As well as $15 on redline gear oil.

[driverman]

I think you just made the best performance mod available for 323, 325, 328 owners.

More torque everywhere and the only parts of the car that are handling the extra torque are the diff, half shafts, hubs and rear wheels. So reliability should remain the same. Plus, no added complexity or gimics. Just more torque to the drive wheels due to shorter gearing.

Congratulations!

[Volasko]

I am very curious to know how this feels vs. stock 6 mt ratio. I don't really think the 3-400 extra rpm at crusing speed really sucks that much more fuel.

[driverman]

Originally Posted by Volasko

I am very curious to know how this feels vs. stock 6 mt ratio. I don't really think the 3-400 extra rpm at crusing speed really sucks that much more fuel.

Since 5th gear with a 3.23 rear end (3.23*1.00=3.23) results in about the same overall gearing as 6th gear with a 3.73 rear end (3.73*0.85=3.17), we can run an experiment to see what the approximate MPG difference will be:

- drive on a level road and set cruise control to 60 or 70 mph
- drive in 6th gear for a while and check MPG
- drive in 5th gear for a while and check MPG
- compare MPG in 5th and 6th gear

The difference in MPG should approximate the difference we'll see on the highway in 6th gear if we swap out the 3.23 diff for a 3.73.

[jamie323i]

I'd say I've maybe lost 50 km per tank of fuel, pretty negligible if you ask me

[Casca]

Originally Posted by jamie323i

It's worth it if you can find the diff for a decent price, hard to explain how the car feels now, not like it has much more power, but just uses it better... also pulls all the way up to redline, where as before there was hardly any useful power above 6k rpm.

cruises on the highway at 120 km/h at 3000 rpm, fuel mileage loss seems negligible

How useful is 1st gear now?

[andrey_gta]

if only there was a simple & cheap way to convert the used higher ratio diffs to an LSD.

I recall a thread from e46 fanatics of someone trying to use e36 parts

[MadJoe]

This has me thinking it's time to track down a 325i AT in the junk yard and slap its dif in my 325i MT.

[andrey_gta]

The Suspension forum has a new vendor offering a new LSD option.
This makes junkyard hunting for Xi or Auto diffs perhaps be as useful...

[GTKrockeTT]

Originally Posted by mapleridge

That is the million dollar question. The 335i and the M3 have the same splines in the hub so you can use the m3 half shafts. Just a matter of ours fitting or not. Although, if you are taking the drive shaft to be shortened and balanced, the same shop could probably mate the 323 half shaft end with the m3 for less than the cost of swapping out the knuckle.

Edit**

These are the hub p/n's between the following models

E90 323i 33416760056
E90 328i 33416760056
E93 328i 33416760058
E90 328xi 33416760058
E90 330i 33416760058
E90 335i 33416760058
E90 M3 33412283020

The flanges/hub numbers between the 335 and the m3 are different but its a direct fit. This retrofit looks like it would also fit an xdrive n52, e93 n52 & 330i as well with just the modification to the drive shaft. Who knows, it may also be a direct fit with the 328/323

maple, do you know what part number is applicable for the E91?

[Casca]

You guys know if a 335i "large" differential cover will fit a "small" pumpkin? Looks like its already confirmed that the 545i/550i finned cover fits the large 335i pumpkin.

Looking at the gaskets for all the different pumpkins, they look like all the same bolt pattern, large and small.