Diff for an E21

[Douge21]

Hello anyone that can help...the diff went in my little baby 323i and I was wondering if I can put a diff out of a 5 or 7 series in it (something stronger and that can handle a bigger engine with ease for when I give it a new engine...like a 540). Or should I just wack in another similar one.

[Jared at Pelican Parts]

First off,


the diff from a later car will not fit without extensive modification to the housing so it will fit. I believe you will also have to change the axle stubs as well, as i don't believe the CV's will bolt up/line up correctly.

You may want to try going to a 320is diff. You can find them on Ebay all the time, and it will give you the result you are looking for. It bolts right in, without any modification.

Hope this helps.

[DuckRyder]

The later diffs are not an easy fit, it's more trouble than its worth,

E21 323i carry a 3.45:1 ratio and they are somewhat hard to come by. 320/6 and us 320i up to and including 79 run a 3.64:1 diff which is in my opinion the best performance ratio. Later 320i run a 3.90:1. (its really too short, causes 1st gear to be nearly useless.) 315 I'm told run a 4.11:1.

"iS" models and some non iS cars have limited slips. You can Identify a limited slip by a large "S" painted on the top or side and an "S" on the metal tag attached to one of the rear bolts.

Try this Al Taylor if you are in the southeast.

http://members.aol.com/atscparts/

Or Dave Varco if you are out west.

http://www.2002parts.com/


Either may even have a 3:45 limited slip for you.

Edit: OOPS, I see now you are in Australia. Cant' help with suppliers there.

[Jared at Pelican Parts]

Originally posted by DuckRyder
The later diffs are not an easy fit, it's more trouble than its worth,

E21 323i carry a 3.45:1 ratio and they are somewhat hard to come by. 320/6 and us 320i up to and including 79 run a 3.64:1 diff which is in my opinion the best performance ratio. Later 320i run a 3.90:1. (its really too short, causes 1st gear to be nearly useless.) 315 I'm told run a 4.11:1.

"iS" models and some non iS cars have limited slips. You can Identify a limited slip by a large "S" painted on the top or side and an "S" on the metal tag attached to one of the rear bolts.

Just to amend your post, duckryder,

In the USA,


320is diffs run a 3.91 ratio and are all LSD, (first gear too short? I could really wind the car out in first, maybe not the same down under?)
320i diffs are 3.64

We also never officially got the 323i here, although there are a lot of them that have been imported. It's hard to find parts for some the owners that call me up.

We also never got the 315 or 316.

[DuckRyder]

Originally posted by Jared at Pelican Parts



In the USA,


320is diffs run a 3.91 ratio and are all LSD, (first gear too short? I could really wind the car out in first, maybe not the same down under?)
320i diffs are 3.64

This is not correct.

ALL 320 iS were limited slip. The limited slip was also a stand alone option all years. Prior to 80 (2.0l) cars all US cars were 3.64:1, 80 on (1.8l) cars US cars (actually 318i rest of the world) were all 3.91:1 whether the car was an "S" package or not has no bearing on the ratio.

This is probably a mute point as the North American market is the only place in the world that there is even such a thing as a E-21 320i from 80 on.

The 3:91 is a good match for the 1.8 4, it is a bad match for the 2.3/2.5/2.8 "baby six". The car runs through the first gear in the overdrive m-60 245 box too fast.

Trust me on this I've run all three ratios behind a basically stock 2.3 and a 3:45 and a 3.64 (all limited slip) behind a Metric Mechanic 2.9. trust me, unless it is a track car, you don't want a 3.91:1

Also the metal tag on the rear cover will indicate the ratio, dividing the two numbers on it will give you the ratio. again the presence of the "S" on the tag indicates limited slip.

[Jared at Pelican Parts]

Duck,

What ratio would you reccomend behind the 2.5 six? Im looking to gain a little acceleration, and the way i always understood it, that to increase the acceleration, you raise the final drive ratio.

I don't mean to argue, but I'm pretty sure that some later E21's came with the 3.64. After i wrecked my 320is, I sold the diff to a friend, and helped him put it in his 83 320i. According to the part number on the diff, cross checked in ETk, it said it was a 3.64.

Then again, i might be wrong about all this.. can you help?

[DuckRyder]

I would run a 3.64:1 limited slip behind most any 6 with an overdrive 5 speed. I'd probably also run that behind a larger 4.

The 3.91 is a good choice for a pure track car, or for a low torque application.

The lower ratios do increse acceleration but you end up doing so much more shifting, its not worth while in most cases. I think the 3:45 is too tall for anything but a 6 with a sport box which has a 1:1 top gear.

I would guess your diff had been changed at some point, as all my experiance as well as the factory literature states that the ratio was dependant upon the engine size, not the sport package, and that there were no optional ratios, at least in the US.

[Jared at Pelican Parts]

my 325is uses 3.73, so if I install a smaller ratio, it will help with acceleration? Do you know if there is a way to simply change out the ring gears without changing the housing?

[DuckRyder]

I don't know that much about E-30 cars, I suspect I'd stick with a 3.73 there, but maybe a 4:11 from a 91 318iS might be a good start?

E-30 cars run a taller tire so I'll leave it to the E-30 guys to tell you what the best choice for one is.

Yes, you can change the ring and pinion, but you will have to import it from AG as NA thinks they are a "Legal Problem". There are a few suppliers who bring them over though.

[Douge21]

Cheers for your help people, I've decided I'll go for a 3.64:1 LSD. I contacted a place (BM Spares) near where I live and they asked me what ratio I wanted. I would have thought they'd know whats in my car! No worries.

Jared, just wondering, when you said a 320iS diff, you meant an E30 right? Do E30's and E21's have the some mounting configuration? As on the other hand, it may look like the way to go over the 3.64:1.

[Jared at Pelican Parts]

Originally posted by Douge21
Cheers for your help people, I've decided I'll go for a 3.64:1 LSD. I contacted a place (BM Spares) near where I live and they asked me what ratio I wanted. I would have thought they'd know whats in my car! No worries.

Jared, just wondering, when you said a 320iS diff, you meant an E30 right? Do E30's and E21's have the some mounting configuration? As on the other hand, it may look like the way to go over the 3.64:1.

In America, there is no E30 320is. I was referring to the E21, I believe they are called the 318 in Australia.

[e21digest]

Sorry, only an E21 diff is going to fit in there. I and others are pushing 200+hp and are not hurting the diff. Just make sure you put a dose of Redline 75w90 in there.

Brandon
www.e21bmw.net

[e21digest]

DuckRyder and I have had this discussion a number of times. I think perhaps the 3:64 (which I have never run) and the 3.45 are both good choices for a six driven E21. Its a trade off between rpms and shifting points and a little more go in lower gears, and or course what's under the hood.

I have a 2.7L six with a 274 cam and a 3.45:1 LSD. I like the combination for many reasons. I think, like was said that a 3.45 is the ONLY diff to go with a 6 and a sport box. I would think that a 3.45 would be a little high for a stock 323i though.

One other thing to consider is wet traction. I would guess that a 6 driven E21 with a 3.64 diff would be a bit more squirrelly in the low gears.


Brandon
www.e21bmw.net

[Douge21]

"One other thing to consider is wet traction. I would guess that a 6 driven E21 with a 3.64 diff would be a bit more squirrelly in the low gears."


Sounds like that the go then!