3.91 vs 3.64 diff

[Murt97]

I’m swapping a low compression 1989 m20b25 in to my 1991 318i convertible, I just wanted to know if the 3.91 open diff from a 1995 320i would work well with my car? (I have been offered the diff for a reasonable price) OR shall i hold out for a more suitable ratio? I have heard that the 3.64 is also an option.

Car will be for weekends and Cruising around town and I don’t think I’d push it any more than 65-70mph on the odd motor way trip.

Thanks all.

[paultv]

I had a 3.73 LSD in my Cabby - and whilst it would pull like the wind, I found 1st almost useless - you run out of revs at such a low speed - and I also found that cruising was a bit painful - revving a bit high at cruising speeds.

Last year I fitted a 3.46 open... and it feels a whole lot better - acceleration is still sharp - and pottering along on the motorway at 80mph is much more relaxing...and the gearing just feels better to me.

I drive a lot of long distances in Europe, so it suits me - I'm not usually tearing up the rubber in the local supermarket car park.

I've got the LSD lurking in case I feel the need for a more aggressive and noisy experience at some point in the future. Also the economy has improved a lot!

...and you will squirt the M20B25 because it will and it can and its fun!!!

check here: https://e30goodies.com/gearing-calculator/

input your possible diff ratios, tyre size and select the 325 getrag 260 ratios.

Paul

[Murt97]

I had a 3.73 LSD in my Cabby - and whilst it would pull like the wind, I found 1st almost useless - you run out of revs at such a low speed - and I also found that cruising was a bit painful - revving a bit high at cruising speeds.

Last year I fitted a 3.46 open... and it feels a whole lot better - acceleration is still sharp - and pottering along on the motorway at 80mph is much more relaxing...and the gearing just feels better to me.

I drive a lot of long distances in Europe, so it suits me - I'm not usually tearing up the rubber in the local supermarket car park.

I've got the LSD lurking in case I feel the need for a more aggressive and noisy experience at some point in the future. Also the economy has improved a lot!

...and you will squirt the M20B25 because it will and it can and its fun!!!

check here: https://e30goodies.com/gearing-calculator/

input your possible diff ratios, tyre size and select the 325 getrag 260 ratios.

Paul

That’s what I’m worried about, I don’t want the car to be too crazy. That being said e30 diffs are so hard to come by, especially cheaply.

So I’m thinking of getting the 3.91 diff and hoping for the best it’s £200 but from a 1985 320i. The seller is convinced it’s a medium case diff but i’m not sure

[paultv]

The small case diff has 6 mount points for the rear cover whilst the medium has 8 - so its easy to check.

For me a 3.91 would be too short with an M20 B25 - but opinions will differ... depends if you like high revs at cruising speeds.

Paul

[Murt97]

The small case diff has 6 mount points for the rear cover whilst the medium has 8 - so its easy to check.

For me a 3.91 would be too short with an M20 B25 - but opinions will differ... depends if you like high revs at cruising speeds.

Paul

Ahhhh it has 6 mounting points! So’ it’s a small case then! I wonder... could I still use a small ccae diff for a 325i engine, box an prop..?

[paultv]

I'm pretty sure it's not up to the job, do a google search and see if anyone risked it !!

It's physically narrower than the 188 so your drive shafts would probably sheer and the internals are smaller so can't be swapped.



Paul

[JungleGus]

The small case diffs work (with the appropriate half shafts) but it's more the longevity when dealing with the torque. If you're just cruising around then it's probably fine as long as you drive smoothly - but IMO I'd stick with the 3.64. I haven't driven an M20 for a while, but I recall that gearing being spot on - I'm using a ZF box with 1:1 5th (vs 4th on an M20 box) and am going from a 3.23 to a 2.93 in an effort to get slightly lower revs on the motorway and make better use of the torque. I would ROUGHLY equate that to going from a 3.7x to a 3.4x type diff...but I haven't bothered checking the maths so use with caution!

[Murt97]

The small case diffs work (with the appropriate half shafts) but it's more the longevity when dealing with the torque. If you're just cruising around then it's probably fine as long as you drive smoothly - but IMO I'd stick with the 3.64. I haven't driven an M20 for a while, but I recall that gearing being spot on - I'm using a ZF box with 1:1 5th (vs 4th on an M20 box) and am going from a 3.23 to a 2.93 in an effort to get slightly lower revs on the motorway and make better use of the torque. I would ROUGHLY equate that to going from a 3.7x to a 3.4x type diff...but I haven't bothered checking the maths so use with caution!

Thanks for your advice guys! I’ll try and find a nice 3.64 medium case diff then. Any tips on finding one? It’s the last major part that I need for my conversion.

One more question... can I use my drive shafts from my 318i cabrio? Will they bolt up to the medium case diff? Or will I need 325i drive shafts? (I’m talking about the two shafts on the right and left of the differential not the propshaft) I get confused between the nomenclature sometimes.

[paultv]

Loads of them here in Germany - but I guess Brexit - Balls makes it problematic to purchase from the EU....

an example:

https://www.ebay-kleinanzeigen.de/s-anz ... 1-223-8511

actually this one is pick up only - but you get the idea... Hit the search button -"Finden" and you'll get a cross section of 188's


Paul

[Lemon98]

It all depends on which gearbox you're using. The one from the 318 or the M20B25?

[Murt97]

It all depends on which gearbox you're using. The one from the 318 or the M20B25?

I’m using the 325i gearbox, getrag 260.

[Lemon98]

It all depends on which gearbox you're using. The one from the 318 or the M20B25?

I’m using the 325i gearbox, getrag 260.

Then the taller final drive will be better I think. My 320i has a 4.10 final drive but a G240 so it works alright and I'm very happy with it.

I think there's a table somewhere on the wiki which lists all of the individual gear ratios for each box and the factory final drive.

[steve_k]

It all depends on which gearbox you're using. The one from the 318 or the M20B25?

I’m using the 325i gearbox, getrag 260.

Then the taller final drive will be better I think. My 320i has a 4.10 final drive but a G240 so it works alright and I'm very happy with it.

I think there's a table somewhere on the wiki which lists all of the individual gear ratios for each box and the factory final drive.

you mean this one?

https://www.e30zone.net/e30wiki/index.p ... Drivetrain

[Lemon98]

I’m using the 325i gearbox, getrag 260.

Then the taller final drive will be better I think. My 320i has a 4.10 final drive but a G240 so it works alright and I'm very happy with it.

I think there's a table somewhere on the wiki which lists all of the individual gear ratios for each box and the factory final drive.

you mean this one?

https://www.e30zone.net/e30wiki/index.p ... Drivetrain

Yeah that's the one

[Murt97]

Thanks for all the help guys. Does anyone know if I have to use 325i driveshafts to match a 325i medium case diff? Or can I use the same driveshafts that are currently on my 318 cab?

[Lemon98]

Thanks for all the help guys. Does anyone know if I have to use 325i driveshafts to match a 325i medium case diff? Or can I use the same driveshafts that are currently on my 318 cab?

I haven't done a swap like this or anything but I remember reading somewhere that you can use the same driveshafts as the medium case is the same width, but has a longer snout so it's the propshaft that might be the wrong length but driveshafts should be fine.

But as I said I have no personal experiences, just from info I read on the wiki I think

[Murt97]

Thanks for all the help guys. Does anyone know if I have to use 325i driveshafts to match a 325i medium case diff? Or can I use the same driveshafts that are currently on my 318 cab?

I haven't done a swap like this or anything but I remember reading somewhere that you can use the same driveshafts as the medium case is the same width, but has a longer snout so it's the propshaft that might be the wrong length but driveshafts should be fine.

But as I said I have no personal experiences, just from info I read on the wiki I think

Yeah I have the propshaft coming In soon, so that’s all good. Just trying to figure out how to go about the driveshafts as the 325i ones are a beefier. I’ll use my 318i ones and see how it goes lol.