Small Case LSD

[flumph]

I have a 1984 325e manual with a B25 swapped in. The factory diff that is in the 'e is a 2.79 open differential. A couple weeks ago, I was at a junkyard picking over a wrecked 318i, and I noticed that it had a 3.93 LSD in it, but it was a small case. I pulled it out and bought it, scraped all the gunk off of it (1/4" layer!), and it's been riding around in the trunk of my GTI since then. I figured that even if I don't use it, it's still probably worth something to someone and it's worth saving from being made into soup cans.

I've done research and it looks like the small case diff should be able to handle pretty much anything a stock B25 will chuck. The small case I have has a way shorter gear, is limited slip, and weighs like 25 pounds less than the factory medium case. I've used the search and found plenty of people swapping from small case to medium case, and it sounds like there's no issue with compatibility, but I never find anyone going the other way around. Is there any good reason why it would be a bad idea to run a small case diff on a stock M20B25?

[TobyB]

Nope, it will work. If you are hard on it, it will not last all that long.

I have run small cases in the 2002 race car (140 ft/lbs, 170 hp) for years,
with quite sticky tires, and they survive ok.

If you can be gentle in 1st gear, you'll be just fine. And yes, they are a LOT lighter!

t

[flumph]

Good to know! I do have an MS3 so launch control is an option... Ok, so it's not totally stock, but I'm not exactly twisting any drive shafts.

I'll do the swap and hang on to my old diff just in case I ever somehow wear out the small case. I'm not chasing power, I just want a small, light, fun car to mob around the city.

I've read about a lot of people using ink to dial in the contact surface between ring and pinion gears, and testing the lockup torque of the clutch packs. I also see that people throw in an extra clutch disc to gain more lockup.

Is checking wear pattern worth it? This would involve taking the cover off of the diff, obviously. The donor car was an '84 and it's in unknown condition. And while I'm in there, I would probably want to put new clutch packs in anyway. But I'm half tempted to just dump some new fluid in it, bolt it up to my car, and go play. Thoughts?

[TobyB]

Go play!

Yes, there are lots of things you COULD do,

but the small- case LSD is not going to overwhelm
you with its ability to lock 'em up in anything but
snow and rain, anyway...

t