small case 4:10 behind an M20

[Jparkr]

With S4:10 medium cases becoming harder and harder to find in the area, I have been kicking around the idea of installing a small case s4:10 behind an M20, for racing purposes. I would like to see a few things.
a. How long would it last compared to a med case.
b. What would be the failure? Clutch packs; gears; or maybe the case itself?
c. Would the smaller components still be able to deliver the power identically to the medium cased counterparts.

Before I waist time testing this, I wanted to see if anyone had real world experience on the subject.

and please, I don't want this thread to turn into a, "I heard on the internet that it won't work, no point in trying" argument. Lets keep it productive.

So. Anyone have any thoughts or experiences?

[Jparkr]

Ill start with how this notion popped into my brain. I have a E36 318ti, It has a open 4.10 small case in it currently. The car also has an S52. While all it seems to be good for is liquifying one of my rear tires, I have not been gentle to that diff in any sense of the word.
It has been 2 months of abuse and the diff still functions without whining noise or any type of failure.
I know a Limited slip will be significantly more pron to wear and tear, especially behind an S52, But it got me wondering could it take a healthy 170 hp M20 for a season (or even half a season).

[Emre]

a. How long would it last compared to a med case.

Not long at all. On the street, you should be OK. But it won't last long under track conditions.

b. What would be the failure? Clutch packs; gears; or maybe the case itself?

In my case, both the bearings and the clutches went bad. This was despite being very, very careful about fluid changes.

c. Would the smaller components still be able to deliver the power identically to the medium cased counterparts.

That didn't seem to be a problem. The only issue was longevitiy.

Before I waist time testing this, I wanted to see if anyone had real world experience on the subject.

I swapped an M20B25 into my '91 318is and kept the stock small-case diff. It died very quickly. We swapped in a medium-case diff from an E30 M3 and it's been going strong for years.

You wouldn't think it would make such a huge difference, but it was night-and-day in terms of durability under track conditions.

[SkiFree]

And article I put together some time ago which you might find helpful....

http://www.bmw2002faq.com/component/option,com_forum/Itemid,50/page,viewtopic/p,1003733/#1003733

I will agree that under race conditions, you'll be hard pressed for longevity. However as shown in the link, there are ways to minimize that risk (one being not to use a E30 318 center section of unknowable health).

[robgoesvroom]

I'd like to know how you track guys feel about a 4:10 LSD paired with a stock M20. I've been running this 3.73 for a couple years now, but I always hear that the 4:10 works best with the M20. I'm running on socal tracks and 15" wheels if that helps

[jlevie]

I'd like to know how you track guys feel about a 4:10 LSD paired with a stock M20. I've been running this 3.73 for a couple years now, but I always hear that the 4:10 works best with the M20. I'm running on socal tracks and 15" wheels if that helps

That rather depends on the track(s) you are running. A really tight track with lots of slow speed corner would be a good match for the higher ratio. But tracks with few low speed corners and long straights would be a better match to a lower ratio.

[Snowmann]

Not long at all. On the street, you should be OK. But it won't last long under track conditions.

In my case, both the bearings and the clutches went bad. This was despite being very, very careful about fluid changes.

That didn't seem to be a problem. The only issue was longevitiy.

I swapped an M20B25 into my '91 318is and kept the stock small-case diff. It died very quickly. We swapped in a medium-case diff from an E30 M3 and it's been going strong for years.

You wouldn't think it would make such a huge difference, but it was night-and-day in terms of durability under track conditions.

I disagree. I am on a summer of street driving and going on season 4 racing my IT car. No issues to date. I change fluid twice a swason. I have logged over 1800 track miles to date.

[djjerme]

The biggest life shortener to diffs is off the line launches; I sent many GM 8.2 and 7.5 rearends in to orbit during my drag racing years.

With the amount of hp/torque our spec classes are limited to, and the fact that Conference doesn't use standing starts like NASA, I would think a small case would fair a bit better. As long as regular maintenance is done and new clutch packs/bearings are installed, and a diff cooler ran - it shouldn't be an issue to keep the small case together John. I am not trying to overrun those who have experience, but if it where me, I would consider the small case before shelling out big bucks for a medium case..

Just my 2 cents - you can tell me I am full of Sh*t at the season opener.. :)

[dashboardmonkey]

John, I have an s4.10 med case if you wanna work something out. 541-653-4895

I'm not asking big bucks, just want a straight trade for a taller ratio for my dd.

[Emre]

I disagree. I am on a summer of street driving and going on season 4 racing my IT car. No issues to date. I change fluid twice a swason. I have logged over 1800 track miles to date.

You're lucky. Everyone I know who has run a small case diff has had trouble.

I was doing 25-30 track days per summer, which comes out to around 100-120 on-track sessions per season. I was driving to events, so several thousand street miles as well. With TMS J-Stock suspension and Toyo RA1's, the diff was getting a workout. The small case barely made it through two seasons before the bearings and clutches were gone. The medium case lasted 4 seasons with nothing more than a fluid flush at the start of each summer.

[djjerme]

John, if you work something out with DashboardMonkey, I can always run over and grab it for you, and when you come back down again (and we meet up for Taco's and Beer) I can bring it with..

:)

Unless you con Ben in to bringing it with him to the Ridge.

[Jparkr]

I talked to Andy, but he needs a trade since he is still running his 4.10. I only have a s2.93 to offer and that is not what he is looking for. Thanks though Jeremy, are you coming to the ridge?

Thanks for all the info everyone, Emre. Your second post seems to conflict your first one. If you ran a small case though that many lapping days plus street driving, and it lasted 2 seasons. I Think that is great longevity.

[Emre]

Thanks for all the info everyone, Emre. Your second post seems to conflict your first one. If you ran a small case though that many lapping days plus street driving, and it lasted 2 seasons. I Think that is great longevity.

The second season, it was basically an open diff with enough slop that if you jacked up the rear so that one side was on the ground, you could turn the other wheel a significant amount by hand. Really, it only lasted one season as a functioning limited-slip diff. The second season, it was already toasted.

The medium-case diff has lasted 4 seasons of constant use and is still as good as new. One season vs. 4 seasons (and counting) is a significant difference in longevity IMNSHO.

If you already had a small-case diff in the car (as I did), then I can see why you'd just run it until it completely sh1ts the bed. But I can't understand going out of your way to buy one (and then spending the time and money to have it freshened up and installed) when you know it's not going to last very long. Pay the extra couple of hundred bucks for the real thing.

[djjerme]

No John, won't be up at the ridge. Car still needs new motor (again) and I have too much baby related stuff to work on this weekend.. Really wish I could though, sounds like it is going to be an awesome weekend.

[Keith M]

John- Steven has one for sale. I could bring it up with me to The Ridge.