Apr 5, 2019 update to the BMW 318is project car

Project BMW 318is: Installing a Limited Slip

The limited slip differential in Rennie’s hand looks decidedly different—mostly because of the clutch pack—than the one inside our old differential.
Cleaned and remounted in the rear subframe, our limited slip differential now has a Powerflex mounting bushing installed.

As you probably know, a limited slip differential causes both rear wheels to power a car out of a corner, as opposed to an ‘open’ differential that just lets the wheel with the least resistance (inside wheel) try and do the work of pushing the car out of a corner. And as you must know, an open differential is not what you want in a performance car.

One of the options available in `91 (and `91 only) for the E30 318is is a factory limited slip differential. As we stated a few updates ago, ours sadly wasn’t ordered with it. If we wanted to ruffle the feathers of an E30 M3 we would need to find one, make sure it was in good condition and then install it in place of our open differential. Easy, right?

Unfortunately, the 318is uses what is called ‘the small-case differential’ and it cannot be easily switched with a larger case differential from any of the six-cylinder E30 BMWs. Bummer. So, we would need to find a unicorn: a small-case limited slip differential from another `91 318is or—even rarer—a 318 sedan in either E30 or E36 variety.

Thankfully our friend Al Taylor, from Al Taylor Sports Cars, had the differential we needed and sold it to us for the gift price of only $400. Retail price on this differential is probably more than twice that figure, especially as these cars get older, rarer and more valuable.

With our prized possession in hand, our buddy Rennie Bryant of Redline Performance then came to the shop to show us how to check and re-shim one of these differentials. While Rennie was disassembling the differential, he discovered that it had already been rebuilt. Score! He quickly put it back together and we installed it in our 318is rear subframe that we still had laying out on the bench. We used the Powerflex rear differential mount, as it is stronger, less likely to tear out and keeps the differential more stationary in hard cornering.

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300zxfreak
300zxfreak New Reader
4/5/19 12:10 p.m.

I’ll be damned if I know how all you Bimmer guys keep all the iterations of them straight, unless you all have one huge cheatsheet. I have a ‘16 328i, and all I know is that it’s an E-something.......I think.

300zxfreak
300zxfreak New Reader
4/5/19 12:15 p.m.

Almost forgot, does anyone out there have experience with the Powerflex bushings? I’m considering them for my ‘90 Z TT, and I like the fact that they have several different durometer products available so I don’t have to live with the harsh Racer Boy units on the street.

DesktopDave
DesktopDave New Reader
4/5/19 12:34 p.m.

What's not easy about swapping a six-cyl limited-slip diff? The Typ 188 "medium case" bolts right in, just needs a different rear cover. Those covers are still available, brand-new, from BMW last time I checked. You can find a wide variety of ratios in many E28, later E24 & E23, most E30. I'm told a few E30 iX were equipped with the viscous-type, and there are a few Z3 fitted with torsen types. I'm running an '00 Z3 diff in my E24 right now (well, not as I'm typing this).

z31maniac
z31maniac MegaDork
4/5/19 12:45 p.m.

Why swap in a larger, heavier diff when your power level doesn't require it? 

z31maniac
z31maniac MegaDork
4/5/19 12:45 p.m.
300zxfreak said:

I’ll be damned if I know how all you Bimmer guys keep all the iterations of them straight, unless you all have one huge cheatsheet. I have a ‘16 328i, and all I know is that it’s an E-something.......I think.

F30, they moved onto the F designation about 11 years ago. In case your curious....

https://store.activeautowerke.com/pages/bmw-chassis-codes-chart

DesktopDave
DesktopDave New Reader
4/5/19 1:03 p.m.

In reply to z31maniac :

Is this car going to be used in parades? My guess is no...and the 'small' differential is a common point of failure on these lovely cars. Mine had had two replacements, and it's never seen a track. Other advantages of the 188mm are the large availability of ratios and a far larger selection of good used units. Furthermore, the weight penalty is very small, I believe it's twenty pounds?

chuckbaader
chuckbaader New Reader
4/5/19 1:58 p.m.

There is a reason the M3 had a medium case diff. If you are making more power/torque than the M3, you need to upgrade to the medium case. Its a bolt in. In my ITA E30, I used a complete 318 rear subframe/axles/brakes with a medium case diff. 

stuart in mn
stuart in mn MegaDork
4/5/19 3:22 p.m.
DesktopDave said:

What's not easy about swapping a six-cyl limited-slip diff? The Typ 188 "medium case" bolts right in, just needs a different rear cover. Those covers are still available, brand-new, from BMW last time I checked. You can find a wide variety of ratios in many E28, later E24 & E23, most E30. I'm told a few E30 iX were equipped with the viscous-type, and there are a few Z3 fitted with torsen types. I'm running an '00 Z3 diff in my E24 right now (well, not as I'm typing this).

All iX cars had a viscous type differential as far as I know (I've owned three of them).  They were only available in the US with 3.91 gears; realOEM shows there were a couple different lower numerically gear ratios available for euro iX cars, but I doubt any of them are still available.

There was also a "large case" differential used in some 5-, 6- and 7-series cars but I don't know if they can be fitted into a 3-series car.

maj75
maj75 HalfDork
4/6/19 9:35 a.m.

It’s simple if you swap the rear subframe from a 6 cylinder car.  I see no upside to going to the trouble of adding limited slip to the small diff.  It’s like putting a viscous limited slip into a NA Miata.

irish44j
irish44j MegaDork
4/6/19 4:41 p.m.
Tim Suddard said:

Unfortunately, the 318is uses what is called ‘the small-case differential’ and it cannot be easily switched with a larger case differential from any of the six-cylinder E30 BMWs.

huh? My 318 originally had a small case, and I have two small case diffs (3.91 original to my car and 4.10 from a '91 318is) and two medium-case (325i) diffs (3.73 and 4.10). They are all a direct swap with no modifications of any sort- all flanges bolt up directly, all mounting points bolt up directly. Just loosen the driveshaft slip-joint collar to make it the correct length since the input flange is a few mm different. 

irish44j
irish44j MegaDork
4/6/19 4:48 p.m.
DesktopDave said:

In reply to z31maniac :

Is this car going to be used in parades? My guess is no...and the 'small' differential is a common point of failure on these lovely cars. Mine had had two replacements, and it's never seen a track. Other advantages of the 188mm are the large availability of ratios and a far larger selection of good used units. Furthermore, the weight penalty is very small, I believe it's twenty pounds?

IDK I certainly wouldn't call it a "common" point of failure. My 1985 small-case LSD was original to the car and to my knowledge never opened when i got the car back in about 2011. Then I ran about 5-6 seasons of rallycross/rally with it and eventually toasted the plates (but replaced them). We usually use the 188's because of ratio availability, but the 3.91 and 4.10 were avialable in e30 small-case diffs, which are nice ratios for certain uses. 

Having just moved all of my diffs across the garage today, I'd be really suprised if the difference is as much as 20lbs. Either way, a few lbs extra low in the car aligned with the rear axles is pretty much the best possible place to add weight I'd say. 

Knurled.
Knurled. MegaDork
4/6/19 5:01 p.m.
Tim Suddard said:an ‘open’ differential that just lets the wheel with the least resistance (inside wheel) try and do the work of pushing the car out of a corner.

 

twitch

 chuckbaader said: There is a reason the M3 had a medium case diff.

 

..It was a Group A homologation special, back when "homologation" meant they had to make/sell a couple thousand street versions and the competition cars could not deviate significantly from that.  If they felt that they needed the larger rear to win, they needed to put it in the homologation model.

 

If they were messing with final drive ratios (can't remember if it was an allowed varaiance or not), that very much will enjoy a larger rearend, because the only way to make a shorter gear (lower gear/higher ratio, however you call it) with a fixed diameter ring gear is to have a smaller pinion gear.  When geometry is fixed like that, the pinion can get so small that things get weak fast.   So I guess the question may not be if you plan on making M3 power, but if you plan on making competition M3 power (250-300+hp) and running 4.75:1 (example) gears?

z31maniac
z31maniac MegaDork
4/7/19 7:28 a.m.
irish44j said:
DesktopDave said:

In reply to z31maniac :

Is this car going to be used in parades? My guess is no...and the 'small' differential is a common point of failure on these lovely cars. Mine had had two replacements, and it's never seen a track. Other advantages of the 188mm are the large availability of ratios and a far larger selection of good used units. Furthermore, the weight penalty is very small, I believe it's twenty pounds?

IDK I certainly wouldn't call it a "common" point of failure. My 1985 small-case LSD was original to the car and to my knowledge never opened when i got the car back in about 2011. Then I ran about 5-6 seasons of rallycross/rally with it and eventually toasted the plates (but replaced them). We usually use the 188's because of ratio availability, but the 3.91 and 4.10 were avialable in e30 small-case diffs, which are nice ratios for certain uses. 

Having just moved all of my diffs across the garage today, I'd be really suprised if the difference is as much as 20lbs. Either way, a few lbs extra low in the car aligned with the rear axles is pretty much the best possible place to add weight I'd say. 

I know it isn't, I've owned multiple E30s. I've never heard of anyone destroying multiple small case diffs in the E30 community, especially people that street drive.

 

I'm with you.

 

I think this may be akin to "these Subaru transmissions are glass" also please ignore that I did 4k clutch drops at every red light.

Knurled.
Knurled. MegaDork
4/7/19 7:35 a.m.

In reply to z31maniac :

The Subaru transmissions could reliably break things on rough roads (like stage rallies) even with nonturbo engines.  Way more rad than just burning up the clutch on the street wink

Tim Suddard
Tim Suddard Publisher
4/7/19 9:21 a.m.

I have not heard of problems with the small case differentials.

And no matter what you do with an M42, it is not going to be a torque monster that readily breaks stuff.

It does depend on how you drive and how you use the car. We also know that these cars are a bit underpowered and saving every ounce of weight would be our key to success on this project.

irish44j
irish44j MegaDork
4/7/19 11:41 a.m.
z31maniac said:
irish44j said:
DesktopDave said:

In reply to z31maniac :

Is this car going to be used in parades? My guess is no...and the 'small' differential is a common point of failure on these lovely cars. Mine had had two replacements, and it's never seen a track. Other advantages of the 188mm are the large availability of ratios and a far larger selection of good used units. Furthermore, the weight penalty is very small, I believe it's twenty pounds?

IDK I certainly wouldn't call it a "common" point of failure. My 1985 small-case LSD was original to the car and to my knowledge never opened when i got the car back in about 2011. Then I ran about 5-6 seasons of rallycross/rally with it and eventually toasted the plates (but replaced them). We usually use the 188's because of ratio availability, but the 3.91 and 4.10 were avialable in e30 small-case diffs, which are nice ratios for certain uses. 

Having just moved all of my diffs across the garage today, I'd be really suprised if the difference is as much as 20lbs. Either way, a few lbs extra low in the car aligned with the rear axles is pretty much the best possible place to add weight I'd say. 

I know it isn't, I've owned multiple E30s. I've never heard of anyone destroying multiple small case diffs in the E30 community, especially people that street drive.

 

I'm with you.

 

I think this may be akin to "these Subaru transmissions are glass" also please ignore that I did 4k clutch drops at every red light.

yeah I'm one of those people who managed to get a hundred seventy thousand miles out of my WRX, tuned and driven hard, and had zero transmission problems lol. Hell my original clutch was still good at 120000 when I swapped it out

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