Diffs.. What to do?

[spookypt]

Hi all, just looking for some suggestions.

After my 2nd Diff failure in 3 Nats events (diff, clutch, diff YAWN) its time for a serious change towards reliability. Having discussed at length with BDR USA it seems the race cars are putting out too much for the little beemer mid case diffs to handle. I understand BDR USA have had a failure recently also. I have 3 options currently, none of which are all that great.

First off, for those of you unaware my rear is essentially a BMW M3 E36 rear end, centre and cradle. All the pick up points are exactly as per BMW. All Backdrafts essentially run this set up. The perfect ratio for my car based on most of the tracks we run at is 4.27. I know this is short but due to such a tall first gear in the Jerico 4sp box anything longer and I fry clutches. Most will have witnessed how its hard to idle around the pits. In fact it doesn't idle full stop. The problem is the shorter ratio you go the weaker in effect they get and I have a pinion gear with no teeth left due to it's size for racing not being up to the task.

My options so far are....

1. Rebuild another mid case diff with 4.27 ratio and have another in the trailer for swapping every 3 meets. Crap idea I know.

2. Do a subframe conversion to accept the Ford 8.8 diff. There is a company in the USA called Vorshlag who make a retro fit kit . They put LS V8's into E36 BMW's. The problem with this conversion is new 1/2 shafts, drive shaft mods etc etc and you can only source 4.30 ratio's which is close enough I guess.

3. These guys Home Page do a conversion kit where they modify the cradle of the BMW mid case diff to suit the larger Beemer large case diff. These centres have a 210mm R&P and are set up to run into the 800HP range. Again, I'd need new 1/2 shafts etc etc but on the down side the larger diff centres used in the BMW 750il V8 and V12 BMW sedans don't come in a 4.27 ratio. Only the 4.10.

Its a bit of a chicken and the egg scenario. Some will say look at your GB selection too as that tall first gear is killing it. Currently at WP Park I wind through the gears 1st to 4th 3 times during one lap.

It all needs sorting out but I'm not sure what the best way to go is without almost ripping out and starting again from scratch!

Any thoughts, advice is appreciated.

Spookypt

[bobcowan]

To get a stronger diff, you'll end up with new axles. So count on doing that no matter what you do.

The Ford 8.8 is readily available (in the US), cheap, and heavy duty parts are everywhere. You can't get a 4.27 ratio, but you can get 4.30. Heavy duty axles are available in 28 and 31 spline flavors. You can also get worm gears from Auburn or Torsen.

I don't enough about BMW parts to speak of.

[Aussie Mike]

Just looking at a few pics of E38 diffs and I reckon the 8.8 conversion wouldn't be too hard. Does your chassis use the BMW bolt in sub-frame or dedicated mounts on the chassis for the diff?

If you can drop the sub-frame out it would be pretty straightforward to fabricate mounts to take the 8.8 diff.

You would probably need to look at axles anyway because once you've replaced the diff then the next weakest link will fail. However depending on the diff dimensions you may get lucky and the stock axles bolt up especially if they use the 6 bolt CV's like the Porsche and Holden rear ends use (Standard bolt pattern)

The 8.8 diff is a good unit and well supported by the aftermarket. Lot's of choices for gears and LSD options. Its probably lighter than the BMW diff to with it's aluminum case.

If you lived closer I'd be happy to mod the cradle for you.

Cheers

[Aussie Mike]

Scotty would be the man to do the fabrication work local to you. He welds like a demon.

[eschaider]

Spooky,

Check out these diffs from Wavetrac, click here => Wavetrac BMW. These are torque biasing all gear differentials.

There are a number of torque biasing diffs that are commercially available. Most have six or eight pinions. These guys use twelve 9310 steel pinions and have a transferrable limited lifetime warranty. They also use a patented wavetrac mechanism that allows the diff to dynamically apply biasing load to the pinions proportional to the traction available at the tire.

With their billet and forged steel construction they are among the strongest diffs you can buy as is attested to by their transferrable limited lifetime warranty. Interestingly the warranty applies to both street and race track use — sort of gives a peek into how robust the mechanisms are. Click the little camera in the BMW 188 axle listing for a pic.

I think you will find them to be quite attractive.


Ed

[spookypt]

Mike, here's a pic. Its the whole subframe that effectively bolts into welded pick up points on the chassis.

[spookypt]

Originally Posted by eschaider

Spooky,

Check out these diffs from Wavetrac, click here => Wavetrac BMW. These are torque biasing all gear differentials.

There are a number of torque biasing diffs that are commercially available. Most have six or eight pinions. These guys use twelve 9310 steel pinions and have a transferrable limited lifetime warranty. They also use a patented wavetrac mechanism that allows the diff to dynamically apply biasing load to the pinions proportional to the traction available at the tire.

With their billet and forged steel construction they are among the strongest diffs you can buy as is attested to by their transferrable limited lifetime warranty. Interestingly the warranty applies to both street and race track use — sort of gives a peek into how robust the mechanisms are. Click the little camera in the BMW 188 axle listing for a pic.

I think you will find them to be quite attractive.


Ed

Thanks Ed. That's very interesting and that's why I love this forum.

[mongrel]

dutchman diff or curry diff both irs 9in or 8.8

[spookypt]

Thanks Mark. I'm not familiar with the first you mentioned but I've pre-ordered ex USA an alloy 8.8 ford irs centre conversion for the m3 cradle with ford/BMW 1/2 shafts but won't get them till April next year as it's still in proto/testing stage so in the meantime I've got a new 4.27 and spare 4.27 centers all made up.

Argh Motorsport....

[Towmaster]

Hopefully I won't have too many problems with the 9" in the Supercar.
One of the things I looked at when buying a race car.... Bullet proof driveline.... Pity the bloody engine live up to the bullet proof bit....

#aarghmotorsport

[750hp]

Mark, what's in the back of yours now? I can't imagine the Jag stuff would be up to the task.

[J.Jensen]

There are "loads" of turbo converted M3 and 5s over here with anywhere from 600 to 1000 and above hp. They seem to manage with BMW diffs. The last one I saw had wheel slip when it passed 800hp/600lbft. They used an E28 745i diff. Others use the alpina turbo diffs.

The M5s are more used than the M3s for the 1k builds, maybe the M5 diff from the E34 fits straight into your frame?

[spookypt]

I'm no officianado but pretty sure the E28 M5,E24 M6 & E23 735i are large case diffs with 210mm r&p and I know for certain will not bolt into the mid case diff cradle.

[Tom Wells]

spookypt,

I'm going through this on a 2003 Mustang Cobra IRS in a (drum roll) 1981 Mercury Zephyr station wagon with a 557 that is intended for open track use.

After reading a bunch about problems on the track with overheating I concluded it needs an oil cooler which I'm in the process of installing. At the same time, I've installed the newly available Ford differential cover: 1999-2004 COBRA IRS AXLE GIRDLE COVER | Part Details for M-4033-G3 | Ford Racing Performance Parts

It has two major upgrades: first, it's waaaay stronger and has diff bearing support caps, and not surprisingly it has all the holes needed to add an oil cooler.

Here's a sample of how some correct the problem (I think this one is pre-new-cover): IRS Differential Cooler:

In your shoes, I might try a cooler first, then if another failure occurs, go to the Ford IRS diff. Don't scrimp on the size of the cooler.

One theory is that the straight axle holds more fluid and has much greater cooling area if you include the axle tubes.

The IRS is kind of shrouded and has only the flexy aluminum diff to get rid of heat. The track guys say the least that happens is that the diff fluid boils and then the limited slip gets cooked. Don't know what might happen after that...

My Cobra has a Jag rear, a very large cast iron chunk with inboard disk brakes and has survived many 30 minute track sessions just fine. Newer is not always better!

Just stuff to think about,

Tom

[Jac Mac]

One of my fellow GT40s members 'GT Bob' posted this compare pic of the BMW units.... but what you 'really' need is a housing which allows 9" heads, few outfits have/do make em in OZ & USA, but not in large numbers hence price is high, I built one, but don't do them on a commercial basis, I simply don't have the time.
With regard to a cooler I could understand that a 'clutch' type LSD might be warrant them due to the heat buildup, but with tru trac or other detroit lockers some means of feeding oil to the R&P contact area is probably more beneficial, but that's more of a driver call as to which unit he prefers.
Other factor that comes into play is the amount of hypoid offset, Ford 8.8 has similar to the BMW unit, 9" has about 0.75" more which can be handy if you want to get overall car height down.

[spookypt]

Thanks JacMac. Very interesting pics. My Mid Case has a different set up to that with a large bolt holding the pinion end to the cradle as per pics below rather than that bolt on bracket assembly.

I also run a cooler so have that issue , if any exists, covered.

[T.L.]

I run the Ford 8.8IRS set up in my Cobra.

I have the torsen LSD set up in there as well. I also sourced beefier shafts from a mob, as well as a few other bits from memory. They list ratios up to 4.56 and 4.88, about 1/4 of the way down the page headed

8.8 IRS Pumkin WITH 4 1/2 INCH BOLT CIRCLE

Brake Rotors Drilled and Slotted,Hawk Performance Brake Pads,Car Brakes,Brake Drums, htcracing

http://www.discbrakesrus.com/axles.jpg

My cobra actually twisted the frame where the diff was held in and then the bolts pulled through and diff moved up. The thickness of the metal used in this area was 1.6 or so, now 5mm plate from memory and handles 400+kw at the wheels.

[spookypt]

Although binned the crown wheel looked relatively untouched. The same cant quite be said for the pinion. Another expensive paper weight to add to my collection.

[Aussie Mike]

Crunchy crunchy.

You should set up your own altar like Burt Munro did

[spookypt]

Originally Posted by Aussie Mike

Crunchy crunchy.

You should set up your own altar like Burt Munro did

Mike the rate I'm going I'm gonna need a lot more shelves!