M42club.com - Home of the BMW E30/E36 318i/iS

DISCUSSION => Engine + Driveline => Topic started by: jakeb on December 17, 2007, 04:04:48 PM

Title: diff question
Post by: jakeb on December 17, 2007, 04:04:48 PM

Will a 4.10LSD from a 325iX fit a 318is?

Jake

Title: diff question
Post by: gearheadE30 on December 17, 2007, 04:26:20 PM

Yes, but the iX has a viscous LSD, medium case size. The m42 comes with a small case diff and an optional clutch-type LSD. The ix is the only e30 that comes with a viscous diff, and it can be a little jarring and inconsistent on a track. Physically, it should fit, though.

Title: diff question
Post by: jakeb on December 17, 2007, 04:44:59 PM

ohhh...

can you explain the differences a little bit?  This car is used for autocross.

jake

Title: diff question
Post by: gearheadE30 on December 18, 2007, 04:50:48 PM

Sure thing. Clutch-type LSD's have clutch packs that limit the speed difference between the wheels. Most factory BMW LSD's come with a 25% locking ratio. This means that for whatever amount of torque is applied to the wheel spinning more quickly, at least 25% of that amount will be sent to the other wheel (generally the one with more traction). Anything more than about 40%, and you will be able to feel the diff catching when going around a tight corner. The fixed slip difference means that the car will consistently catch in the same place every time. A viscous diff, on the other hand, is a type of torque-sensing limited slip. Basically, the diff is filled with heat-sensitive fluid. As the wheels spin at different speeds, the fluid is heated up, becoming thicker. This is called an increase in viscosity (hence the name). The hotter the fluid, the thicker it gets, and the more of a locking facter you will have. The problem is, in an autocross, if you enter a corner on the gas, the diff will act like an open diff (spinning one wheel) for a short time until the fluid heats up. Eventually, it will heat enough to stop the pinning of that wheel. Because fricion and heat are no longer being generated, the fluid will cool and become thinner. The inside wheel will begin to spin again, and the cycle will start over.

Hope this helps. it is kind of hard to explain over the internet ;)

EDIT: just found this, and it happens to be BMW specific. Good idea of how clutch-type diffs work:http://members.aol.com/agspeed/LSD-lock.htm (http://members.aol.com/agspeed/LSD-lock.htm)

Title: diff question
Post by: jakeb on December 18, 2007, 09:35:37 PM

yes that makes sense, thanks!!

Title: diff question
Post by: Weeman on December 21, 2007, 01:41:25 PM

To follow up on this, as i also am in need of a new diff:

Do Z3/MCoupe diffs physically fit the E30 318iS? What mods (such as output flange change) are needed?

Do E30 325i LSD or Automatic LSDs fit the E30 318iS? Any flange changes required as well?

Thanks guys,

Lincoln

Title: diff question
Post by: WarrenHBrown on December 21, 2007, 06:24:45 PM

I thought the medium case required a different cross member than the small case 318i units.  This may be totally incorrect but I have seen it several times.

I think any (E30) 325i, M3  and Z3 use a meduim case not sure about the M Coupe.

Warren

Title: diff question
Post by: tjts1 on December 22, 2007, 12:05:57 AM

Both the Z3 and 318ti used the E30 rear suspension and differential. This article has a nice outline of what involved in going from one to another.
http://www.318ti.org/notebook/diff_conversion/index.html