What's the Diff?

Bill and I have been discussing the relative merits of rear axle bits, sort of a "Differential Diagnosis," (see what I did there). So today's Bullit University thread relates to the types of differentials or "center sections" as they are often called.
First there is the open differential, characterized by one wheel spinning while the other does not. This was an inexpensive way to produce rear axles and works fine for everyday driving, except in mud, snow or other low traction conditions. The center carrier has no clutches, just side and spider gears which "walk" around each other.
Second type is the limited slip differential, characterized by a clutch system of some kind, discs, cones, worm gears etc. The Bullits use the Ford traction lock diff, which uses spring loaded discs to minimize slippage under power.
Works well when new, requires rebuilding under heavy use. They are about $250 for a whole new section or rebuild kits run about +$100. They can chatter if you don't have the friction modifier added and will show progressive slippage as they wear.
The Torsen differential is a "torque sensing" unit which uses worm gears to tighten up and reduce slippage. They have a torque ratio based on the slipping wheel and can produce about ~2X the torque or 4X the torque depending on the model. Ford uses these in the Boss 302 and Shelby 350 and 500 and apparently the new Bullit. They are more money, about $600-900 depending on the model. Note: Torsen diffs. require frequent fluid changes, every 12 hours of high speed or track use. They can be a little noisy, but with friction modifier added are fine.
There are other limited slip differentials that have good reputations, such as Yukon, Eaton, Auburn etc. All limited slip differentials do not provide 100% lock up, hence the name "limited." (Prices from $350-900+)
The third type of differential is the "locker." People usually think of the "Detroit locker," which is now made by Eaton and can provide 100% lock up under power. They can be noisy and lock with a bit of a bang. Not ideal for street use. A lot of racers like these, as they can help "rotation" through the corners.($700)
And, the last type is the spool. This basically is a solid center section with no differential capability. Used for drag racing primarily or short track racing. 100% drive, all the time.($300)
No matter what type of differential you choose, rebuild time is a good opportunity to change all your seals and bearings, and carefully inspect or replace your axles and wheel studs/nuts.
Also, a good reinforced differential cover is cheap insurance and facilitates easier fluid changes. (about $200)
Differential rebuilding or installation should be done by an experienced pro.
As always, your questions, comments, likes are welcome. Over and out.