Originally Posted by AYBABTU
FYI - looked at the door code - 9H
So it's a 9" LS
So with a stock limited slip, what can I expect?
Worth buying a locker or similar product?
Depends on how worn the limit slip is, but even new they aren't perfect.
H9 axle code is the same as mine, it means 3.55 ration limited slip. It doesn't say anything about which axle housing it is. If it has the 5.4L like mine, its the 9.75" housing. V6 and some 4.6L are 8.8. Ford hasn't put 9" rear ends into new vehicles for quite some time.
Limited slip differentials (posi-trak being the GM brand name for it) don't send power to the wheel with the most traction like someone said above. The factory limited slip works by using spring pressure and clutch packs to establish a minimum torque threshold between the rear tires before it will let one tire spin relative to the other. If both tires are on similar traction surfaces, both will spin if pushed past the limit of adhesion (this is how you get nice two wheel burn outs). If one side is on a slick surface (ice) and the other is grippy (asphalt) and this traction difference causes a high enough torque difference, the wheel on the slick surface with the least traction will
spin by itself. While this is happening, you are rapidly wearing out the clutch packs in the differential, so don't keep your foot planted, let off and try agian with less loud pedal.
Limited slips, like all differentials (excluding spools), have to allow the rear wheels to rotate at different speeds when turning corners. If they don't, one tire will hop and screetch around the corner while causing driveline bind on pavement. The spring pressure and clutch type determine the torque threshold mentioned above and how easily the truck turns corners. Because the factory designs the truck to be driven mostly on the street, so this threshold is realtively low. This means off-road where you are likely to encounter different traction side to side on the truck the factory limited slip will give up and let one wheel spin fairly easily, but no where near as easily as the "zero" torque difference of an open differential. Aftermarket LSDs use stronger springs and "grippier" clutches to raise the torque threshold and provide better off road performance at the possible expense of increased turning effort.
This is different then lockers which physically lock the rear wheels together. Automatic lockers (like the detroit true track) use some mechanism to sense the torque difference between the wheels, unlock at the appropriate threshold and allow differential action, such as when going around a corner. The Detroit Locker uses a "ratchet" mechanism with a spring and ramped locking teeth for this. Selectable lockers (like the ARB air locker) operate in the opposite way. They remain unlocked and act like an open differential until the driver locks them in. At this point both wheels turn at the same speed and are given the same torque, just like a spool. Spools are always locked and always send the same torque to each wheel but won't let them rotate at different speeds. They are for off-road or drag strip use only.
Notice none of these systems can "sense" which tire has more traction and send it more torque. For that you either need an electronic, ABS driven traction control system which brakes the spinning tire, forcing torque over to the other one, or possibly a torsen gear style differential. Don't ask me how those work, its been a while sense I studied them.
For your situation, go slow and if you notice you are lighting up one wheel a lot, consider switching to an after market LSD.