I guess that I should have been a little more clear in saying that you need a tight diff. There is a diffrence between having the diff effect tight and the spur gear not move under acceleration. The latter is what we are looking for. It is possible to keep the spur from spinning and still have a free diff. Pan cars are a little diffrent from touring cars since they have a straight axel for the rear. I tight diff will make that end of the car break traction. The outside tire has a greater distance to travel than that of the inside. By tightening the diff it slowes down the amount of rotation of the outside wheel causing it to break traction. That is why people that run a spool in the front of their T/C say that it has a bit of a push that you have to get used to. Diff settings are just another tuning aid to get the car to do what you want.
Hope this helps!!!
Jeff Antle
Corally U.S.A.