I bought a spohn torque arm and am going to put it on tomorrow. It came with a magnetic angle finder but I misplaced the papers that came with the torque arm. Do I connect the angle finder to the driveshaft or the rearend housing? If it is the rearend housing...where?

 

Both. The pinion angle is the difference between the driveshaft angle and the pinion plane.

Put the angle finder on the driveshaft. Lets say you get 3 degrees down towards the rear. Now put the angle finder on the torque arm mount. That's the closest your going to be so that it's lined up with the pinion. If you're someone like me and don't have a torque arm mount you use the end of the pinion or the yoke then subtract 90 degrees. If the pinion is pointed 2 degrees down towards the front you add the 2 measurements to have a total of 5 degrees pinion down angle.

The pinion angle is in relation to the driveshaft. It has nothing to do with the ground so just measuring the pinion plane means nothing.

 

So what angle should I have? I hear that -4 degrees is best for hooking.

 

It depends on the car and how it's driven. For a street car you don't want to go more than -3. zero to -1 is a typical factory setup. For all out racing -5 works better.

It's all going to depend on how much traction you have and how much torque is put down the driveshaft. Under load the front of the diff will want to rotate up. The goal is to have the pinion angle at zero when under load.

More than -5 degrees can start eating u-joints.