I'm looking at a little more hp than stock so I'm concerned about the limits of my stock C4 and 8 inch 28 spline axel. Does any one have experiance with the torque limits - breaking point?

Unless you are racing with slicks, or going to huge tires, the stock tire will limit the strain on the C4 and rearend.

There are many posi units available for 8 inch rear ends and better than Ford axels. A great upgrade. If your car has an open rear end, you will have a hard time breaking anything with an automatic.

The C4 in stock form will be just fine unless you really start hammering it. There are many upgrades to the C4 and due to its light weight it is the choice over a C6 to build up.

Rob

I had a shift kit put in my C-4. Love it.
Neil

When I had my trans rebuilt, the shop owner put in a shift kit. He also said he was upgrading the spline to a 31? I think that's what he said in order to create a stronger output. I don't think that was absolutely necessary but that's what he thought would be best. Anyway, it worked out great and performs well.

I'm changing out the alien FMX in my car and putting the original C4 back in - with the shift kit - the C4 is a great transmission you shouldn't have any trouble

Steve,
31 spline output shaft as you thought. A bit beefier than the orginal 28 spline. Similar to axle splines. A nice upgrade for your tranny!! Need to change the driveline yoke at the same time. Lets you run a stronger u-joint also. Bet you got all of that!

Sky is the limit on a C4. It can be built for huge horse power and is the choice of racers now a days. Everyone used to chase those C6's that came behind Clevelands in the early 70's. They had a small block bellhouse. Not anymore!!!

Rob