If I have a 28 stock spline in my mustang and I upgrade it to a ford racing 31 spline what will that do?
it is a way of making a sleeve and a bar lock together and slide as they turn but it can also be used to let them turn left and right
Your talking about the axle splines. The rear axles (left and right axle shafts) are splined on one end and they slide into the differential. This is how power is transferred from the axle differential to the axle shafts... to your tires.
By going up in spline count (from 28-31) you have more surface area on the axle shaft. Because of the increased surface area, the splines are stronger and less likely to become stripped by a high horsepower/torque motor.
THe 28-spline axles are just fine for the car, even if it is modded some. Besides, 28 to 31 isn't very much difference. I would only do it if I tore up an axle shaft, but just upgrading it to say you have them is pretty much pointless.
Your not going to get more power, or more speed from them. They are not a 'direct' performance enhancement. They are just a stronger component than stock that will handle a harsh driving environment and increased power output more than the stock axle shafts can.
I agree with Boogie, unless you are getting into serious drag racing with slicks, not much reason to upgrade to 31 spline axles.