I might buy a 1986.5 928 auto, the timing belt snapped on the original motor. He bought an S4 motor to replace it.

Will the S4 motor bolt in? What is different?

I have access to the motor that was in the car before. I could rebuild it, but which is the best path?

And finally, if I put the S4 motor in I have plans to supercharge it, will the transmission and drivetrain be up to it?

 

It will bolt right in to any 83+ car, you just need the computers that go to the S4 engine if you are swapping in the entire S4 engine. And yes the transmission can take around 400+ ft/lb of torque very easily, I would rebuild the drive train while it is easy to get to so you don't have failures from the newfound power . The S4 engine has bigger valves and higher compression. I would put the heads from the S4 on the S3 engine block because of the engine's thicker walls which can take more abuse from forced induction from the supercharger, it also means less hassle with computers because you can still use the S3 computers.

 

Doesn't the 86.5 have the same drive train, including transmission, as the S4?

I would expect engine harnessing to be different too as the S4 has slightly different sensors than the S3.

Will the CE panel need to be changed or modified?

 

for simplicity, i would rebuild the S3- sell the S4. CE panel, Harness, would need to be re done.