rotor faces
 

I was searching and just realized that all s4 rotors are cast and all s5 rotors are machined (the recessed tub in the face).

I have been running an s5 motor with cast faced rotors. the counterweight is from that cast faced motor as well. A shop in south jersey weighed them and sold them to me as s5 rotors. They said that mid way through the s5 they changed. apparently that is not the case according to many members. The motor they came off of had an s5 turbo on it. It also had s4 fuel rails. so it may be a frankenmotor. I know they are a good honest shop so I never questioned the rotors.

Did they make s5 turbo rotors with cast faces in early production (89)?

Is it ok to run them on an otherwise all s5 t2 motor (e-shaft? flywheel?)?

thanks

I have never heard of s5 rotors being cast.
As for using it, there is not problem. The s4 rotors are a bit lower compression and a bit heavier, but other than that they are the same. The compression difference doesn't matter for an NA engine, and shouldn't really matter for a turbo until heavily modded. I ran an s5 motor with my s4 ecu with no problems.
All 13-B e-shafts are the same, so thats not a problem. And the counterweight is what must be matched to the rotors, so it you have an aftermarket flywheel with the auto counterweight, then you are fine.

Well I am running a heavily modified motor (street port, BW s361, microtech and all supporting mods). I am kind of disappointed to know that I downgraded my motor without knowing it. Any benefits of running s4 rotors (slightly safer at higher boost?)?

If you have a standalone, there is no problem, I was just talking about a stock ECU.

And yes, the only advantage I know of is slightly safer at high boost. FWIW, I think J-rat uses s4 rotors, and he's well into the 500's