whats the differnce in S4 and S5 turbos?

 

,

 

The compressor housing, compressor wheel and turbine wheel are identical. The differences are in the turbine housing and the manifold.
The S4 uses an undivided manifold with an ECU-controlled flap in it to close off the larger of the ports in the turbine housing below 2700rpm. This causes faster spooling. The turbine housing has a very small wastegate port.
The S5 uses a divided manifold with equal size ports and no flap. The A/R ratio of the turbine housing is slighter higher. There are two wastegate ports (one for each exhaust port) that are much bigger than the S4's.
The S5 turbo also has electronic boost control, but that's seperate from the actual turbo.

 

If the S5 turbo has electronic boost control, do can you still control the boost using an aftermarket boost controller on the actuator hose?

 

The S5 wastegate actuator hose has a tee in it that goes to the boost control solenoid. You just pull the hoses off the turbo and run new ones to the EBC solenoid. The factory stuff becomes redundant.