i belive the egr ports are diffrnet

 

the s5 and s6 have the L spark plug hole lower, this does in fact "retard the timing" as well as flame front change.



i woudlnt use one s4 and one s5/s6 housing

 

Timing can be looked at as the rotor position (measured in degrees) at which point the spark fires. Since this does not change when the spark plug position is changed, the ignition timing does not change, only where combustion begins. Timing is not retarded with the S5/S6 housings versus the S4 housings.