I have searched and found they both are 550cc, but are the S5 better because they are high impedence or what, bit confused?

 

No, technically low-impedence fuel injectors react faster.
But, they all run through a resistor circuit, so it doesn't matter.

S4 = rectangle Bosch plugs
S5 = oval ND plugs
The plugs are not interchangable.


-Ted

 

87.5-88 S4 injectors are also high impedance. Only early S4's used low impedance injectors.