are they? besides the spacer on one side of the s4 primary and the injectors, could i use s5 prim. and second. on s4 tii? or s4 primary and s5 secondary?

 

Yes, they are interchangeable but the S5 rail would not sit correctly. Just bolt it semi tight. If you are planning to use the S4 injectors and an S5, than you will have a little bit of problems with the connectors and the impedence. What year of an S4 do you have. If it is 86-87 than its a low impedence 88-92 are high imp..

 

thanks, im aware of the issues with the injector impedance and clip spacing, ill probably end up using S4 primary and s5 secondary to rule out mounting issues ill post comparison pics laters