Have just replaced the cracked housing and manifold on my s4 with s5 parts, apparently they'll make a reasonable difference and bolt straight up. Can anyone explain how the s5 twin scroll system works though? My 'rotary bloke' said it works without any actuation or anything... this is correct?

 

s5 turbo's don't have the twin scroll system. Instead the exhuast inlets that mate up with the mannifold are equally sized.

 

S5 does have "dual scroll" turbo as it is a divided turbo and exhaust housing that has two scrolls that are differentially shaped to direct the exhaust gasses at the turbine wheel to maximize boost response AND total power.

It has no actuation like the S4 dual scroll system. The WasteGate hard line on the front of the compressor housing DOES have a take off nipple for the Boost Controll solenoid that MUST BE CAPPED or the waste gate will not open and you will have massive boost creep.

You will also have to block the vacuum line that went to the old '87-88 style dual scroll actuator or solenoid so you don't have a vacuum leak.

The best parts about the S5 conversion are it has a MUCH better dual wastegate design so you can run a much more free flowing exhaust w/ out boost creep and it simplifies the turbo system- no dual scroll actuator/solenoid etc. I'm not sure if it really makes any more HP, as a matter of fact I bet many cars will lose a little HP since many have done exhaust mods that increase boost over wastegate setting (6.5psi?).