"S5 to S4 MOTOR SWAP"
 

Im in the process of buying a S5 block to install it in my 88 T-II. My question is can I still use my S4 airflow meter,and wiring harness ? I know the intake manifold for the S4 will work with the S5, but I would like to know which other parts are interchangeable from The S4 to the S5 motors. I heard the fuel rails, and the injectors from the S4 can be swaped into the S5.
Could someone please tell me exactly what I need to do so this swap comes out as smooth as possible. Any help is greatly appreciated. "Thank You" :D

ideally you would want to swap everything from your old S4 motor on to the new S5 block, including the front cover (so you can use your mechanical OMP).

As Icemark said, you basically want to swap everything over (some people prefer to keep the series5 fuel rails as they say they don't suffer from leaking as often as series 4's) But, since the series5 wastegate is better, I'm a fan of keeping the s5 turbo, which will require swapping some oil lines (not too difficult) as they're a little bit different length.

So basically Icemark what your telling me is I can use my wiring harness, and ECU on the S5 block with no problem ? Also the S4 airflow meter can I use it ? I thought the S5 AFM is diffrent than the S4 ? I knew about swaping my OMP to the S5 already as I have heard it is better than the electric OMP on the S5 cars ?
Why is this ? Are the S5 OMP a piece of crap or what ?
Also SonicRat metioned the S5 fuel rails are better than the S4 fuel rails is this true ? If so why ? I know that the S5 turbos and wastegate are better than the S4 as Ive heard this on the forum from "A Million People" already

Well, for one, unless you feel like swapping a million harnesses, you can't use the s5 harness. Which means the s5 omp can't be used, because it's electric and there's nothing to hook it up to! Moving along, the AFM's are different per harness, thus needing the S4 afm (which works with the series4 ecu) Just have to match up the parts with the series ECU you're running, series 4 omp, series4 TPS/manifolds, series4 afm, series4 pressure sensor, etc. Only s5 stuff you want is the block, turbo and exhaust manifold :)

The engine driven OMP does cost you a horse or so, but they have been much more reliable than the electric OMPs. The other reason though to use the S4 OMP is the S4 electrics are not compatible with the S5 OMP. If you were going to S5 electronics, then you could still use the S5 electric OMP, and have the ECU go into limp mode if it ever did fail.

The S4 AFM sould be fine, since you are retaining the S4 ECU.

The fuel rail thing is a matter of personal opinion. The S5 Fuel rails use a PD (Pulsation Dampener) integrated into the fuel rail. While this is a manufactureing cost advantage, there is no real world advantage. However some people still believe that the S5 PD is more reliable than the S4 PD. Personally (again) I don't agree as I have seen more S5 and S6 PDs fail and leak than S4 PDs (but I do know that I am in a minority on that).

The one big advantage that the S4 rail/PD assembly has is that the PD can be replaced without replacing the whole fuel rail (as you are forced to do with the S5 PD) and at considerably cheaper cost per part.

So if it was me, I would continue to use the S4 rail and PD, and just replace the PD every 10 years (or 120K miles) rather than having to replace the whole S5 rail and PD every 10 years/ 120K miles.

SonicRat Your the man Thank You for the info.
Now everything is more clear, and I got the answers to my questions, your help as well as Icemark is greatly appreciated.