Okay.. I'm thinkin about buying one of those JDM motors off ebay. I have an S4 NA (88 GTU) shell and I could get a S4 Tii motor.. But wouldn't an S5 be just that much more fun?
It comes with a motor, tranny, ecu, and uncut harness. I'll find the MAF/boost sensor/solenoid/etc.

So, my engine, transmission, and ECU should all talk fine, right?
I can shorten a driveshaft (i do stuff like that for a living) or get my hands on a rear end, so that's no big deal. Any other fitment issues i'll run in to?

What about the interior? Will my gauges (tach, for example) still work properly? Will my inside-the-car wiring harness fit into the ECU?

If anyone has done this swap, I'd sure love to hear what problems you ran into so I can plan ahead.

Thanks in advance!

 

bump for bored at work and still searching
I'm seeing S4 NA -> S5 NA and S4 NA -> S4 T2..

 

Read this active thread: https://www.rx7club.com/2nd-generation-specific-1986-1992-17/s4-s5-engine-swap-check-list-884624/

There's no real point in using an S5 engine. Once you turn the boost up with a controller, the two engines make nearly identical power. Mixing series just introduces wiring hell, unless you aquire all S4 TII electronics to make life easier.