87 10.2:1 compression motor into an '86 924S (9.7:1 originally)
 

Are there going to be any electrical incompatabilities?

Not sure about in UK but, '87 engines were not 10.2. Only '88 were.
The engine code has to be M44/09 or M44/10. Or look at the pistons. http://forums.pelicanparts.com/uploa...1326900580.jpg

87 in the UK and ROW were all 10.6cr and use the pistons on the right. US were still 9.5cr in 87.

No, there should be no problems as long as you swap all of the existing wiring and sensors onto the new motor. The 924S uses the late DME system, but the early wiring harness style. Also, the dash gage temp sender is early style, but I believe the holes and the threads are the same and the senders will swap over. It's a win situation, as the 2.5L desperately needs that extra compression, although all 944 motors had 10.6:1 pistons in Europe. Maybe the 924S was a special case and ran the 9.5:1 pistons even in Europe. If you are increasing the compression, ou may need to swap the chip (or the entire DME would do it) out of the 87 DME into the 924S DME for best benefit, but the tuning may be the same between the models anyway. Another plus is the cam in the 87 motor should be the later spec cam with slightly more exhaust duration and overlap for slightly better performance and should also be used in the car.

The extra compression may be good for 10-15hp. The DME programming doesn't even need to change to support that. Electrically (except for the possible lack of O2 sensor) they are identical.

i do a simil change from a 86' 944 USA engine to a 84'944 EURO engine.You can feel the difference in every gear and RPM,s but is not so dramatic....
If you have a good 87' engine is OK and it have the autotensioner for timing belt.

I had differences on the block and "swap" from the engines this parts:

-alternator/AC bracket
-temperature sensor(gauge) not the same hole
-metal water pipe(on the exaust side)
-chack the exaust manifold there are diffferent models in the years,beter is the more linear one
-don't now is 924s have the idle speed control(motor) or bimetal one? so maybe put it from your engine

for the rest is the same,maybe your engines have same brackets but just take care of this,is easy when the engines are out.
I prefer to check clutch(better change it) and do a head gasket job so you are sure that no need to do anythigs in next years....

If you are in doubt about the engine is good to lock main,rod bearings and balance shafts,they are not expensive and you never look oil 0,0 bars on idle in a hot day if you want to keep the car.