Engine compatibility question.
 

Is the engine in an 86 944 a drop in replacement in a 83-84 car? If not what mods need to be done. Thx. Mark

The 86 engine would have a different DME and engine wiring harness. The harness on the late cars goes in through the firewall by the battery compartment, with the DME under the passenger footwell; the early cars it's kinda under the steering wheel, so I'm not sure how you'd route the wiring.

But basically, yeah, it's a drop in replacement.

You could probably get away with using the early harness and DME, but it might not run as well.

I can't think of any other differences.

The bare long block of the 86 is a direct replacement.
Re-use the wiring/DME/AFM/sensors/hoses/manifolds from your 83-84 car.

The only real difference to be aware of when swapping is the coolant gauge temp sensor mounted in the block. The later style has a sensor with two wires, your early car has only one wire for this sensor. No problem - you just hook up your one wire to one of the 2 prongs on the 86 sensor to make your gauge work.

Otherwise the engine is more or less identical aside from internal details/revisions which do not affect the swap-ability.

You said "long block"? Should I use the intake off earlier car or leave 86 whole and change sensors from 83 into the 86 engine? I am not familiar with these at all.

To be safe I would re-use your 83 manifolds.
There was at some point a minor revision to the intake manifold between 83 and 85 where they moved a vacuum port.

You could use all the 86 sensors for your 83 car if you wanted.