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Quick and dirty:
The Early 300ZXs came with an injected sohc two valve iron block Aluminum
headed V-6 (87x83 mm), which produced 160bhp@5200 and 175lb/ft@4000(non-turbo).
The only engine option was a turbo, with a 40hp advantage.
The 84 turbos have no cooling for the turbos, and fail more frequently than 85-89
if they are not cared for.
The main bearing caps are all cast as one unit, with two bolts per bearing,
providing extra block rigidity. Aluminum pistons, with two compression rings
and one oil ring, are connected to the crankshaft by steel rods.
Other than the one-piece caps, there aren't any major technical highlights
in the early V-6s, apart from their bullet proof reliability.
As far as maintenance goes, timing belt every 50k miles. Check your plugs, wires, etc, etc at the same time. Flush the cooling system every once in a while, the radiator is a pain to get out should it need to be re-cored. Oil every 3k miles, no synthetic, no generic stuff either. Change the gear oil in your transmission and differential every year or two also. If you have an automatic, fresh fluid is the key to it's long life. Transmission fluid is cheap compared to transmissions.
Since a few people have asked:
The computer injects fuel once every crank revolution, half the total
needed amount, naturally. The computer determines how much fuel
the engine needs from the array of engine sensors: mass air flow meter,
engine temperature, engine rpm, throttle opening, oxygen sensor, and
a few others. This is all standard fare on cars now, but was quite
advanced in 1984. The engine sucks air through the air flow meter which
has a thermistor and very thin wire strung through the path of the
inrushing air. The thermistor tells the computer the ambient temperature
of the intake air, and the wire is heated by a current from the computer.
The air rushing over the wire cools the wire off, the faster the air goes,
the more heat is transferred from the wire, thus changing it's resistance(and current draw),
which is what the computer measures. The computer also knows the position,
velocity, and acceleration of the crank from the crank angle sensor
in the distributor. The engine temperature is read from the cyclinder
head temperature sensor, and the throttle switch tells the computer if
the throttle is closed(and fully open on automatic transmission cars).
From this data, the computer calculates the correct
amount of gas needed by the engine and fires half of this amount every
crank revolution. With the oxygen sensor, the computer determines
the amount of oxygen in the exhaust, making small corrections to the mixture
when in closed loop mode.
The fuel is shut off when decellerating in gear to conserve fuel.
The computer also cuts off fuel at 6500 rpm on all models, and at
220kmh(137mph) on turbos, both for "saftey reasons".