Stands for High Capacity Actively Controlled Steering (source: FOD since I haven't had time to source the Nissan definition yet). Wikipedia gives High Capacity Actively Controlled Suspension but since it is definitely a steering system, "Steering" makes more sense. Wikipedia also states that HICAS is the hydraulic only version while Super HICAS is electro-hydraulic. I know that early Z32 300ZXs (like mine) are the hydraulic while later ones were electronic. One source on the ever-useful www.300zx.org.uk gives hydraulic up to Series II Sept 1993 and electronic from Series III thereafter. This below article, from 1990, appears to contradict Wikipedia as it is about the first Z32s and yet specifically uses "Super HICAS". My money is on Ray Hutton being accurate.

Here's an extract and some pics from the ever-useful 300ZX Enthusiast's Companion by Ray Hutton:

The 300ZX Turbo has one extra technical feature of which the engineers are especially proud: Super HICAS. Other Japanese manufacturers offer four-wheel steering and in the case of Honda and Mazda it is two-phase, assisting low-speed manoeuvres as well as enhancing high-speed stability. Super HICAS is also two-phase, but of more subtle character. It is not arranged to make it easier to squeeze into parking spaces. The rear-wheel steering, which never exceeds plus or minus 1 degree, is purely designed to improve the car’s response in medium and high-speed swerves.

The rear-wheel steering system, which is integrated with the multi-link suspension and the front-wheel steering, is electro-hydraulic. It operates like this: electronic sensors detect the car’s speed, steering angle and steering wheel movement, and if they exceed predetermined values, the computer directs a hydraulic actuator at the rear axle to steer the rear wheels via the rearmost lower suspension links. In normal driving the movement is rarely more than 0.4 degree.

But, unlike other 4WS systems which, at speed, steer the rear wheels in the same sense as the front ones, Super HICAS first introduces a twitch of counter-steer before settling with all four wheels pointing in the same direction. The degree or suddenness of this touch of opposite lock is reduced as the speed rises.

Super HICAS automatically produces a precise form of the rally driver’s technique of flicking the steering to the outside of the corner before turning into it. The result for the 300ZX is very sharp ‘turn-in’, with better stability through the classic lane-change manoeuvre which reproduces emergency evasive action at speed. An ordinary car tends to swing its tail in such circumstances, but 4WS allows the front and rear tyres to develop their slip angles simultaneously to the benefit of handling.

If it is so good and they are so pleased with it, why doesn’t Nissan fit Super HICAS to the normally aspirated 300ZX as well? The answer is about economics but NTC’s testers say that the real benefit of the rear-steer become apparent at over 120mph and that they are happy with the handling of the regular car without it. The 300ZX Turbo, with its considerably higher speed potential, needed Super HICAS if it was to achieve the ambitious handling and stability standards they had set themselves.

© 1990 Ray Hutton and Motor Racing Publications Ltd, Unit 6, The Pilton Estate, 46 Pitlake, Croydon CR0 3RY
Extract from their excellent publication “Nissan 300ZX The Enthusiast’s Companion” which should be on the bookshelf of every Z32 fan.

Some recommend removal of the HICAS effect - usually by fitting a HICAS "eliminator" which locks the rear steering mechanism so it can't move - for a number of reasons: letting you remove all the HICAS gubbins for weight reduction; drifting and whatnot. All of these are fair enough except for just not being arsed to fix the system properly when it gets wobbly.

And there is the point that if the test drivers think it doesn't give "real benefit" until 120mph, why did Nissan fit it to the JDM Turbo which is restricted to 114mph?

However, I stick to the fact that it's part of the original design of my Z and if the designers wanted it on the car, that's good enough for me.