BGTV8

A lowered Z34 has rapidly increasing rear camber in bump. If the car is rolling into a corner with no rear bar, you may find you are only getting half the rear tyre on the tarmac in mid-corner and you are trying to put power down thru 5 or 6 inches of rubber.

It may be that standing the tyre up a bit might be the solution

I have a 500hp (hubs) Z34 that runs 250/650R18 Yokohama slicks on a 10" rim and I have good balance with an OEM rear bar. I run MCA double-adjustable shocks (made in Australia by a former Australian Rally champion) and I'll check my current rear spring rate when I get home tonight (on the train on the way to work at present).
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We found we had to lift the rear given the camber curve once we'd lowered the rear ride height to where we initially thought it should run. Eventually we found balance but our early lesson was "you can go too low" with OEM suspension dynamics

Softer rear spring will only exacerbate camber increase in roll but harder spring rate will kill squat which gives you drive and the end result will be a very twitchy car.