About half way through 93 the gears in the transmissions were revised to be quieter and as a result the transmissions went from a torque capacity of approximately 450 ft-lbs to 400 ft-lbs.
The first 232 or so ZR-1's made in 93 had the same higher torque capacity transmission as the 90-92 ZR-1's.
http://www.zr1netregistry.com/ZR1_faq.htm#zfgears