I realize this is a bit of an old thread but hopefully I can still help. I'm new to VQ engines but I have a bit of experience tuning older Z's and the fundamentals are pretty much the same when it comes to tuning EFI.
Approaching 85% duty cycle is usually a sign that larger injectors are required, especially if you are consistently going over 80% DC. A max of 80% duty cycle has always been the rule of thumb that I've followed when setting up engines. Start going over that (using a solid tune) and you run the risk of your injectors not being able to respond quick enough and behaving as if essentially wide open. This limits your ECM's ability to accurately control what your injectors are doing during the most critical time, WOT and high RPM.
I don't know how modified your car is, but have you made significant mods that would require more fuel than stock? What are your AFRs when duty cycle approaches 85% and when does this occur, i.e. long WOT runs, slamming open throttle, etc?
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