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Unfortunately there is not a lot of hp margin in the stock 964 injectors. I’ve heard 300-305 hp mentioned by some shops. I don’t think I’m hitting more than 285 hp and I’m seeing well over 90% duty cycle at ~13:1 air:fuel. The recommended max is 80-85%. I haven’t monitored fuel pressure at the same time so maybe my fuel pump is getting tired.
Anyway, you definitely don’t want to run lean under power. You’re likely okay with a 3.8 (plus a little exhaust work) on the stock injectors if the rest of the fuel system is tip-top. A dyno pull with an air:fuel monitor would give you a rough guide. Even better if your shop knows how to use an oscilloscope to record the duty cycle during a pull.
I haven’t seen any vendors successfully modify the chip for larger injectors, but I have seen some bad attempts to try. The entire chip is essentially written around the stock injectors. The only other solution is moving to a stand alone fuel system, which is no small expense or effort.
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