plumbing
How long are your lines from the pump to the solenoid and nozzle?
Do you run a pressure accumulator of any kind?
You may have just enough pressure build up delay that your not getting good atomization on that nozzle just as it turns on.
My guess is the engine rev's fast enough through the critical max torque rpms, that your too lean ( or have inadequate water flow) as the pump comes up to pressure and fills and pressurizes the lines. I would suspect that you need to trigger the pump a bit sooner to give it time to build pressure.
With a return bleed system your pressure rise might not be fast enough to get the water spray working at design flow in time to prevent detonation.
As a test I'd suggest you try lowering you trigger point so the nozzle has more lead time to get up to flow. With that larger nozzle you may have slowed pressure rise in the system just enough to push you over the edge.
Larry
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