cheers jason o, just fitted car with ngk pfr 6a plugs which were in previous and was hesitant then around 4-5000 revs and still is. who supplies iridium plugs and are they worth the difference. also leaded or super unleaded?
I had PFR6A's in my car and they did hesitate around the 4.5-5k rpm mark, even when gapped at 0.8mm, the iridiums definately cure it. Contact Rod Bell as he is the most local specialist for you, and he will be able to supply the iridiums.
I regularly see more knock on my APEXi PFC when running Optimax than what I see when running BP Super UL.
I'm yet to be convinced of the sustainable quality of Shell Optimax, and FWIW Mycroft also is unsure of it's quality, just check the Octane Booster thread for his opinion.
Running with ordinary UL will cause detonation (knock) and will end up with you having a large bill to replace some melted pistons.
You may even find that poor fuel could be the cause of the hesitation, as the ECU will retard the ignition when it detects knock.
The stock ECU in the R33GTS25T ups the boost from 0.5bar to 0.7 bar at about 4.5k rpm, it changes the cam timing at this point too. This is the point the plugs seem to have a bit of bother maintaining a spark if the gaps are too big.
It may also be that your fuel filter could be blocked, and therefore not supplying a quick enough rise in fuel flow when the boost rises to 0.7bar.