Help! Bizarre Fuel Pressure Behaviour....

[kismetcapitan]

just wrapped up my BNR32 rebuild two weeks ago or so, and the car is mostly sorted, except for massive explosive misfiring under WOT load at 6000rpm and above.

main bits of the car: Tomei Poncams 260/260, 9.15mm lift/Garrett 2860-5 turbos (like HKS 2530s), and all supporting mods. The fuel pump is an Apexi in-tank model, spark plugs are NGK Iridium grade 8 gapped at 0.6mm, about 2000km on them but they've fouled three times in the past two weeks. New Splitfire coilpacks and stock fuel rail with Nismo fuel pressure regulator. The charcoal fuel vapour canister up front was removed and the lines blocked off or rerouted.

The overfueling has been sorted and the plugs aren't fouling anymore (a fuel tank breather hose was misrouted). Now however, fuel pressure will not fully rise with boost. No matter what I set the base pressure at, during driving, it will follow boost up 0.7bar, then start to gradually, and then rapidly (depending on rpm and rate of acceleration) fall 2 bars. Below 3bar fuel pressure, the engine starts to misfire. Boost is set at 1.5bar at the moment.

During dyno mapping, the fuel pressure ran normally (climbing in sync with boost) and I made 496rwhp. But I can't replicate this ever since they "sorted" the breather line problem, which would cause the car to massively overfuel when the tank was topped off, but run normally on 1/4 tank or less.

I suppose I could just set the rev limited to 6000rpm and live with it. But then, that wouldnt be much of a GT-R now would it??

Any thoughts/ideas would be appreciated!