Filled up with bp ultimate this morning(the station is close by and has a big turnover) had about half a tank of shell vpower(nearest station is a 40k round trip away) left in it at the time,headed out for a nice quite drive to the upper yarra dam, about a 70k round trip. During week days this road is very quite, anyway at around 5k rpm in third and 15psi I hear detonation, not bad, but not good either. At first I thought the BC solenoid valve had started to cycle early instead of at 18psi, it makes a racket when its bleeding air, which sounds a little like detonation. So I turn the controller of, actuators are set to open at 14psi, at 5k rpm on flat road in second detonation again, and also detonating in third at the same rpm. I'm starting to think vacuum leak, regulator, low fuel pressure, checked this out when I arrived home, Idle is stable at 750, cant hear any air leaks,fuel pressure is good at idle, and doesn't leak down quickly. Up shot is I poor some nulon octane booster in and no more detonation. The only thing other than some mechanical, vacuum, fuel issue, timing is @15deg solid, that I haven't uncovered, is the fuel I put in it this morning. Tell me vpower is the way to go, and ultimate is rubbish. Your thoughts.. experiences.. please!