[Marv D]

Your issue is definately compression. The advertised "12.5"1" compression ratio is typically assuming the old 64cc heads and a stock 0.025" deck. Being aftermarket pistons for the 0.030 bore it is 'likely' to be the 12cc dome typical of the TRW 12.5 pistons.

Thing is,,, bore it +.030,,, Zero deck that motor (as we know it has been decked but not how much) and it's 13:1. You need a MINIMUM of 108 - 110 MOTOR octane to keep it from eating it's self alive with detonantion.

I'd say your first step is to find out how N.Z. rates fuels. Here in the US they use R+M/2
If NZ shows only research octane at the pump,, (which is always considerable higher than MOTOR octane in pump fuels) YOu need to find theh motor octane of what they are pumping there MOTOR OCTANE is the ONLY number you should consider in a motor on the street, or the strip.

All that said,,, ~84 MOTOR octane typically works well with the typical mild hydraulic performance cam (i.e. the Comp 268H - 274 EX grinds) with a 9.0 to 9.5:1 mechanical compression ratio, with iron heads.

To get there with all the assumptions in deck and such,,, your going to need a 10cc dished piston. If you can find a 'D' dish which will keep a tight quench on the closed side of the chamber,, would be ideal. I think Mhale has one,, but they are pretty proud of them at $700 a set. OUCH