A couple of questions for the experts.

1. In a situation where you suspect that preignition is going on, the answer is usually to shim the head or reduce nitro. How about running a colder plug?

2. It seems many european engines are designed to run strong on FAI fuel. Even though nitro is not as expensive in the US, why not do the same thing here? FAI fuel IS cheaper after all.

I bought yesterday 1 kg Nitro (they don't sell it per liter). This costed me 20€40....

A colder plug should certainly help with preignition because it'll retard the timing slightly but it might then make the idle a bit more erratic.

Any engine, no matter where it's made, will run perfectly well on zero nitro but the slightly higher compression of the European engines will help them get a bit more power from low nitro fuels than a lower compression would. It's always safe to add shims but taking out shims needs a bit more care because you have to be certain there's enough (squish) clearance between the head and piston. I don't like to go below about .010" clearance. As a guide, the optimum compression for zero nitro fuel is about 13.5:1 or a bit more if you play with cold plugs.