Important for engine tuning is the correct heat value of the spark plug, otherwise the cylinder will overheat or the spark plug will be constantly sooted. A cold spark plug gives off more heat to the cylinder head, the electrode tends to soot because the cleaning temperature is not reached. A hot spark plug gives off less heat to the cylinder head, be careful here, as the cylinder can overheat - danger of piston jamming. Many tachometers need a suppressed spark plug to work properly. Without interference suppressed plugs, extreme jumps or even absurd revs often occur. If you own a rev counter, then you should use the somewhat expensive interference suppressed plugs. Your tachometer will thank you for it.