In general sport flying, the hotter the fuel and higher the compression, the colder the plug. Lower compressions and colder fuel need hotter plugs. For a Super Tigre, hot fuel is anything over about 12% Nitro fuel. For a Dub Jet, it might be close to 20%. ABC engines are usually higher compression engines. Generally one just has to work it out. Regardless of the popular hype, I still prefer Idle-Bar plugs for sport and scale. Sport and Scale, I like the McCoy 50 and 59. For racing a McCoy 9 generally fits the bill. In summer 5% nitro with a good M-50 IB or a non-IB M-59 works just fine for most engines.

edited to change a verb.