The NGK spark plugs GR8DI and GR9DI have proven to be very robust. Nevertheless, being exposed to cold ambient temperatures in combination with rain or mist increases the risk for GR9DI spark plugs to foul especially on DD2 engines. Fouling means that the spark plug does not ignite the fuel anymore and thus causes the engine to stop. Travelling at low speed on track (formation lap) or operating the engine on the trolley without load encourages the fouling process.
To minimize the risk of fouling at cold ambient temperatures Rotax recommend installing the GR8DI spark plug which has a higher calorific value compared to the GR9DI model and thus runs hotter. To increase the ease-of-use for end customers racing an engine out of the box Rotax will from now on supply all new Rotax 125 MAX evo engines with the GR8DI spark plug.
The GR9DI spark plug, which is still available as a spare part, has a slightly lower calorific value and thus performs better at higher ambient temperatures.