When the 27% Extra 330S was in development, we used spark-ignition (gas) engines and 72 MHz with no problems through a few different prototypes of that model. Now, I have my own 27% Extra and am using 2.4 GHz, also with no problems. A minimum of 10" is always maintained between the engine and any electrical components associated with it (ignition module, ignition battery, switches) and the rest of the radio gear.