Spark plug thermal characteristcs were investigated for both propane and butane at various altitudes. These tests show that under ordinary conditions, if the correct spark plug heat range is installed at sea level, the selection does not need to be changed for higher altitude operation.
Operating spark plug temperatures were lower with butane as compared with propane and may warrant, in some instances, a hotter heat range. Knock-limited spark advance increased with altitude, but decreased with increasing compression ratios. There is no significant spark plug voltage requirement change under running conditions, as altitude is increased from roughly sea level to 8000 ft.