Tests on a single-cylinder engine have shown that the flame speed and lean extinction limir may be increased by the fitment of a “vortex generator” in the engine intake. This consists of a nest of delta wings which generates a vortex system in the inlet air flow. The results show that this vortex generator should be of variable geometry and replace the conventional throttle.The main instrumentation on the engine consists of a flame logger which accepts amplified signals from ionisation probes set in the engine combustion chamber. The logger is gated to count those flames which travel across the combustion chamber during a set crank angle period. Information from this instrumentation is presented in the form of graphs describing the effect of vortex generator blockage ratio, mixture strength, engine speed, load and spark timing upon the percentage number of flames which cross the chamber and apparent flame speed.