I agree with RysiuM. 1.2 volts is plenty. I just competed 4 long flights on my FETS (Flying Engine Test Stand). And everything worked very well. Easy starts, good ecceleration, good top end. I was running plain regular gasoline (87 octane) and 50:1 Amsoil. I used the original OS F plug and it is still working fine after about four 14 ounce tanks of gas. I did notice a fine white coating on the plug but it still glows bright. It was about 85 degrees with normal humidity for here in Oklahoma. I tried everything I could do to get the engine to fail but it just kept running until the last flight. I had made a homemade plug connector and that failed in flight. I didn't know that it had failed until I slowed down to land and the engine quit. After I landed I noticed the wire hanging down and reallized that I had lost the glow. What I just said was that the engine kept running at full throttle with the glow off. Didn't quit until I tried to throttle down for landing.

So in conclusion, I still havn't found the down side of this system on my Robin 24.5cc conversion engine. You must keep the plug glowing at all times. Reliablity depends on the glow plug. I will continue to test the plug to failure.