I have been doing some more reading into this and let me know if i am correct.. If u run higher nitro % say over 50% on the big engines there is so much slow buring nitro in there u need the extra glow plug to get a higher burn % ..If u run a lower nitro % say under 50% u can run a single glow plug and get a good burn..I have 3 old gp style CMB 90's.. 2 with single plug heads and 1 with a dual set up..Bought them used.. I wanna learn on an old motor so when i burn it up or blow it up no big $$$ lost..the dual is for high nitro % and the single is for low nitro %.. Lower % burn faster than higher %.. If i am in-correct in my thinking...Let me know what ya know..The old rule of thumb being the bigger the motor the lower the nitro % the smaller the motor the higher the nitro % because the volume is too big in the bigger motors for 1 glow plug.. Smaller motor less volume = less glow plug needed..Is there any posted info on this already??? Any info anyone has i will read it !!!