Both used dual Bosch magnetos firing twin spark plugs for ignition. Their superchargers, intercoolers, dual magnetos, and two spark plugs per cylinder reflected their aircraft origins. The alternators that use permanent magnets are specifically called magnetos. These were referred to as "magneto-electric machines" or magnetos. Special vacuum chambers tested spark plugs and magnetos. Ignition was by a single spark plug per cylinder, served by two magnetos. Bicycle dynamos are actually magnetos and produce alternating current. Problems encountered included misfiring magnetos and breaking primary chains. Early in the race, one of Shelby's ignition magnetos failed, harming his car's pace. These plugs are fired by two 44-ampere magnetos. Magnetos were changed for Simms units. The accessory housing supports two magnetos, a starter, a generator, and a dual tach drive. By 1918 1,000 employees were making vehicle electrics and aircraft magnetos. Magnetos thus established a small niche for themselves as lighting generators. Early magnetos had one coil, with the contact breaker (sparking plug) inside the combustion chamber.