varies in its intensity with the speed at which the magneto is turning. When the engine is being started, either by hand or by a starter, the speed of the magneto is necessarily low, and the spark accordingly weak – even weaker if a low charge battery is driving a struter and is barely able to get the engine around. Some other engines have an impulse coupler on the magneto which accelerates the magneto as it passes the spark generation point and creates a hotter spark. But not the Stearman.

The addition of a “Booster” coil significantly improves the starting characteristics of the W -670 engine. The “Booster” expression implies that it intensifies the spark from the main magneto, but it does not. It is a separate and auxiliary supply of intensified ignition current which is conducted to a separate booster segment on the distributor finger. This booster segment acts as a second finger and tails the distributor finger to give a more retarded spark for starting. Thus the Booster fires late, just past TDC, while the engine-driven mags are timed to fire early or 29 to 32 degrees before top dead center for normal operation. The reason for this arrangement is to prevent the engine from “kicking back” when it is being started with the booster coil.

The Stearman magnetos are designed to receive the high tension lead from the Booster Coil (sometimes referred to as a “Shower of sparks Coil), through the Magneto case and into a specified hole in the Magneto Block.

The booster coil is then energized with 12 volts in parallel with and at the same time as the engage solenoid on the electric/ inertia starter. With a direct drive E-80 type starter, the Booster is energized at the same time and in parallel with the starter. This gives a hot shower of sparks even at low engine turning speeds and an increased reliability of starting.

The picture shows a Bendix Booster Coil that can be adjusted for maximum spark at either 12 or 24 volts.

Remember, however, that you cannot turn the engine over with the starter· for some trouble shooting purpose, with the mags off thinking it won’t start. The Booster Coil can still be active and provide spark to the engine

.stearman-book-A Magneto 1