The difference between the TIM 6 and the CH unit is that on the Tim 6 only the battery voltage is applied to the coil. In the CH unit there is a circuit that boosts the battery voltage up in the hundreds of volts and that goes to the coil for a much hotter spark. This is called Capacitive Discharge and is the system used on modern cars. Either should work OK but the CH circuitry should be better. Aso CH has a device that will automatically retard the spark for starting with out any mechanical retarding system. In RCM in the 80's there was an article for building a unit like the CH basic unit.