I used a old model A dist turned with a small variable speed heater motor for a show and tell.to show the firing of of plugs and reversing the coil voltage and monitoring the plug voltage( picked up a 40 k voltage Meter at Hershey a couple yrs ago) also the current draw of coil.Used several old capacitors to subustute to see what happens,,around .33 mfd worked the best for hi voltage.lower the mfd the voltage dropped,, all at same rpm.heating the cap.made a big drop in voltage also hi rpm caused voltage to dec.,don't know how scientific all this is but heat and cap do make a diff.Iithink I'll go for the Napa
RR175. The Vertex are a little pricy I think