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I have a 355 SBC with HEI ignition and just got a new set of Edelbrock RPM heads and the RC-12YC plugs Edelbrock recommends. Should I use .035 or .045 gap with this HEI system. Please advise as I have been told to use both gaps at different times. Thanks
 

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the coil capacity rating determines practical max gap for a resonable coil life

as the gap increases, the coil has to build to a much higher voltage before it is able to arc across the gap

.035= you need a coil rated at 30kV, actual spark voltage is approx 10,000V
.045= you need a coil rated at 40kV, actual spark voltage is approx 15,000V
(those are really rough #'s to illustrate)

HEI is 45-50kV
 
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