I was wondering how you know which balast resistor to buy from the parts store once you've converted to the MP ignition kit? I was just wanting to buy a few to keep in my cars but there are different part numbers.

Thanks for any info.

The part number for a borg warner select ballast reistor from oreilly auto parts is RU12 that is for the 2 prong resistor. The part number for the 4 prong borg warner select ballast reistor is RUE1.
Whatever part store you go too to get a ballast reistor should be able to cross refernce those part numbers and get the correct ballast from there store for ya.

Ipersonally only use ether Borg warner or Echlin or car quest premium blue boxed ignition parts, i have found the rest of it to be junk. Masterpro and napa mileage plus and duralast especially.

It depends. I converted to EI and also got a new M&H harness that came with a four prong resistor-style plug and a five-prong control box plug. To keep from cutting it up I just bought a four prong resistor but one side of it won't be doing anything since I have a four prong MP orange box. In an older setup the fifth wire would have gone from the fifth post of the control box to the other side of the ballast resistor, but now it's just going to sit there looking pretty.