I just went through this on my ignition system. When you go to an aftermarket coil with lower pri resistance...this will actually DROP the amount of voltage it gets with the stock ballast.

I blindy replaced my coil with the MSD Blaster II and ran it but never checked the voltage it was getting which was only ~4V! This ended up translating to only a consistant 10-15kV at the plugs when I tested it! I had my car dynoed like this and I wonder if it cost me any HP/TQ?

Either way when the resistaance across the coil is dropped (aftermarket) it is much less in comparison to the ballast resistance so the balalst will "take the lion's share" of the voltage! In my case with 14V on the ballast, it dropped 10V and left only 4V for the coil.

This means, when you go aftermarket, asjust the ballast accordingly. I went to the .8ohm ballast and now I'm getting 7-8V on the coil and it gives me 35-40kV+ at the plugs all day.

Oh yeah, unless specifically designed for it, don't connect the hi side of the coil directly to battery with no ballast resistor. That is one way to cook your coil fast!