It's either stock coils, stock coils on a voltage amp, or Spitfires.

I run my Spitfires on 21psi all the time and I never have a hiccup. The coil pack upgrade also smoothed out my idle and low-speed response. I can tell I'm getting a more complete burn because I don't smell like exhaust/fuel as much anymore! haha. It's a great all around kit.