If both are built with forged internals, the 420A is newer and more efficient...period. the biggest advantage the GST/X guys have over us is their ECU, IMO.www.TeamKLR.com1997 Silver RS12.3@118mph,20psi, race gas(drag radials)Wiseco 10.5 PistonsEagle RodsT3/T4 TurboCustom FMICGreddy Exhaust440's