the system uses stand mounted secondary injectors ,which have a duty cycle that starts at 6-7000 rpm , (mind you these engines are meant to run at above 10,000 rpms e.g the TRD group A (AE101) 4AGE revs at a phenomenal 11,000rpm , top or side feed injectors just can't keep up with the optimum air/fuel mixture at these high speeds, so these kick in at high rpm's.