the power of the '28 is resticted in 2 main places;

a, in th exhausts, the headers have tiny inlets, fixed with race pipes
b, in the pgm4/card, the timing is retarded more than a std '21, fixed with the HRC card and wiresplice, cheap workaround involves the '21 flywheel.

also general consensus is the airbox is quite restrictive.

There is no plain splice (i.e. just a wiring mod) like on the '21 as the restiction is actually in the card, the '28 splice is to allow the running of the HRC card without the HRC loom, running the loom means losing all road gear, lights, indicators the lot.

Wheather a 28 running the '21 flywheel has a top end speed restriction or a gear related power tail off (like the '21) I don't know.
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