Non-American RC51's do not have the soft rev limiter and are able to rev all the way to the hard limit of 10.2K RPM where the ECU cuts the ignition.
For whatever reason, Honda created a soft rev limit on American RC51's which retards the timing at 9.2K RPM. They still have the hard limit as well, but the HP fades quickly at 9.2K.
By bypassing the soft limit and allowing the engine to rev to the hard limit, you can gain over 5 HP depending on state of tune or exhaust system.
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