I would never build an engine like yours and expect to run safely on 91 octane. Your heads won't support a cam that would let that happen. Your rod and piston choice wouldn't support the RPM the correct cam would need to operate.
I have no idea what the specs are on that cam but my guess is someone is thinking the extra exhaust duration is going to kill off cylinder pressure. That only works on a lawn mower while your pulling the rope. It's the intake valve closing point which has the most control over cylinder pressure and the Thumper cams close that valve earlier than normal and help build cylinder pressure.
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