Knew it was coming! What's the compression ratio in your single plug 2 valve race car?
When have long strokes, not big bores, low compression by modern standards, 4 valves per cylinder for efficient cylinder filling and cooling, and as I'd that's not enough, 2 plugs in each jug to light her off from both sides at once. No high octane ever needed. Yamaha worked their butts off for that.
HD uses high compression to make bigger torque, and are using old head technology. That's why they MUST run high Octane to run right.
Run 87, a
nd enjoy yet another benefit of owning a Road Star!