Is it because HD EFI has them sat up to run lean/hot? Sort of to compensate for detonation that would happen if a normal car hi compression engine rated for "premium" fuel, uses "regular" gas.

 

Many reasons, air cooled eng vs water cooled, high octane = slower burn rate. Compression rates are not equal between air and water cooled engines

 

For instance, the CVO 110"ers are approx. 9.2 cr.
Combine that with that super-short, upside down, 255 cam, and you have 205-7 ccp.
You'll want premium fuel with those.
Scott

 

One other thing to add into the mix is the 10% ethanol mixes (soon to be 15%). Stoich with that fuel is down to the 14.2 to 14.4 range so a bike tuned to run on pure gas will be even leaner runing the blends. Premium is absolutely necessary unless you decide to tune to the fuel you want to use. You can tune to run 91 octane (at close to sea level but you will be pulling timing and in the end leaving hp and torque on the table.