This is a curiosity thing, and i'm hoping the more advanced gearheads here can explain...

Harley's have a pretty low compression ratio, stock, yet require premium fuel. My understanding is that premium fuel has additives that keep it from detonating under compression. High compression (performance) cars use premium fuel for this reason.

Why do Harley's need the premium despite having a low compression ratio??

 

its a good enough question - although I believe technically they require 91 which is mid grade where I'm from.

Only thing I can think of is that air cooled engines run hotter and are more likely to suffer pre-detonation, so thus the 91 requirement.

 

Large bore with only 2 valves = VERY low compression and 87 octane, or low compression and 91 octane.

 

Would bore really matter? Compression ratio is just compression ratio, so the whole volume (bore x stroke) should come into play.

Same deal with # of valves... I think?

 

Help me understand this as well. Will 91 octane ignite slower than a lower octane gas?

 

Doesn't it say in the manual 87 octane or higher?. That doesn't sound like a premium requirement to me.,,

 

Nope, it says 91

 

SPECIFICATION
RATING
Pump Octane (R+M)/2
91 (95 RON)

 

Yes, exactly correct. "The higher the octane number, the more compression the fuel can withstand before detonating"

http://en.wikipedia.org/wiki/Octane_rating

Compression creates heat. My belief is that because air cooled engines run hotter, they are more prone to detonation, thus higher octane requirement.

 

If the engine was water cooled it would likely only require 87 octane. The operating temperature simply swings too much so 91 is required just in case
the temp gets high.