nope, compression does not change.
but the ONLY benefit to higher octane fuel is that it resists early ignition (pinging, pre-detonation, knocking, etc) better than a lower octane fuel.
and our engines generally will run just fine on lower than called for octane rated fuel... as long as you are not subjecting the motor to hi load conditions- hi air temps, hi engine temps, low rpm, lower elevations, and heavy weight loads. the easiest thing to do under such circumstances is to .. drop a gear, and go.
Ride Safe, Ride Lots
After reading the comments above, I'm not sure if I interpreted the poster's use of the word "load" correctly. In any case, rather than debate or question it, I found this link (http://www.lafn.org/~dave/trans/ener...asting.html#s5) which actually does a pretty good job of explaining engine performance and efficiency topics including BMEP (Brake Mean Effective Pressure) and BSFC (Brake Specific Fuel Consumption). If one looks at the fuel maps including in this link, you can appreciate that fuel economy can worsen by shifting up and having the engine operate at lower RPM. Right or wrong here, I am interpreting load to be essentially analogous to BMEP.
Wasn't it somewhere said that german octane ratings are different from US/Canada ratings?
In Germany - and in my owner's manual for my '89 R100GS - the fuel grade is specifedi as 91 ROZ (or RON) in the US. Regular unleaded in Germany is rated as 91 ROZ and 82.5 MOZ (or MON in the US).
In the US the number stated on at the pump is AKI = (RON + MON) / 2 which makes regular unleaded in Germany comparable to 86.75 AKI.
That makes me feel better when I occasionally have to fill up with '85'.
[Edit]Scott, you were at the same path at the same time.
I've been using regular grade gas in motorcycles that called for premium since 1975. I've never had a problem with any of them that was even remotely related to fuel.
'You can say what you want about the South, but I almost never hear of anyone wanting to retire to the North.
On a recent trip in the southwest I could only get 85 octane fuel for my 1200 GS. I didn't notice any difference in the perfomance of the bike. I don't ride hard or fast so all was well. I did however notice a hugh difference in the taste of beer after it got hot on the way to Monument Valley
If I am heading off on a road trip, I usually throw a bottle of octane booster in the side case. I don't ever rember seeing anything less than 87 octane in Canada myself, but in places like Nebraska I have seen 85 octane. Anything less than 89 octane, I give it a slug or two from the bottle and I have never noticed any knocking or pinging. My bike is 2100 R1200GS; minimum requirement is 89 but I put in 91 if it is available.