The water cooled engine on my 05 Goldwing is a low compression engine and runs fine on 87 octane gas. The water cooled engine on my 05 Honda Shadow Sabre was also low compression and also ran fine on 87 octane gas. So, my question is, why does the low compression Harley engine require higher octane like 91 or 93? Is it because it is air cooled or is there some other reason? Just curious.