I was just wondering (Fuel Question)

Mjoseph320 wrote:
Yup, my daughter's BMW, my SILs BMW, and my BIL Mercedes all require premium gas. My other DD has a Nissan and it is recommended but not required so she uses premium every 3rd or 4th fill up to prevent knocking.

OK, I have trouble understanding the effectiveness of that choice. Higher octane will only prevent knocking when that is the fuel that is in the combustion chamber. This means that you will only be having an effect, every 3rd or 4th fill up. In between fill ups will be just as prone to knock as if one ran low octane all the time. Even if you are filling up at a half tank or above, means the dilution of the octane boosters will have no real effect. Secondly, the computer will prevent knocking anytime the octane is too low. Any modern engine recognizes the knock, and detunes the engine, so it won't knock. Detuning reduces available power. So what that means is that every 3rd or 4th fill up, they have increased available power. You probably don't notice it unless you run wide open throttle, where the air pressure in the cylinder would be high enough to cause knock. Otherwise, you will never know the difference.