Originally posted by Lobster
And here I thought you were just ignoring my ignorance! I was questioning due to some dyno time(22 hp twin air cooled Briggs on an eddy current crankshaft dyno) and different fuels we tested waaaaayyyy back when and didn't see a benefit from additional octane when we mixed our own. However, I notice you always add "race" in your replies, and a 700cc twin making 22 hp isn't a race engine!
Sorry, I wasn't ignoring it, I just MISSED the question the first time. :ugg:
For what it's worth I've seen instances where higher MON octane fuels will knock far
sooner than some lower MON octane fuels, especially on air cooled engines like RD350s and modified XR600 engines. I was never really sure WHY that was the case until Dr.Dave Redszus from Phillips explained it to me. He clued me in to the signifigance of piston crown and combustion chamber temperature on radical formation. The lower octane fuels I used had higher END POINT evaporation temperatures than the flat curve 118 octane stuff. The higher temps of these heavy ends would allow some liquid droplets to reach the combustion chamber and leech some heat. Reaction rates are sensitive to temperature, so even though the lower octane fuel had less stable hydrocarbons in it's mix, the lower temps helped to limit the formation of active radicals. I believe this is why Honda specs AvGas as the fuel for customer RS125s. AvGas has a fairly high end point and provides an excellent piston crown cooling effect when tuning at the ragged edge, as road racers are prone to do :)
Those conversatoins with Dr. Dave certainly made me appreciate the value of trying to understand as much of the chemistry of combustion as possible ( clearly I have a LONG way to go ;) ).
The moral of the story, octane doesn't MAKE power, it just can help establish a less hostile environment for power to be made in, and it's just ONE SMALL PART of the whole fuel equation. Controlling temperatures plays a key role in controlling knock.