Ah, but higher octane fuel does ignite in buld less readily -- it also has slower flame probagation rate, so in an engine that does not require it, it CAN reduce power output and economy from poor combustion. Higher compression (the main reason for needing higher octane) generates higher heat, so the higher octane, less volatile and slower to ignite fuel is more volitalized, more easily ignited, etc.

RVP is an "average" for the fuel, not the volatility of the entire fuel bulk, and volatility and burn characteristics are related in complex ways. Gasoline isn't a simple mixture by any means, and most of the compounds that burn slowly (ie resist detonation more) are unsaturated, with lower vapor pressure and lower volatility than fast burning compounds (that resist detonation less). The highly volatile compounds will always vaporize, so the spark will ignite the fuel, but the bulk of the fuel will ignite less readily (ie have less tendency to detonate).

The whole point is to have highly combustible fuel that ignites only with the timed spark that will also burn competely in a controlled manner in a short time (before the pressure and temperature drop too far). Mixtures that ignite very easily will ignite under compression heat and the influence of "hot spots" (spark plug tips, hot deposits on exhaust valves, etc) before or simultaneously with the timed spark. TWO flame fronts in a cylinder at the same time isn't a good thing, and premature ignition of the mixture (one form of detonation) isn't good either.

Part of the price we pay for having the power output MB engines produce is a requirement for high octane fuels. No way around it, as you risk serious engine damage from running low octane fuels -- if it knocks or rattles, use higher octane fuel. Otherwise, it's gonna be valve job time soon.

Incidentally, octane requirement can go UP as engines age. Deposits from various sources build up on the pistion crown and on the exhaust valves, and this can both raise the compression some and provide red-hot spots. Spark knock can then increase the deposits from poor combustion. Using higher octane fuel controls the knock and can reduce the deposits -- I've had several 150,000 mile plus engines do this.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!