A high compression engine will generate more power with high octane fuel than with low octane fuel, however this has nothing to do with the energy content of the fuel. If anyone would like you to belive high octane fuel has "more energy" it is the oil comanies right? Here is what Chevron has to say about it:

"Don't Buy a Higher-octane Gasoline to Improve Fuel Economy

Octane and energy content are not related. Premium-grade gasoline doesn't necessarily have a higher energy content, especially if it is oxygenated.

The exception to the above advice is when a lower-octane gasoline doesn't satisfy the octane requirement of the vehicle's engine. The abnormal combustion that announces itself as knocking reduces engine efficiency. Using a higher-octane gasoline that eliminates knocking will improve both engine performance and fuel economy.

Many newer vehicles with an electronic control module (ECM) also have a knock-sensor device. When the sensor detects knocking, the ECM retards the engine's ignition timing to eliminate the knocking. This happens so quickly that the driver never hears the knocking. But retarding timing decreases power and fuel economy. A higher-octane gasoline may improve the performance of knock sensor-equipped vehicles that have less power than when new."

I question their last statement, but they are trying to sell the stuff right? It is a classic "might" claim. "might", "might not".

Here is the link:

http://www.chevron.com/products/prodserv/fuels/bulletin/fuel_economy/

Mike

__________________
1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra