Higher octane fuel WILL produce more horsepower if it allows more ignition advance. Same BTU content, but the engine develops more power with more advance on the same amount of fuel.

However, if you already have full advance, there will be no advantage to higher octane fuel.

In old, dirty engines, higher octane can help because there is so much crud in the combustion chamber the compression ratio is increased and you get knock on lower octane fuel. Knock robs horsepower, so milage goes down.

Inside those limits, there can be an increase in power and fuel milage. Higher octane fuel in a properly running engine that does not require it will be wasted money, and can in fact cause running problems from poor combustion.

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!

I tried different octanelevels on my previous car..
My conclusion (I drive a lot!) is that the M102 (230E) was smoother, a tad bit faster and had lower fuel consumption with 98 octane vs. 95 octane.
The difference in price between 95 octane and 98 octane is appr. 6 cents pr. liter gasoline (1 gallon is 4,5 litres)..(6 cents is half a norwegian krone)

In my opinion I rather pay a little bit more, and have a car that runs smoother, and at no time experience engine knocking..



ES