There are myths and truths to everything. To begin with, you are correct that the higher octane prevents knock and increases power(by virtue of allowing you to runn more spark advance than on a lower octane fuel). You are incorrect in believing that the higher octane improves mileage UNLESS your engine was pinging severly on the lower grade fuel.

There is little truth that higher octane fuels produce more deposits. Actually, there are probably less deposits since most of the majors over dose their premium grade gasoline with detergent based additives.

It is correct to say that the optimum sustainable performance/power from an engine occurs right on the verge of pinging.

And, it is correct to say that you can safely use lower octane fuel to run your car as long as you do not hear predetonation (ping) and are satisfied with any perceived degradation in power.

The problem with pinging is that you may be experiencing predetonation, but not be able to hear the audible pinging sound suggested. Past evidence suggests that engine life is markedly shortened if the engine continues to be operated in a pinging mode.

Mercedes has safesided the issue with its older cars by calling for premium fuel. In reality, you can probably get away with running midgrade (89 octane) with no perceived loss of power or reliability. Dropping to regular grade is much more risky.........particularly in an older car that does not have knock sensors.

Engines with knock sensors will retard engine timing when they "hear" engine knock, so there is little chance that you will cause engine damage by running regular fuel. I doubt that your 1985 vintage car has knock sensors, so you can't be certain if damage is occurring or not.

Assuming a price spread between gasoline grades of about 5 to 10 cents per gallon, your incremenal cost of running a car about 15,000 miles per year (assuming an average fuel economy of 15 mpg) is between $50 and $100......not enough reward to risk a loss in performance or reliability for your engine. Since the price spread between gasoline grades has remained in the 5 to 10 cent range, independent of the overall fuel cost..........one should think twice before switching to a lower grade of fuel.

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Ernie Rossi

1989 Porsche Carrera Targa
1995 E320 Special Edition
2000 Lexus RX300
1988 190E 2.3