The gasoline you use to power your Corvette down the road is blended with several different goals in mind. The blend of gasoline is changed for the seasonal weather changes, plus the federal, state, and local environmental regulations that may mandate the use of reformulated gasoline. There is also a push to increase the use of ethanol or alcohol-based additives to both reduce fuel-related emissions and lessen our dependence on foreign oil. This means the blend of gasoline you use will not only vary from brand to brand, but also will change by the season and the part of the country you buy your gas in. As the level of ethanol or other alcohols is increased, the fuel efficiency of the engine will decrease. The fuel efficiency will decrease because gasoline has more energy per gallon than ethanol or alcohol-based fuels. An engine running on alcohol-based fuel runs at a richer air/fuel ratio than an engine running on gasoline. The stoichiometric or chemically ideal air/fuel mixture for an engine running on gasoline is 14.7/1, for an engine running on E-85 (85-percent ethanol and 15-percent gasoline), it is an air/fuel mixture of 10/1; an engine running on ethanol is an air/fuel mixture of 9/1.
A modern fuel-injected engine computer will make the air/fuel mixture adjustments needed so the engine will run its best with the blend of gasoline you are using. The ethanol and other alcohol-based additives that are blended into today's gasoline will cause a noncomputer-controlled carbureted engine's air/fuel mixture to go leaner as the percentage of alcohol-based additives is increased. The reformulated gasoline of today burns at a different rate than the leaded gasoline of days past and will cause the air/fuel mixture to be at least 3- to 5-percent leaner than it was with leaded gasoline. (Please see the tech articles in our June and July '06 issues on tuning a noncomputer-controlled carbureted engine to run on reformulated gasoline if you want more information on this subject.)
Power-Robbing, Fuel-Related Deposits
As your engine consumes the fuel it uses to make the power that moves your Corvette down the highway, deposits are building up in the combustion chamber and on the engines valves that will reduce the engine's power. Most of the gasoline that you buy at your local gas station has additives that are designed to reduce the formation of these power-robbing, fuel-related deposits. However, it may not be of high enough concentration to prevent or remove any existing deposits. The use of a fuel-system cleaner, such as Techron Concentrate Plus from Chevron, on the same schedule that you change your engine's oil should be part of normal vehicle maintenance on both fuel-injected and carburetor-equipped engines. A high-quality fuel-system cleaner will help clean and prevent the carbon deposits that form in the combustion chamber and on the intake valve as the engine burns the gasoline it uses to make power.
The fuel-related carbon deposits that form in the combustion chamber and on the intake valve can create a wide variety of engine-performance problems. The engine will produce less power as the deposits on the intake valves and intake ports restrict the airflow into the cylinder. These carbon deposits can also absorb gasoline like a sponge when you are starting a cold engine, and thus cause the engine to have extended crank times before it finally starts. The carbon deposits that build up on the piston and in the combustion chamber can cause the engine to have detonation or pinging problems. This will cause the knock-sensor system on a computer-controlled, fuel-injected engine to retard the timing.