Tuning Your Pontiac For Gasoline Density And Performance

The density of the gasoline directly impacts its heating value or energy content. This is defined as the heat released when a known quantity of fuel is burned under specific conditions. In the United States, the heating value is usually expressed as British thermal unit (Btu) per pound or per gallon at 60 degrees F.

For gross heating value, the water produced by the combustion is assumed to be recondensed to a liquid. For the lower net-heating value, the water is assumed to remain a gas. Because all engines exhaust water as a gas, the net-heating value is appropriate for comparing different fuels.

In most parts of the country, street gasoline is diluted with 10 percent ethanol, a grain alcohol derivative. This is done to reduce emissions and replaces methyl tertiary butyl ether (MTBE). Ethanol has approximately 27 percent less energy per gallon than pure gasoline. Though gasoline's energy content varies with density, an average value of 115,000 Btus at 60 degrees F is considered commercially acceptable. The same gasoline with 10 percent by volume ethanol has only 111,100 Btus, or a 3.4 percent reduction in energy. It must be noted that these are average values. If the fuel you purchased happens to be at the lower range of the specific-gravity scale (0.720), the energy content is considerably less.

Because gasoline is sold by volume (gallons), it's customary to express heating values per unit volume, specifically Btus per gallon. This is the value that correlates with fuel economy because fuel usage is expressed per unit volume as miles per gallon.

The density of the hydrocarbons in gasoline varies over a much wider range than their heating values per unit weight. Consequently, compositional changes that result in density differences are accompanied by alterations in heating value per unit volume. If the density of gasoline decreases, its heating value or power produced per unit volume also decreases.

Modern pump gas has oxygenates added that also decrease the heating value. Oxygenates have a lower heating value than hydrocarbons on either a unit-weight or unit-volume basis. The decrease depends on the amount and identity of the oxygenates.

In practical terms, your Pontiac will produce less power and go a shorter distance on a gallon of fuel with lower density. For this reason, you must recognize the density of the fuel and make changes to the carburetor jetting to obtain the best track performance.

When considering the flow of fuel through a carburetor jet, the coefficient of discharge for normal hydrocarbon fuels is virtually constant above a critical value of the Reynolds number. This defines the proportional ratio of the inertial force to viscous force in a flow system. The critical Reynolds number corresponds to the transition from turbulent flow to laminar flow as the velocity is reduced.

Mass flow is a function of the density of the fuel. Increasing the density increases mass flow and lowers (richens) the air/fuel ratio. This is partly offset by the lower level in the float bowl that occurs with heavier fuel. Therefore variations in the density of the fuel influence the air/fuel ratio unless there is a change in carburetor jet size to counteract the diverse specific gravity. Since an engine performs best at one given air/fuel ratio, the density must be checked for the gasoline in the tank. The engine digests fuel and air as a mass even though we look at fuel as a volume, so the lighter the fuel, the more jet the carburetor requires to create the same air/fuel ratio. Conversely, if the engine is tuned with lighter fuel and then run at the track with heavier gasoline, the jet(s) need to be smaller or the mixture will be richer than optimum.

Checking Fuel Density - Fortunately, checking fuel density is not hard thanks to the efforts of Kinsler Fuel Injection and the company's fuel analyzer kit. Packaged in a wooden case, the fuel test kit includes a glass sample cylinder, two hydrometers, a thermometer, and two small sample bottles. The temperature reference chart we show in this article is included. For approximately $220, the base kit is a good investment for any Pontiac racer.