According to Webster's College and Home Dictionary, the noun "enthusiast" is defined as: one who is filled with enthusiasm; one who thinks himself to be inspired; a visionary; fanatic. Not to be an apologist for this primer on fuel density, but if you don't fit the definition of an enthusiast, skip these pages and move on to the other exciting content High Performance Pontiac has to offer this month.
Understanding, checking, and tuning your Pontiac for fuel density might be on the fringe for some readers. But for those who are detail oriented and want to gain every last bit of performance and/or consistency from their Pontiac racer or street/strip car, adjusting for fuel density, as with other technical topics that have been represented in HPP, is as important as setting the ignition timing.
A Quick Lesson In Gasoline
The unleaded gasoline burned in a street Pontiac is considered a commodity and thus traded in the marketplace. There are many different brands of gasoline, and they can be broken down into two distinct groups: producers and marketers. What is referred to as "no name" or "unbranded" gasoline is available through companies that are strictly gasoline supermarket/department stores, which have no manufacturing or refining capabilities. These marketers purchase refined gasoline and distribute to the end user. In contrast, the established oil companies, such as Exxon/Mobil, Chevron, Texaco, Shell, and so on, have the ability to complete the entire process, from drilling for crude oil to filling the tank of your Pontiac.
There are a few aspects of gasoline that aren't common knowledge which need to be understood. Since there isn't a refinery on every street corner, oil companies purchase what is known as "base product" from each other. As an example, in your part of the country, most of the refined gasoline may come from one facility even though it's sold as different brands. Gasoline is made brand-specific by the additive package that is blended into the base product, which is usually done at the distribution depot.
Recently a good deal of refined gasoline is imported from other areas, mainly the Middle East and South America along with Canada. This is due to environmental laws that have prevented building a new refinery in America since approximately 1975. So, even though you may buy your gasoline from the same pump at the same gas station, there is a good chance that each delivery the retailer receives was produced at a different refinery and from a variety of crude oils. The only constant may be the additives, if any.
The old question now arises, is there a difference between name-brand and off-brand gasoline? The simple answer is yes. Federal regulations require that all gasoline in the United States contain a deposit control (DC) additive. The lowest additive concentration to pass all the EPA-required tests is called the LAC level. In most instances, off-brand gasolines are at the LAC level. As a result, name brand fuels with higher additive levels provide better deposit control than cheaper gasoline with minimal additives. In addition, fuel such as Chevron with Techron and others like it use DC additives that are more effective, have fewer side effects, or are mixed at much higher levels. So when it comes to gasoline, there is a reason why unbranded fuel is usually cheaper: It doesn't have the extra additives.
DC additives are not to be confused with octane. This term describes the fuel's ability to resist combustion through pressure and heat and wait for ignition through the arcing of the spark plug. Higher-octane fuel can withstand higher temperatures and pressures. For a more in-depth analysis of octane and its impact on detonation, reference "Knock, Knock ... Go Away," HPP, Sept. '06.
What Is Density?
When discussing gasoline, density can be described as the mass of a unit volume of material at a selected temperature. For example, the density of water is 0.9990 gram per cubic centimeter at 60 degrees F. Relative density (RD), also called specific gravity, is the ratio of the density of the subject gasoline at a selected temperature to the density of a reference material at a selected temperature. For the relative density of petroleum products in the U.S., the reference material is water at 60 degrees F.
The American Petroleum Institute (API) often uses API gravity instead of relative density. While API gravity measurements may be made on liquids at temperatures other than 60 degrees F, the result is always converted to the value at 60 degrees F, the standard temperature. API gravity is inversely proportional to relative density. API gravity increases as relative density decreases.
For our purposes, we need to understand the density of the fuel measured as a specific gravity corrected to 60 degrees F and not be concerned with the API value.
Unleaded street gasoline, regardless of the octane rating, should have a specific gravity of 0.720-0.780. As was established earlier, no two loads of fuel delivered to a gas station have the exact same density, so the Pontiac hobbyist needs to recognize this fact.
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.
Once the specific gravity of the fuel is identified, a decision for jet tuning can be made. As with determining and using air density ("Weather or Not You Go Faster," April '06 issue), good notes and a baseline need to be determined. For example, if you ran your Pontiac at the track with 0.712 fuel and the best performance was obtained with a certain jet, use that as a baseline. On your next visit, the fuel checks at 0.736, so a jet change is in order or the engine will be richer than required for peak power. The calculation is:
0.712 – 0.736Using a carburetor manufacturer's jet-area chart (Demon Carburetion has one in the company's catalog), decrease the jet area by approximately 3.25 percent.
If your Pontiac requires dedicated race fuel, it is also important to acknowledge the specific gravity of the gasoline. Upon request, racing fuel dealers provide an information sheet on the fuel they sell. Due to the small-batch production of race gas, along with a higher level of quality control, huge differences in specific gravity on a load-to-load basis are unusual. If you're a traveling racer, then it's important to check and adjust for variations in specific gravity of different brands of race fuel. For example, if you race in New Jersey using Sunoco Cam 2 and travel to an event in Ohio where only VP is offered, it's good practice to check the density of the unfamiliar gasoline.
The actual fuel-check procedure is better explained with images, so follow the photos and captions as HPP shows you another trick to keep your Pontiac ahead of the competition.
Carburetor Icing
Carburetor icing occurs when the intake air is chilled below the freezing point of water by the vaporization of the gasoline. The ice forms on the throttle blade(s) and in the venturi and can cause the engine to stall as soon as the gas pedal is released to idle. This ice formation restricts any air from passing by the throttle plate(s), choking the engine.
Icing can be acute when the air is moist (70 percent or higher relative humidity) and the ambient temperature is between 35 degrees F and 55 degree F. These weather conditions are common during the fall, winter, and spring in many parts of the country and can last into early summer in coastal regions. Carburetor icing is lessened when the intake air is much below freezing since it's common for the humidity to be less.
The extent of carburetor icing doesn't depend on the weather alone. It also involves carburetor and vehicle design and the mechanical condition of the engine, in particular, the components that affect warm-up time. Thermostat, choke, intake-air heaters, and heat risers all help to eliminate carburetor icing. Part of the solution also involves gasoline volatility.
A good index of the tendency of a gasoline to cause carburetor icing is the 70-percent evaporated-temperature specification in the distillation profile. The lower this temperature, the more severe the icing.
Carburetor icing is not as big a problem as it used to be. For emission control reasons, most carburetor engines built since the late '60s are equipped with intake-air heating systems, which generally eliminate carburetor icing. But keep in mind that many aftermarket intake manifolds and headers eliminate these components. Also, race gasoline, which is not designed for cold-weather startability, has a tendency to create more icing than street fuel.
The symptom of carburetor icing is the engine loading up shortly after its started, much in the same manner as with a faulty choke pull-off or stalling when cold and the throttle is released.
