Tuning Your Pontiac For Gasoline Density And Performance

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.