What Classic Car Restorers Need to Know about Gasoline
The Clean Air Act of 1990 resulted in regulations which forced the introduction of "reformulated gasoline" in, at first, nine states where air pollution — specifically ozone — was at its highest levels.
Gasoline is a very complex "witches' brew" of compounds which vary widely in their physical and chemical properties, and these properties need to be carefully controlled in order to affect pollution amounts. The fuels now in widespread use are known as "reformulated" fuels (RFG).
There are many specifications involved in the production of gasoline: Antiknock Index (Octane), Volatility (Vapor to liquid ratio), Copper Corrosivity, Stability, Sulfur Content, Metallic Additives, Temperature for Phase Separation (Water tolerance). The two most important of these, when it comes to driveability, are Octane and Volatility.
Octane in fuel used to be measured in the laboratory by a designation known as RON, or Research Octane Number, and those ratings are the familiar numbers at the gas pump 10 or more years ago. Another, equally legitimate method of measurement is the Motor Octane Number. These ratings typically are 8-10 numbers lower than RON and were historically not used at the gas pumps. Today, all gasoline octane ratings are an average of
RON and MON, hence are lower than what we used to see. Eighty-seven octane of today is the same as 92 octane of years gone by!
Tetraethyl Lead used to be the additive which was used to increase octane. After 1971, fuels gradually phased out lead additives and they are now only found in some farm equipment, aircraft and racing fuels. Reformulated gasolines do not use lead additives but do use oxygenates (alcohols or ethers, including ethanol, methyl tertiary butyl ether, tertiary amyl methyl ether, ethyl tertiary butyl ether) and these additives are sometimes blamed for problems, in most cases incorrectly.
Volatility of fuel is the second important factor for driveability. Too low a volatility level means poor cold starting, warmup, cool weather performance and increased combustion chamber deposits. Too high a volatility means higher evaporative emissions, vapor lock and poor fuel economy. Gasoline volatility levels are now tightly controlled with additives which are blended for seasonal temperature levels and altitudes. While these seasonal blendings minimize problems, they cannot eliminate them in such situations as unseasonably warm spring and fall weather, etc. Carbureted vehicles will experience the greatest driveablilty and starting problems under these conditions. It should be evident that in cars that are driven only periodically, fuel storage is a concern. It's best to run out each tank of fuel within 60 days of filling.
The other specifications of gasolines — corrosivity, stability, metallic additives, temperature for phase separation — affect fuel system corrosion, storage life, emissions, catalyst deterioration and water tolerance, in that order. These specifications are so tightly controlled today that there should be no concern on the part of any vehicle's owner, whether the car is new or old. Reformulated fuels are not different in composition, just in the acceptable range of each ingredient's volume or weight.
A few myths have grown around the public's knowledge of gasolines. Here are some:
Myth: The greater the octane rating, the more power the fuel will produce.
Truth: Octane-boosting substances in gasoline actually retard the burning rate so that the heat of compression won't pre-ignite the mixture, causing "knock", and there is no performance improvement measurable by using higher-than-specified octane fuel in your car. The engine's compression ratio dictates the octane requirement. No engine (older than 1980) with a compression ratio of 10:1 (that's quite high) should need fuel with higher octane than 100 RON. That's 94 octane in today's rating system. RON octane requirement is reduced by 5 points with each 1 point of compression ratio (so, 9:1 requires 95 RON or 89 at today's pumps) Octane in fuel is no different than it ever was. It's just measured differently.
Myth: Reformulated fuels cause bad mileage.
Truth: In the worst case, reformulated fuels might get 3 to 4% less mileage than in the past, but running your tires at 10 pounds less than specified pressure will do the same.
Myth: Reformulated fuels cause driveability problems.
Truth: No reliable evidence exists for widespread problems under regular use. People tend to assume a specific problem is due to fuel when so many other factors can be the cause. Scientific testing has yielded no such problems.
Myth: Reformulated fuels deteriorate fuel system parts and cause vapor-lock (hard starting in old, "classic" cars).
Truth: The new fuels are compatible with all modern and most older fuel system parts. Restored cars have experienced failure of old-style, synthetic rubber gaskets and diaphrams, but newer replacements are available. Vapor-lock is and was always a problem with certain old cars and is not any more so with the new fuels, unless stored in the car for long periods. All fuel systems, old and new, are susceptible to corrosion of metal parts, especially the fuel gauge sending unit, due primarily to sulfur.
Myth: Gasoline has a short shelf-life and will turn to "varnish" over time.
Truth: This used to be somewhat true, especially when lead was used in fuels. Today's gasolines will last several years without too much problem, but the volatility additives will leach out. It is better to use the fuel quickly or add some aftermarket substance, such as StaBil, to insure better starting and driveability.
Myth: The new gasolines cause more water to be precipitated in the tank and foul up the system.
Truth: Actually, water appears in the tank because of temperature changes over time. The new fuels are more likely to suspend moisture and carry it out of the fuel system because they contain oxygenates, which are alcohols.