Additives, other than gasoline components, are blended in for special purposes. Typical additives are: anti-oxidants and metal deactivators (both inhibit gum formation and improve stability), deposit modifiers (reduce deposits, spark-plug fouling and pre-ignition), surfactants (prevent icing, improve vaporization, inhibit deposits and reduce certain emissions), freezing point depressants, corrosion inhibitors and dyes (for safety or regulatory purposes). Additives are used in very small amounts, usually 50-100 pounds per thousand barrels of gasoline.

A major difference between many pump gases and most racing gas are oxygen-bearing chemical compounds, or oxygenates. “Two oxygenates are used.” Tim Wusz told us, “One is MTBE–Methyl Tertiary-Butyl Ether–and the other is ethanol. MTBE is an ether compound and ethanol is an alcohol. Both were originally blended into gasoline to increase octane but later found to reduce exhaust emissions. Both have oxygen in their molecules and reduce emissions by enabling more efficient combustion.”

The Clean Air Act of 1990 mandated oxygenated gasoline and its newer, more complex sibling, reformulated gasoline (RFG), in parts of the country with air quality problems. The intent was hydrocarbon and carbon monoxide emissions reductions beyond those possible by ’80s emissions controls alone. Widespread distribution of gas oxygenated with MTBE began in the early-90s but, in the late-‘90s, after MTBE’s classification as a “possible human carcinogen” and being documented as a ground water pollutant, public opinion built to eliminate it.

By the mid-’90s, improvements in combustion chamber design, emissions control hardware and engine controls software accomplished significant HC and CO reduction, in spite of RFG. With the amount of pre-mid-90s vehicles on the road decreasing, RFGs are becoming a solution looking for a problem. That and political pressure has, in most states, made MTBE a piece of history. California outlawed it on Jan. 1, 2004. A dozen other states have also outlawed it since then and the rest of the state will follow in 2005.

Enviromentalists never give up, so ethanol has become the new mainstream oxygenate and it’s here to stay. California requires at least 5.8% ethanol in pump gas. Other states may differ but, generally, ethanol content varies from 6-to-8 percent and goes as high as 10%. While cars built in the last 5-7 years don’t require oxygenates to achieve low emissions, ignorance or disbelief of that by environmental activists along with resistance to ethanol’s elimination by its producers and that it’s the oxygenate-of-choice for RFG in areas where MTBE has been be banned make its presence in gasoline inevitible and permanent.

A downside of RFG can be overly-lean air-fuel ratio when burned in 1970s Corvettes with stock carburetors having lean calibration to reduce emissions. Performance and drivability may be compromised by existing calibration combined with additional leaning due to RFG. The solution is to slightly richen the calibration. Engines built before the late-’60s often ran slightly rich and have little problem using oxygenated fuels.

Two additional caveats apply to ethanol RFG in some older vehicles. It may release scale from the walls of old fuel tanks. The solution is either frequent replacement of fuel filters while the scale is going through the system or a new tank. Ethanol is incompatible with some types of fuel hose. If hoses date to before 1985, replace existing hose with modern products, all of which are compatible with RFG.

What’s Up with Octane

In the combustion chamber, after the spark plug lights the air-fuel charge, a “flame front” burns away from the plug. This burning needs to be controlled if the engine is going to perform well and last a long time.

“Detonation” is rapid, uncontrolled combustion. It occurs after ignition, when the unburned charge ahead of the expanding flame front is compressed to the point of auto-ignition. If a significant amount of unburned charge auto-ignites, detonation will be audible and will generate intense pressure waves which cause the chamber walls to vibrate. You hear that as a knocking or pinging sound. This pressure builds quickly, before the piston reaches top dead center. When downward force builds before the piston changes direction, stress on it and other parts is significant as is the performance loss. Detonation also sends combustion temperatures soaring. The stress and temperature make even moderate detonation problems capable of damaging the engine in your Corvette.

 “Octane” or “antiknock rating” is a measure of a gasoline’s resistance to detonation. Two ratings are common: “research octane number” (RON) and “motor octane number” (MON). Tests for both use a single-cylinder engine having a variable compression ratio. The engine is run on a gasoline to be rated and the compression ratio is varied to obtain a standard knock intensity measured by an electronic knockmeter. The octane of the sample is determined by comparing its knock tendency with that of reference fuels having known octane numbers.

The MON test, because of faster engine speed, higher mixture temperature and variable spark timing, better simulates conditions in an automotive engine and is, Tim Wusz told us, “... more relevant to the enthusiast trying to understand gasoline. In a real world engine, MON is necessary at wide-open throttle. It is an important number for high-performance engines since they spend a high percentage of their lives running at high speed under high-load.”

The Federal Government requires octane of gas sold for road use be rated by an average of RON and MON (“R+M/2”) and that number must be on a yellow sticker applied to the gas pump. In many places, regular unleaded is 87-octane, mid-grade is 89 and premium varies from 91 to 93 octane. In high-altitude areas, you’ll find lower octane gas. “Pressure and temperature in the combustion chamber are less at altitude.” Wusz stated. “Engines needs less octane so Government allows lower octane fuel in counties having a large majority of their territory above 4000 feet.”

Will high-altitude gasoline damage an engine requiring a higher octane? Not if you stay in the high country or, if you drop below 4000 feet, you don’t run the engine hard until the high altitude gas is used or diluted with “sea level” gas.

Do refiners save money selling lower octane fuel in mountain areas? Nope. They loose any savings in transportation costs. Most refineries are near coastal areas so most gasoline is transported to interior states. The farther it’s moved, the more expensive it becomes.

So...how much octane do you need?

Only enough to keep your Corvette’s engine out of detonation. More than that offers no performance advantage. How do you determine an engine’s detonation threshold? By testing and the first test instrument is your ear. If you hear detonation at wide-open throttle, you have a problem. If there’s no engine-related trouble (ie: too much spark advance, lean mixture, etc.) causing the detonation, then you need more octane.

Plug reading can also identify a detonation problem. If you see tiny flecks of aluminum on the plugs, that’s evidence of detonation. Severe detonation may melt electrodes and/or crack the center insulators.

All 1982 or later Corvettes, have feedback control of spark advance and will be equipped with a knock sensor (KS). If the KS “hears” detonation, the engine computer’s software retards the spark enough to stop the detonation. The ’82 or later cars offer access to the computer’s serial data stream for diagnostic purposes. You can use a “scan tester” (either software-based, such as “Diacom”, or a dedicated piece of hardware, such as the Vetronix TECH 1A, TECH 2 or Mastertech) to view the KS signal and/or the spark retard value. If you can read either of those on a road test and you see detonation with the tester and know there’s no engine-related trouble causing it; then you need higher octane gas.

Get the Lead Out.

The octane and valve-seat-durability enhancing qualities of alkyl-lead compounds (chiefly tetraethyl lead, aka: “TEL” or just “lead”) were discovered in 1922 by General Motors. By the late-’20s, “leaded” gas became available and, by the early ’50s, TEL was in virtually all gas sold in the U.S. By the late-’60s, “super premiums” averaged 3.5-grams TEL per gallon and were around 100 RON. While TEL was a cheap way to improve engine performance and durability, it is toxic, both unburned and in lead-oxide-particulate form in exhaust gases.

In the late 1960s, concerned with environmental effects of TEL, the Federal Government legislated its phase-out. Gasoline retailers had to make unleaded gas available by July, 1974. Following that, stepped limits on alkyl-lead (“low-leads” of the ’70s and ’80s) were enacted starting with 1.7 grams per gallon in 1975 and dropping to 0.1 g/gal. in 1986. To meet emissions regulations for model year 1975 (MY75) some manufacturers added exhaust system catalytic reactors to their vehicles and GM did that with Corvette. By MY80, all light duty vehicles had them. Also known as, “catalytic converters”, “catalysts” or just “cats”, these reactors convert pollutants to non-polluting substances. This ability is destroyed by alkyl-lead so any catalyst-equipped vehicle had to use unleaded gas. As a result, leaded gas use peaked in 1977, then declined significantly. In December, 1995, use of gasoline with more than 0.05-gram alkyl-lead per gallon (in a practical sense, all leaded gas) by any on-highway vehicle built after MY75 was outlawed.

Lead’s phase-out forced octane reductions. While it didn’t cause detonation in new Corvette engines, because, starting in MY71, GM had already lowered compression ratios as a technical answer to government’s restriction of oxides of nitrogen (NOx) emissions; it did cause detonation in ’50’s and ’60’s high compression engines. The phase-out, also, eliminated alkyl-lead’s valve seat protection, forcing an industry-wide, cylinder head manufacturing process revision: the addition of induction-hardened seats to the cast iron heads common back then. Aluminum heads have no problem because they use hard, steel valve seat “inserts.”

The car had 15 gallons of Chevron, premium unleaded in it when we added one bottle of NOS “Racing Formula”, drove it 5 miles to mix the booster thoroughly then put the car back on the Auto-Dyn. This time, in spite of the IAT climbing to 115°F, the Mastertech showed a maximum of two degrees retard and, on three of six passes, it read no spark retard at all. Run #5 was the best with power at the rear wheels up almost nine horsepower because the gasoline’s octane was, now, just high enough to tolerate full spark advance. Clearly, boosters with enough MMT to be effective are good for occasional, limited increases in octane. Applications might be: 1) a stock, pre-’71 Corvette having a compression ratio between 9.5:1 and 11:1, 2) a late-model C4 or C5 modified with a low-boost, streetable supercharger kit or a “mild” nitrous oxide injection system or, 3) a late-model car, such as our ZR1, that experiences loss of performance on hot days when its engine controls retard timing due to detonation. If you want to make pump gas into “racing gas” for engines with 11:1 or more compression, high-boost superchargers, big doses of nitrous or any engine run on a race track at sustained high-speed/high-load; forget it. No canned octane booster in any quantity will fail to stop detonation under those conditions. If you “read” spark plugs to tune your engine, the redish-brown MMT residue makes useful readings impossible. Levels of MMT octane boosters just moderately beyond the recommendations of booster manufacturers will foul spark plugs, damage oxygen sensors (O2S) and plug catylitic converters. High percentages of MMT contaminates engine oil and leaves hard metallic deposits in the combustion chambers, piston tops and upper end of the cylinder walls such that engine wear is greatly accelerated. Do not use them in concentrations higher than suggested by their manufacturers.

NOS Racing Formula should be mixed one bottle to about 16 gallons of gas. Lab testing and our dyno test showed this product to be a useful octane booster but, in most cases, not as economically attractive as mixes of racing gas and pump gas. Image: author.

Continued - Next Page >