Gasoline Digest
What you need to know about fuel for Corvettes and other high-performance cars.
by Hib Halverson
©2002 all rights reserved
No use without permission

Image: Frank Hough

You’re leaning against your Corvette, holding the gas nozzle lever open, watching the counters flash. Sound familiar? Beyond reading the price and the octane on the pump, ever wonder about what goes into your Vette’s tank? If you have; read on. We’re going to cover some high-profile, gasoline issues of interest to Corvette enthusiasts.

Gasoline is a mix of volatile, flammable, liquid hydrocarbons. "Volatile" means it readily evaporates. "Flammable" means its vapor is combustible. "Hydrocarbons" are compounds of hydrogen and carbon. When hydrocarbons are burned in an engine, expanding gases apply force to its pistons and that’s what makes your car go.

Many hydrocarbons are in crude oil. To extract specific hydrocarbons that make gasoline, "crude" is processed or "refined" by one or a combination of: "distillation", "cracking," or "polymerization". The first boils crude in a vacuum to separate it into various factions, of which gasoline is one. The other two chemically modify hydrocarbons to give them desired properties.

Research for this article led The Idaho Corvette Page to Tim Wusz, Performance Products Engineer at The Phillips 66 Company’s "76 Performance Products Division". Wusz has worked for Phillips and its predecessors, Tosco Corporation, Unocal and Union Oil Company since 1965, spending much of that time developing racing gasolines. Wusz is a former professional drag racer, a long-time musclecar nut and a former Corvette ZR-1 owner.

The Basics of Blending

"A ‘barrel’ of crude oil," Tim told us, "is 42 gallons and about 50% ends up as gasoline. After refining, we’re left with gasoline ‘components’ or ‘blend stocks,’ segregated in tanks connected to a ‘blend header’ containing computer-controlled valves which meter flow from each tank. Gasoline is mixed in this blend header then subjected to quality control such as ‘on-line’ octane testing and sampling for chemical analysis.

"During blending, we select various hydrocarbons, depending what we want from the gasoline. For example, with gas for street applications, warm-up is critical. If the engine hesitates or dies during warm-up, exhaust emissions and customer satisfaction are impacted. For racing applications, we’re more concerned with resistance to vapor lock."

A typical oil refinery, the Phillips 66 facility south of Los Angeles in Wilmington, California This is a major source of gasoline for southern California. The tall tubular structures are distillation and cracking towers. Image: author.

To burn in an engine’s cylinders, gasoline must vaporize. "Volatility" describes gasoline’s tendency to vaporize. Pump gas is blended for slightly more volatility and better warm-up. Racing gas has less volatility to resist vapor lock. Gasoline blends are changed seasonally to give consistent drivability. In cold weather, volatility is increased and in warm weather, it’s reduced.

You might think cars stored during winter shouldn’t have full tanks because of vapor-lock problems when using "winter" gas the following summer. Tim Wusz told us moisture in an empty tank is a bigger problem than vapor-lock and he recommends storage with a full tank. Once spring arrives, it won’t take long to dilute the winter gas with summer gas.

We asked Tim about "shelf life." "Gasoline is designed for cradle-to-grave of about six weeks. In normal use, it is seldom stored for longer than that, however, it has enough stability so storage for a year is not a problem."

2000 Winston Cup Champion, Bobby Labonte, and all other NASCAR Winston Cup and Busch Grand National drivers depend on the quality of 76 Performance Products racing gasoline to give them the performance they need race 500 miles. In a pit stop near the end of the NAPA 500 at Fontana, California in April of 2002, Labonte’s #18 Interstate Batteries Pontiac, gets a dump can full of 76 Competition 110. Not only does Labonte want a racing gasoline with high resistance to vapor lock because NASCAR requires mechanical fuel pumps, but he also wants consistency in performance and continuity in blending. He gets all that and more with 76, the official gasoline of NASCAR. Image: author.

You’d think the challenge in blending gasolines would be quality control but, according to Tim Wusz, "The biggest problem is meeting government requirements for volatility, octane, oxygen content, distillation curve and other things. I don’t know if any one requirement is more difficult but the combination makes the blending engineer’s job challenging.

"California has its own requirements and is the only state that does. The rest of the country comes under slightly less restrictive, Federal requirements. At one time, we felt handicapped by California’s requirements, but now, gasoline for California is better performance-wise than gas sold in any other part of the country.

"This benefit carries to unleaded racing gasolines sold for street use, too. 76 Competition 100 is refined to California specifications and is capable of slightly better performance than if it was refined to Federal specifications."

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.

76 Performance Products Engineer, Tim Wusz, at work in the 76 Performance Products’ gasoline quality lab. Most racing gasolines get additional quality control verification steps over what’s done at a refinery. In the case of 76, steps are performed by Wusz at the Performance Products facility in Yorba Linda, California. Image: author.

By the mid-’90s, improvements in combustion chamber design, emissions control hardware and engine controls software accomplished HC and CO reductions, 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 will eventually have MTBE a piece of history. California Governor Gray Davis announced in mid-March 2002, that it would be outlawed in his state, effective Jan. 1, 2004. Other states will probably follow suit.

At this point ethanol’s longevity is pretty much assured. While cars built in the last 5-7 years don’t require oxygenates to achieve low emissions, misunderstanding or disbelief of that by environmentalists along with resistance to ethanol’s elimination by producers and that it’s the oxygenate-of-choice for RFG in areas where MTBE is to be banned will have it around a while.

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 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 is on the yellow sticker applied to a 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.) 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 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 going 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 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. Today’s aluminum heads have no problem because they use hard, steel valve seat "inserts."

Red Line’s Lead Substitute is the ideal solution for engines that don’t need octane any higher than that of pump gas but do need the valve seat protection once available with leaded fuel. Red Line’s other gasoline-related product, Complete Fuel System Cleaner, is a solvent-based fuel injector and fuel system cleaner that is one of the most effective products of that type on the market.
Images: Red Line Oil.

By the mid-’80s, car companies learned to better control emissions and refiners developed higher octane unleaded, so compression ratios began to climb, again. These modern unleadeds are blended with aromatic hydrocarbons, which raised their antiknock rating. To a lesser extent, the addition of oxygenates also increased octane.

If an engine using unleaded gas has heads lacking induction-hardened seats or hard seat inserts, "valve seat recession" may occur. Metallic oxides resulting from lead’s combustion leave a protective coating on valves and seats. Without that coating, when valve temperatures are high, during severe duty or extended operation with lean mixture, microwelding transfers softer seat metal to the valve face causing the seat to recede into the head. Wear debris cause additional damage to valve stems and guides. Exhaust valve rotators, used in some engines, accelerate valve seat recession. The end result is poor valve sealing and eventual cylinder head repair or replacement.

Heads lacking hard seats on Corvette engines burning unleaded will have durability better than urban legend leads many to believe. If you’ve got old heads on a show car or weekend cruiser that sees low annual mileage and rare high-rpm/high-load use; problems from valve seat recession are unlikely. If the car sees high annual mileage or you drive it hard on a regular basis, then you need to either: retrofit hard seats to your heads, install later heads with hard seats or add something to gasoline to inhibit valve seat recession.

There are pour-in additives that address this problem. The Idaho Corvette Page has test data showing Red Line Synthetic Oil Corporation’s "Lead Substitute" is an outstanding solution to valve seat recession. Lead Substitute uses a sodium-based chemistry which forms sodium oxides upon combustion offering protection similar to that of TEL. Red Line Lead Substitute comes in 12-oz. bottles, is mixed 1-oz. per 10 gallons of fuel and should be used in every tank of gas for best protection.

Octane boosters–do they work?

While a few canned, liquid, "octane boosters" can raise octane, their practical benefits are often overstated. Common claims by these products’ manufacturers are: they increase octane by so many "points." These "points" are seldom quantified, but the implication some manufacturers hope consumers will make is: one point equals one octane number. Sometimes these "points" are tenths of an octane number, ten times less than what some consumers may believe.

Manufacturers may not admit the tests supporting their performance claims were done with 87 octane gas. Most boosters, if effective at all, work better with regular gas than with the higher octane, premium unleaded to which most people will add them.

The Idaho Corvette Page interviewed Jim Bell, who runs Kenne-Bell Performance Products, a manufacturer of aftermarket supercharger kits. In developing blower kits and supporting the customers who buy them, Kenne-Bell has tested many octane boosters. "Don’t waste your time with boosters that use alcohol because they don’t do anything," Bell told us. "The ones that say they have lead in them, don’t work, either, because the amount of lead is so small. The only boosters we’ve found to be worthwhile are those that use MMT and one we recommend is the NOS brand."

MMT stands for–try this tongue twister: methylcyclopentadienyl manganese tricarbonyl. Once viewed as a possible replacement for TEL, while not as potent, it still increases octane and, in large quantities, can eliminate valve seat recession. While it’s used in Canada, MMT is ignored by refiners in the U.S. in favor of other antidetonants, mainly because it’s illegal in areas where RFG is required and a few refiners feel its long-term use might compromise engine life. MMT is shunned by car companies due to durability concerns about components in on-board diagnostic and emissions control systems and questioned by the EPA as a possible health hazard. Nevertheless, in 1995, MMT was allowed on the U.S. market with some restrictions after its manufacturer won a Federal court case against the EPA. At this writing almost no U.S. refiners add MMT to gasoline, but it is the key ingredient in a few canned octane boosters.

The Idaho Corvette Page acquired octane booster test data from an independent research laboratory. The first test was straight 92-octane unleaded gas from a Chevron station in southern California. It tested at 96.3 RON and 88.3 MON for an R+M/2 rating of 92.3, 0.3-oct. higher than the rating on the pump. To a second sample of Chevron 92 from the same station, the lab added "104 Octane Boost". The octane of the gasoline modified with this booster was unchanged. The lab tested a third sample of Chevron 92 and NOS brand "Street Formula", a MMT octane booster, mixed 1:170 (12-oz. bottle in 16 gal. of gas). The results were: 96.8 RON, 88.4 MON and 92.6 R+M/2, a measurable change but, clearly, as the MON went up only 0.1-oct, not a practical improvement. NOS’ most potent booster, "Racing Formula", another MMT-based product, in Chevron 92, tested at 98.5 RON, 90.4 MON and 94.5 R+M/2, a credible but modest improvement.

Before we get farther into testing, we should advise the reader that some of the research for this article was done in 2001, just before a change in premium unleaded fuel in the western United States from 92-octane to 91-octane. Some of the testing done for this article was with 92-octane fuel, however, some additional testing and price research was done with 91-octane fuel. We apologize for the confusion but, unfortunately, we had no control over it.

That NOS octane booster lab-tested reasonably well intrigued us enough to give it a practical test. We picked a 1995 ZR1. In hot weather, the ZR1’s LT5 engine, when run on premium unleaded pump gas, will have spark retard as a result of knock sensing. We demonstrated this running the car on the Dynojet 248H chassis dyno at K&N Engineering while monitoring the engine controls data with a Vetronix Mastertech scan tester. The engine intake air temperature (IAT) was 108 degrees F. Between peak torque and peak power, the Mastertech showed 5-8 degrees spark retard on each of several dyno tests.

The car had 15 gallons of 76, 92-oct. 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 K&N Engineering’s Dynojet. 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 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 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 cats. 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.

That some automotive manufacturers believe MMT causes problems with the second generation on-board diagnostics (OBD 2) on ’96 or vehicles has us concerned about long-term use of MMT boosters in OBD2 engines. We would not use a MMT octane booster in a Corvette with OBD2. In fact, we’re not even comfortable with long-term, regular use and any vehicle with 02Ses and cats, OBD2 or otherwise.

While some of them have a practical benefit, the economics of octane boosters aren’t very good. We priced NOS Racing Formula on the Internet and at retail vendors. It averaged $12.99 per bottle. At time this article was posted on the Internet, 91-oct. was going for $1.69.9@gal. at local gas stations and we found a 76 station selling 100-oct. unleaded for $3.85@gal. Sixteen gallons of 91 boosted to 94.5-oct. with a bottle of NOS ran $46.75. Sixteen gallons of 94.6 octane gas mixed 3:2, from 91 and 100 unleaded cost $40.95.

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