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
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."
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.
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."
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.
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
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),
MTBE in RFG
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,
(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
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.
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.
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.
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.
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
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
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
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
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.
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."
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
MMT stands for–try this tongue twister:
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
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
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
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.
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.
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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