What is "Octane"?
Written by Paul Huizenga
There is a lot of confusion about what the "octane number" on the gas pump means,
what higher octane numbers really mean, and why some engines need or can take
advantage of a higher octane fuel.
In a nutshell, the octane rating of a fuel is a measure of its ability to resist
detonation, ping, pre-ignition, or knock. Although purists will quibble, all of these
terms can be described as an incorrectly-timed burning of the fuel/air mixture in the
combustion chamber. Any motor fuel will spontaneously ignite under the right
conditions - too much compression, hot spots from leftover carbon deposits,
excessive boost, too much ignition advance, or a number of other causes can provide
the source of ignition. When the flame front from this ignition event meets the one
created by the spark plug, the result is sky-high cylinder pressures and a destructive
shock wave that can shatter spark plug insulators, hammer rod bearings, break
rings, or even put a hole in the piston. The octane number is a way to express the
resistance of a fuel to this type of unwanted ignition.
Octane Myth #1 - Higher octane fuel is "more powerful" and "burns hotter"
The difference in total energy between high octane and regular gas is insignificant. In
fact, race gas often has slightly less energy by weight than pump gas! High octane
fuels are generally harder to ignite, and burn more slowly than ones with low octane.
In some cases, an engine optimized to run on 87 octane gas will produce less power
when fed racing gasoline due to the slower ignition and combustion characteristics of
the high octane fuel.
The number we most often associate with octane is the "Anti-Knock Index", or the
"Pump Octane" number. This rating is an average of two different measurements;
the Motor Octane Number, and the Research Octane Number. Both of these
measurements are taken using a special single cylinder test engine that has a
variable compression ratio. The RON measures the knock resistance of a fuel during
low RPM, light load conditions, while the MON is representative of high-speed, high
load operation. As a result the MON will always be lower than the RON, but for our
purposes the Motor Octane Number is more significant because it represents the way
racing engines are operated.
Octane Myth #2 - Putting premium and/or race gas in my car will give me better
performance
Engines are designed to run on gas with a certain AKI. Putting premium in a car that
was designed to run on regular gas and doesn't knock is just throwing money away.
If an engine doesn't knock on 87 octane gas, there is no advantage to using gasoline
with a higher octane number. However, many engines with electronic controls have a
knock sensor as part of their computer system. This sensor actually listens for knock,
and can change the engine timing or even cut boost in a factory turbocharged car
when it detects knock. Some anti-knock systems are sensitive enough that it's
possible for them to reduce the performance of the engine without you ever hearing
audible detonation. The bottom line - Read your owner's manual to determine the
correct octane rating for your car and see if it can take advantage of premium fuel.
Obviously, an engine's octane requirement can change. Increasing the compression
ratio, adding a turbo or supercharger, changing the air/fuel ratio, or advancing the
ignition timing can all cause knock. Even without modifications, over time
combustion chamber deposits build up and can lead to a need for higher octane fuel.
The best way to determine what your engine needs is to listen to what it's telling you
- Audible knock is a sign that damage to the engine is imminent! Better still are scan
tools that can read the engine control computer's built-in knock sensor, or a standalone system that detects detonation. There's also the old fashioned but most
accurate method is reading your plugs for signs of detonation. Whatever the case,
it's worth the time to determine what your engine's real octane needs are. Too little,
and you flirt with engine damage. Too much, and you're wasting money and quite
possibly reducing performance.
Octane Myth #3 - My brother went to high school with this guy who had a Chevelle,
and he went to the airport and filled it up with jet fuel. That gas was so high octane,
it blew up the motor!
While aviation gasoline is indeed higher in octane than gas destined for street use,
jet fuel is actually kerosene, which has an extremely low octane number. Jet engines
have absolutely no use for high octane fuel, since their combustion process is totally
different from that of gasoline engines. Putting jet fuel in an internal combustion
engine is a sure way to destroy it immediately - not because the jet fuel is "too hot",
but because it has no knock-suppressing capability. Incidentally, aviation gas also
has a lot of tetraethyl lead in it. Despite the fact that the pump is labeled "100
Octane LL (low lead)" it still has more lead in it than the worst 1960's automotive
gas. Running this through a car with an oxygen sensor or catalytic converter is sure
death for those parts.
So what is the correct octane rating for your car? There's a delicate balance between
many factors that determines how much knock suppression you need. Simply adding
higher octane gasoline can keep your car from knocking, but it may mask other
problems like an incorrect fuel/air ratio, ignition system trouble, or too much boost.
Ideally, the car's state of tune should work in conjunction with the proper fuel, rather
than using high octane gas as a band-aid to cover up some other problem. It's often
cheaper in the long run to determine the root cause of knock instead of switching to
a more expensive high-octane fuel. On the other hand, tuning an engine for
maximum power will often demand the added knock resistance of higher octane gas.
Only by understanding how and why knock occurs, and the interaction between
ignition, compression, fuel delivery, and the fuel itself can you reach the state of
optimum performance.