Extracted from Wikepedia...
ORIGINAL: Mister Sinister
Octane ratings exist purely to describe the anti-knock characteristics of the fuel, or it's tendency to burn in a uniform or controlled manner, and have nothing to due with the energy characteristics of the fuel. The energy value of fuel is a much more complex science. The higher the octane, the lower the energy value. This is further compounded by Ethanol, which adds octane at the expense of energy value. It sounds backwards, but that's how it works.
Most modern vehicles are desingned to run on 87 octane, but factors like driving habits, vehicle condition, weather, etc. can have an effect, so sometimes higher octane is needed to prevent knock. Some performance cars (particularly turbo/supercharged) need higher octane fuel, because the pressures inside the cylinders is higher than that of normal cars. Lower octane fuel can light off when it's not supposed to in these vehicles (detonation), and cause all sorts of chaos.
Many argue that there is no benefit to be had running a higher octane, but there's also a lot of people who say they can feel a difference. My Wrangler pings on 87 octane, and it has since day 1, even though that's what it's designed to run on. So Irun 89 or 93 in it. Jeep sayus the ping is normal and within the realm of acceptable, but Iprefer to not have it ping at all. It costs me maybe $2 extra, Ifeel it's worth it. In my 55 Chevy, Ihave to run 93 octane due to it's higher than normal compression and timing curves.
Spark ignition engines are designed to burn gasoline in a controlled process called deflagration
. But in some cases, the unburned mixture can autoignite, which results in rapid heat release and can damage the engine. This phenomenon is often referred to as engine knocking
or end-gas knock. One way to reduce knock in spark ignition engines is to increase the gasoline's resistance to autoignition
, which is expressed by its octane rating.
Octane rating is measured relative to a mixture of 2,2,4-Trimethylpentane
) and n-heptane
. There are different conventions for expressing octane ratings, so a fuel may have several different octane ratings based on the measure used. Research octane number (RON) for commercially-available gasoline varies by country.
The octane rating became important as the military sought higher output for aircraft engines
in the late 1930s and the 1940s. A higher octane rating allows a higher compression ratio
boost, and thus higher temperatures and pressures, which translate to higher power output. Some scientists even predicted that a nation with a good supply of high octane gasoline would have the advantage in air power.
Heptane is one of the primary “Power” ingredients, not “Octane”…
80/87, which is dyed red, had the lowest lead content prior to commencing its phase out in the late 20th century, with a maximum of 0.5g per U.S. gallon (0.13 g/l). It was used in engines with low compression ratio
. Currently commonly called Avgas 80, its availability is now very limited.
100/130 avgas, now commonly called Avgas 100, is dyed green. It contains a maximum of 4g of lead per U.S. gallon (1.1 g/l). 100LL has replaced 100/130 in most places, but Avgas 100/130 is still sold in some parts of the United States
and New Zealand
.[dubious – discuss]
91/96 & 115/145In the past other grades were also available, particularly for military use, such as avgas 115/145 (dyed purple) and 91/96 (dyed brown). Limited batches of 115/145, commonly called Avgas 115, are produced for special events, such as unlimited air races; in the past 115/145 was used as the primary fuel for the largest, boost-supercharged radial engines that needed this fuel's anti-detonation properties.