What does the octane rating really mean?

A BRIEF INTRO

Octane in this sense is just a rating system for a fuel's knock resistance. Knock happens if fuel ignites too early or burns unevenly, which can cause a knock or pinging sound. A higher octane rated fuel can resist this much better and is a benefit to engines that are trying to run advanced timing or high compression.

DO I BENEFIT FROM HIGHER OCTANE?

If you're asking this question, the answer is likely no. Your car is tuned for a specific octane rating, typically 87; if not, most dealers or sellers make this well-known when selling the vehicle. Running a higher octane can make the engine perform poorly, become less efficient, and can even cause fault codes. Running a higher octane is typically only recommended if you have dirty fuel or water in the fuel; these issues can cause the octane rating to be reduced, running a higher octane fuel can even it back out.

HOW THE NUMBER IS CALCULATED

Two reference fuels are used: Iso-octane and n-heptane.

• 90 octane would be 90% Iso-octane and 10% n-heptane.

• 0 octane would be 0% Iso-octane and 100% n-heptane.

• 100 octane would be 100% Iso-octane and 0% n-heptane.

But remember, gasoline is not this mixture; this mixture is only used for the knock resistance test.

US GAS PUMPS

Gas pumps in the US use the (R+M)/2 method, which is the average of

• RON (Research Octane Number) - measured under light load.

• MON (Motor Octane Number) - measured under heavy load.

This rating is the AKI (Anti-Knock Index).

WHY IS IT CALLED THE "OCTANE" RATING?

Octane is in gasoline, just not in the amount that is being led on. In the early 1900s, when fuel testing was being standardized, scientists needed a consistent way to test fuels knock resistance. Octane and heptane were chosen as the reference points for this test. Heptane had an incredibly weak resistance to knock, so it scored 0, and Octane an incredibly strong resistance, scoring 100.

Scientists needed a name for this test, and "octane" just stuck; marketers loved it, and it was easy for the public to remember.

WHY NOT JUST ACTUALLY MAKE IT OUT OF 87% OCTANE AND 13% HEPTANE?

If 87 octane gasoline was actually made of 87% octane and 13% heptane, it would evaporate in cold weather, not ignite properly in certain temperatures, and burn significantly less effeciently. Gasoline has several other molecules to allow it to operate properly in cold temperatures, prevent deposits, and pass emissions standards.

HOW DOES IT GO ABOVE 100?

You may have noticed some race fuels going above 100 like 106 or 108. This is because scientists in the early 1900s were using octane and heptane as a rating scale, and mixtures have been developed that test better on the scale than octane itself. For example, Ethanol E85 has an octane rating as high as 105 if blended properly.