Ocatane ratings

While filling up the tank of my gas guzzling SUV today I took to reading the labels on the gas pump ( I know, I know, I need a hobby).

Most of them were easy to follow and understand, though I probably would never drink gas whether the sign said it was safe or not.

It was the octane rating that left me confused. It said

87 minimum octane rating. Calibrated using the
(R+M)/2 method.

The what?

And now that I know the minimum, what’s the maximum ocatane rating for my gas? For all gas? For any gas. It’s things like this that keep me awake at night playing Diablo II instead of snuggling up next to Mrs Welby and getting some booty.

The octane rating depends on the fuel/air mixture. The (R+M)/2 method takes the average of octane rating when the mixture is rich (high fuel-to-air ratio) and the octane rating when the mixture is lean (low fuel-to-air ratio).

Pure isooctane has an octane rating of 100. Certain additives can raise or lower the octane rating. That is about as far as I understand it, though others can no doubt give a more detail explanation.

There are two standard ways to measure octane rating–the Research method ® and the Motor method (M). The pump ratings are the average of the two, mathematically expressed as (R+M)/2.

I don’t know the details of the two testing methods, but I’d be surprised if the difference is between a rich mixture and a lean mixture.

That is the way I heard it. Maybe I misunderstood the explanation.

In UK normal unleaded petrol is 95, and Superunleaded is 97. 87 seems very low.

Well, Dr. Lao, you got me curious enough to Google it. On this site (http://www.commerce.state.mn.us/pages/WeightMeasure/OctaneFacts.htm), which probably answers all the OP questions, the difference is indicated as engine under load vs. free running engine.

Octane ratings as high as 95 in the US are quite rare and about four times the cost as typical gasoline (so about as expensive as in the UK!). This high an octane fuel is usually reserved for extrememly high performance engines (professional racing) and difficult to find commercially.

I suspect a different rating scale is used in the UK.

I read about some Russian troop-carrying military vehicle which could use 76 octane fuel. It also had a hand crank that could be used to start the engine if the battery or starter weren’t working. That would be a nice feature, actually.

Last summer I saw a gas station in Connecticut selling 100 octane gas. It was practically $5 a gallon! Why on earth would anyone need that kind of gas? I thought only race cars used fuel that powerful, and I don’t think many of them are traveling Connecticut’s public highways (heck, the speed limit is only 50 in some parts of 95!).

Is there a racetrack, like a dragstrip or such, near where you saw the 100 octane gas for sale?

  1. There is no “maximum” octane number for gasoline; economically, however, it makes little sense for the refiners to put in more additives than they have to.

  2. (R+M)/2 does indeed refer to the average of the Research Octane Number (RON) and the Motor Octane Number (MON).

  3. In the UK, it’s typical to use only the RON; this gives greatly inflated octane numbers, as the RON is as much as 12 units greater than the MON. Some high octane fuels are labeled with the RON in the U.S., like that 100 Octane stuff Elwood mentions.

  4. Octane number has nothing to do with power; high octane fuels burn more slowly and more evenly, which reduces knocking. They’re usually in demand for older cars, which still require lead, and racing, which I’ll explain next.

  5. While high octane fuels add nothing to engine power in themselves, they do permit engines to run at higher compression without knocking; racers will adjust/machine their engines with this in mind.