What does the formula on the gas pump mean?


Dear Tom and Ray:

What is the formula on the gas pump all about? (R+M)/2 -- what's the R? What's the M? -- James

TOM: It's all about octane, James. Octane is a hydrocarbon that, when burned in an engine, has a very high resistance to engine knock, or pinging.

RAY: But since it's very expensive, you never actually burn octane in your engine. The gasolines we use contain no octane. The octane rating simply measures how closely a gasoline compares to pure octane in suppressing knock.

TOM: So a fuel rated 93 octane will resist knock and pinging like a mixture that's 93 percent pure octane.

RAY: There are two different methods used to determine the octane rating of a fuel. There's "R," which stands for "research." The research octane number (RON) is determined in a lab with a test engine running at 600 rpm, which represents a low-compression, low-knock situation.

TOM: Then there's "M," for "motor." The motor octane number (MON) uses a test engine, also in a lab, at a higher rpm. That's supposed to represent higher-speed, higher-temperature operation, where knock is more likely.

RAY: Why is the second one called "motor" when it's also, technically, "research"? Done in a lab, too? We have no idea. Why not L+H for "low" and "high"? Or S+F for "slow" and "fast"? Or if it's going to be meaningless, why not S+R for "Siegfried" and "Roy"?

TOM: Anyway, the way they get the number on the pump is by averaging "R" and "M." Or, put mathematically, "(R+M)/2." So, if the RON of a fuel is 93 and the MON is 87, the octane rating you see on the pump is 90.

RAY: And remember, all the octane rating tells you is how much knock protection you get. A higher-than-necessary octane rating doesn't keep your engine cleaner, make the car go any faster, make your engine last longer or keep your hairline from receding. It just costs more. So use only the octane required by your manufacturer to prevent knock, and no more.

Tags (Browse All)

You must be logged in to leave a comment. Login / Signup
Support for Car Talk is provided by:

Donate Your Car,
Support Your NPR Station

...and get a tax break!

Get Started

Find a Mechanic

Promo tile

Rocket Fuel