High Octane Fact And Fiction

So... You shuck out the big bucks for the high-test when you fill up your mower? Adds a bunch of pep to that sucker, that you really feel in the ol' butt-dyno, don't it? Don't it? Well, yes... And no, but mostly no. Seriously.

Here's what you should probably know about higher octane fuel, and why you're likely wasting precious dollars that you could be spending on tires, parts, or maybe even groceries.

Let's start with the definition of "Octane", and it's implications. Simply put, the octane rating of a fuel is it's resistance to detonation. Yup, you read that right- resistance! Put another way, it's more stable- it has less of a tendency to go "BANG" until you really want it to. The higher the octane rating, the harder it is to ignite. Read THAT again, too!

Now- we all know that combustion occurs in a gasoline engine when a compressed air/fuel mixture is ignited by a spark. And we've learned that higher octane rated fuel is more resistant to detonation. So, what is detonation? Boom, but in a bad way. Detonation occurs when that compressed a/f mix somehow manages to ignite either before the spark, or just too fast after it. But how? Well, like a diesel, really. Given enough heat and compression (which makes more heat) that mixture, if unstable enough, can light itself. Not only can this rob power from your engine, it can actually break it if it's bad enough. A higher octane rating simply means that the fuel is less likely to go boom until your spark plug tells it to, or no explode too quiclkly for the engine's setup.

So, where's the problem? The problem is with an engine with a higher-than-normal compression ratio. Such engines (generally, race engines or other high-performance applications) can eat themselves pretty quickly if not protected from detonation. Thus, a higher octane rated fuel is to be used. Being more resistant to detonation, it'll ignite when it should- and only then.

So what does this mean to you, and your favourite mower engine? Nothing, unless the engine has been built with a whack of high dollar race parts. That's right, nothing! There is absolutely no benefit at all to a higher octane rating fuel in a low compression engine. Forget what your friends tell you, or what you heard from That weird neighbour with the glass eye. It's a total waste of money.

How high is your compression? Well, for a factory stock, average L-head mower engine, it tends to be around 6:1. For an OHV, it's a little higher, up to 8.5:1. Still, well below a high-octane requirement. In fact, the Briggs manual suggests a minimum octane rating of 77. The lowest I've seen at the pumps anytime recently is 86 or 87. More than enough.

Now, in the general interest of fairness, there sometimes CAN be a very slight benefit to buying the "good" fuel. Depending on where you live, SOME fuel suppliers will sell low-grade gasoline (say, 87 octane) with up to a 10% ethanol content, and the high-test stuff, (let's say 95 octane) may contain 5% ethanol. Gasoline has a per-gallon energy content of 114,000BTU. Per gallon, ethanol boasts an energy content of about 76,000BTU. Brace yourself for a math-induced headache...

A gallon of low-grade gas (10% eth) will then contain 102,600BTU in gasoline and 7600BTU in ethanol, for a total energy content of 110,000BTU.
A gallon of high-grade gas (5% eth) will then contain 108,300 in gasoline and 3800BTU in ethanol, for a total energy content of 112,100

Therefore- IF your gasoline supplier puts 10%eth in the low-grade, and 5%eth in the high-grade, a gallon of the high-grade will have an energy content of about 1900 more BTU. Total difference? 1.02% more energy content in a gallon of high-grade. IF they use 5% less ethanol... This is an extremely small value.

Food for thought when you pull up to the pump...

*please do note that this is a very simplified explanation- great volumes have been written about the topic matter, this is just a condensed, basic FYI.

Doc Sprocket wrote:

So what does this mean to you, and your favourite mower engine? Nothing, unless the engine has been built with a whack of high dollar race parts. That's right, nothing! There is absolutely no benefit at all to a higher octane rating fuel in a low compression engine. Forget what your friends tell you, or what you heard from That weird neighbour with the glass eye. It's a total waste of money.

Honesty and truth for once on the internet!

I agree with you and you can call me stupid but right now i been running 111 octane through my v-twin. I really consider for people to listen to you cause im a crazy Racecar Guy. All we run is 111 and Methanol through are race engines.

I think a lot of it is just the ethanol rating in high octane. But they make millions of treatments for ethanol gas and its fairly inexpensive too. I add Sta-Bil to my gas but thats more of a preservant than a corn-juice eater. But with gas at $3.46 at minimum E85 is all I buy at the pump.

111 is like around 6-7$ a gallon /: