All About Octane: What’s in a Number?

A neighbor of mine — who is a chronic used boat shopper — finally got himself a runabout with a V8 engine. After owning nothing but outboards, four-bangers and V6s, he couldn’t be happier. I’ve never seen him wash any of his boats more often than this one.

The other day after he got home from the lake, I popped in to see how he was enjoying his new-to-him boat. As you might expect, now that he has a V8 engine, he’s all about top speed. He was a bit stumped as to why he couldn’t get it to go over 60 mph.

Octane labels

Octane has to do with an engine's resistance to knock. Higher octane doesn't increase an engine's power.

“If I put 93 octane fuel in my boat, it’s supposed to go faster, right?” he said to me.

“Uh, no, not exactly,” I explained.

It boils down to numbers. One gallon of pump gas contains about 125,000 BTUs of energy, whether it’s high-octane 93 or regular old 87 octane. What is the difference? High-octane fuel simply burns at a slower rate than regular and is less likely to preignite from the heat of compression. Putting premium in a boat with an engine designed to run on 87 octane fuel is largely a waste of money, and it won’t make it go any faster. It can’t.

According to the American Petroleum Institute, “Octane number is a measurement of fuel’s resistance to engine knock.” What’s engine knock? “Engine knock is an abnormal combustion associated with using gasoline with too low of an octane number. Ordinarily, your [boat] will not benefit from using a higher octane than is recommended in the owner’s manual.”

On the other hand, if my neighbor wanted to experience what a difference in BTU makes, I told him all he’d need to do was run a tank of E-85, the “flex-fuel” blend of 85 percent ethanol and 15 percent gasoline. Boat owners don’t need another excuse to avoid ethanol, but it helps to illustrate my point about the difference between octane and BTU.

“Regular” E-85 has an octane rating ranging from 100 to 105, compared with the 87 octane rating of regular pump gas. However, ethanol contains 85,000 BTU per gallon, an energy loss of about 25 to 30 percent. That translates to a 25- to 30-percent decrease in power and a similar decrease in fuel economy. Unless your family owns a few thousand acres planted with corn, that kind of power loss is unacceptable to most boaters.

I then explained to my neighbor that what high-octane fuel will allow you to do—if your engine-management system permits—is advance the ignition timing without fear of preignition knock. Advancing the timing will give you a small boost in power and perhaps a couple of mph on the top end. The downside is that as long as you keep the timing advanced, you will have to run the high-octane stuff, which isn’t getting any cheaper.

I think I kind of blew his mind a bit, but at least he won’t be wasting money on fuel that won’t make him go faster.

Ten bucks says he comes back asking about some other thing that will make his boat go faster.

- Brett Becker

 

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