Small Engine Gasoline Guide

I'm a man who likes quality, but spending more money for something doesn't always mean it's better.

When you fill up your car at the gas station, there are usually three different types, or grades of gasoline to choose from. Often, you’ll see Unleaded, Midgrade, and Premium. Each will have a number, typically ranging from 87-92.

If you’re unfamiliar with what these octane ratings mean, you might pay extra for premium gas, thinking it “sounds” better. The same goes for filling up your lawn mower, snow blower, or other gasoline-powered equipment. Maybe you think you’ll get better efficiency, but that’s largely a myth unless your manual specifically calls for it. 

In this article, we'll talk about everything you need to know about gas and your small engine. 

What is an Octane Rating?

In order to understand octane or anti-knock ratings, you should understand the basics of how an engine works.

Let’s use a 4-stroke engine to illustrate:

1. The Intake stage (piston moves down) draws in air and fuel into the engine’s cylinder.
2. In the Compression stage (piston moves up), the engine’s piston compresses the air/fuel mixture.
3. In the Power stage, the spark plug ignites the air/fuel mixture, creating an explosion that forces the piston downward, (which drives the machine to work).
4. In the Exhaust stage, the piston comes back up to force exhaust gases out.

In a 2-stroke engine, these steps still occur but are streamlined into two strokes (the piston goes up once and down once).

If we look at the compression stage specifically, then we can see how octane affects performance.

Today, manufacturers are increasingly building car engines with high compression ratios (previously found mostly in the racing world), which means the air/fuel mixture is put under more pressure and is more explosive.

Higher compression translates into more efficient use of air and fuel, but it makes the mixture burn much hotter than lower compression. This can cause the air/fuel mixture to spontaneously combust (without a spark) in a phenomenon known as knocking, or pre-detonation.

Knocking can occur before the piston reaches the top of the cylinder, forcing it down prematurely, thereby wrecking the engine’s timing and inflicting serious damage.

High octane gasoline is specially refined for higher compression engines to prevent knocking. With a higher octane, the air/fuel mixture can endure higher compression without combusting. Thus, octane is the measurement of a fuel’s resistance to knock and is on a scale of 1-100.

What Octane Rating Should I Use?

For those wondering “Is premium gas better for small engines?” the answer is most likely no. That’s because small engines are not built with high compression ratios. There is no threat of engine knock taking place.  

Sure, you won’t hurt your lawn mower or string trimmer by putting in Premium gasoline, but you’ll be wasting your money since it’s more expensive. Contrary to popular belief, Premium gas does not improve fuel efficiency or performance in motors that don't require it. 

Certain gasoline suppliers claim that their Premium fuel is cleaner and better for the engine, and people like my cousin swear by it. However, these claims are largely untested. If you perform regular maintenance on your equipment, (keep fresh fuel, clean the carburetor, etc.), then Regular octane gas will work just fine. 

You only need Mid-grade or Premium gas (89-octane or higher) if you have a vehicle with a high compression ratio, which will be noted in your owner’s manual. In that case, using regular gas could cause knocking and damage the engine. 

What is the Best Gas for Small Engines?

If octane doesn’t really matter for small engines, then what does? There are a couple of considerations to make when choosing gas:

Buy Fresh Fuel

By far, the most crucial step is to use fresh fuel. New gas is critical for a small engine to operate properly. There are many gas station companies out there. Choose one that does its own refining (typically name-brand stations)—that way the fuel will be as fresh as possible.  

Buy Quality Fuel

As I said, more expensive isn't always the best, but don't go for the cheapest of brands out there either. I recommend going with a Top-Tier fuel, which contains extra detergents that help prevent gum buildup and carbon deposits in the engine's critical parts and maintain performance. 

Once you buy the fresh fuel, store it in a plastic gas can, and add fuel stabilizer to keep it fresh. A good tip is to only buy a 30-day supply of fuel at a time and run the carburetor dry if you won't be using it for a while. That way you’ll use what you bought and not leave fuel stagnating once the season ends.

Avoid Ethanol

You might hear about ethanol, a corn-based fuel, in the news, and it’s becoming more and more popular thanks to its clean burning properties. Ethanol is mixed with gasoline in a proportion, typically 10 or 15 percent. You may have heard of E85, which contains 85% ethanol.

Despite its rising popularity, ethanol is a poor choice for small gas engines. Ethanol attracts moisture from the air, separates from the gas, and sinks to the bottom of your engine’s fuel tank. This watered-down ethanol is then fed into your engine and can cause ignition problems. Ethanol is also corrosive and burns really hot, which can cause overheating and irreparable damage. 

It’s best to use ethanol-free gas in small engines. If you do decide to use ethanol, don’t use a fuel that is more than 10% ethanol (E10), and be sure to add fuel treatment to prevent the fuel from separating.

Fuel Up!

Don’t overthink fueling your small gas engine. As long as you have fresh fuel, use a stabilizer, and avoid ethanol, you’ll get peak performance out of your machine.

Now that you also understand octane, you know it’s not necessary to waste money on Premium or Midgrade gas unless you have a high compression engine or your owner's manual specifies it.

Using Regular gas in well-maintained equipment will keep your lawn trim in the summer and the driveway clear of snow during winter. Quality without the price tag--that's something I can go for!

NEXT: What is Engine Oil?