Fuel For Pro Saws

Adding Mix Oil

Today's pro saws have special fuel needs. If you are new to the industry or haven't been updated on what is necessary to get peak performance from your pro saw, the following information will be helpful.

To start, both Stihl and Husqvarna recommend the use of high octane unleaded gasoline. Both brands of pro saws are designed to burn fuel rated at 89 octane or higher. Most regular grade fuel has an octane rating of about 87. This is not enough. We also don't recommend the use of high octane aviation grade fuel.

What Pro Saw Users Need to Know About Octane

Low octane fuel may cost less, but it diminishes the power and performance of your pro saw. Chain saw powerheads are high performance engines. They produce more power per pound than many race car engines -- and they need high octane fuel to do it.

Some pro saw users don't realize that low octane fuel not only reduces a pro saw's performance, but can damage its engine. Most people think the fuel/air mixture should "explode" when it is burned in a saw engine's combustion chamber. This is not true. The fuel/air mixture should start burning at the spark plug and progress across the combustion chamber until it reaches the cylinder walls. True, this happens quickly, but it must be orderly. If one can imagine throwing a rock in a lake, the ripples that occur after the splash are similar to the way fuel should burn in a combustion chamber.

Another thing most pro saw users don't realize is that the saw's spark plug doesn't ignite the fuel when the piston is at the top of its stroke. Using degrees of crankshaft rotation as a reference, pro saw ignition systems typically ignite the fuel at 20º before top-dead-center. Even when the fuel is ignited early, peak cylinder pressure should not occur until 15º degrees after top-dead-center. This means the crankshaft rotates 35º during the time it takes for the flame to progress across the combustion chamber -- and this is good. If fuel explodes early and peak pressure occurs before the piston starts its travel down the cylinder, engine performance is reduced. It is also easy to imagine how much pressure there is on internal engine parts when this occurs.

Another factor most people don't comprehend is, for maximum power, it is not just how much pressure is pushing on the top of the piston, but also how "long" it pushes. If you have ever been slapped or punched, you know the difference. A slap is a hand and arm motion -- a quick event that may sting, but rarely does much harm. A punch is a longer event than comes from a boxer's whole upper body. It hits with such force, it can knock an opponent out. To get top performance from your pro saw, you want "punches" on the top of the piston, not "slaps."

Octane Rating

The octane rating of a fuel relates to its ability to resist knocking. Knocking is the sound of the air/fuel mixture "exploding" inside a engine. Most everyone has run low grade gasoline in their car or truck at one time or another and heard a rattling sound come from under the hood. This knocking robs power from the engine and causes moments of extreme pressure on its internal components. It can also cause heat to build up. The same thing can happen to a pro saw engine, but you aren't likely to hear it knock. Cutting noise masks the sound of detonation. If you are running low octane fuel, you may not realize what is occurring until it is too late.

Piston Damaged by Low Octane Fuel

This piston was damaged by detonation which was likely caused by running fuel that did not have enough octane.


The only thing you really need to know about octane is today's pro saws need fuel with a minimum octane of 89. We suggest you run 91 octane, which is readily available as "supreme" at most gas stations. So, unless a saw is modified to have more compression and spark advance than factory specs, fuel above 91 octane offers no real advantage. In fact, some of the additives in these fuels are undesirable.

Detonation & Other Combustion Problems

Detonation, pre ignition, pinging, or dieseling -- these are all words used to describe combustion problems. Without getting too technical, you know fuel should burn at a steady rate across the combustion chamber, but it sometimes doesn't. When fuel "explodes" it creates extreme pressure inside the engine. Even if the pressure occurs at the right time, it doesn't last long enough to make good power.

What Pro Saw Users Need to Know About Oxygenated Fuel

You have probably heard of "reformulated gasoline" or "oxygenated fuel." A few years back, this fuel caused problems for owners of pro saws, but today, most have been overcome. Today's mix oil both blends better and lubricates better than the oil we sold just a few years ago. Some of it now has a fuel stabilizer built right in, adding to mixed fuel's storage life. Manufacturers have also made a number of improvements in the saws themselves. Better fuel lines, carburetor components, and coatings on metal parts help offset the affects of this fuel. While saw manufacturers have done much of the work to make this fuel compatible, pro saw users will have better results with it if they understand a few things about it.

Piston Damaged by Low Octane Fuel

Some fuel lines, especially on older saws, are not tolerant of today's fuel. Also notice what a dirty fuel filter looks like compared to a new one. Fuel filters should always be "bright" white.


E10 Fuel

E10 is what most of us think of as common gasoline. This is available at almost every gas station. E10 runs in most gas engines without the need for any modification. In this fuel, up to 10% of it can be ethanol, which is a type of alcohol. This can be used in today's chain saw engines.

E15 Fuel

E15 is another type of fuel that is gaining acceptance for use in automotive engines. This may also be found at gas stations, too. This is also a blend of gasoline and alcohol, but with E15, the alcohol portion can be 15%. Currently, this does not work well in today's chain saw engines. Don't use it.

E85 Fuel

E85 fuel is becoming more available. You may be tempted to buy this because it is usually less expensive than E10. The problem is, it is made of 85% ethanol and 15% gasoline. Some of today's cars and trucks are designed to self adjust to this and other blends of gasoline and alcohol. Their electronic engine control system is flexible enough to automatically adjust to a variety of different fuels. Your pro saw lacks this ability. Don't use it.

Shaking a Fuel Container

It is always good to shake a container of mixed fuel to ensure the mix oil has blended with the gasoline. It is important to know that as some of today's fuel ages and is subjected to moisture, its phases can separate, making it impossible to blend even with vigorous shaking.


Filling Stations to Avoid

When you purchase fuel, don't buy it from a filling station that permits different blends to be dispensed from the same hose. Even if you are careful to buy only E10 fuel, you have no control over what fuel is left in the hose from a prior customer. If the customer before you purchased E85 and you are filling a small container, you may get fuel with more ethanol that you realize.

More About Oxygenated Fuel

For a basic understanding of how oxygenated fuel works, it helps to understand some simple chemistry. Here goes. The carburetor on a pro saw blends hydrocarbons in gasoline with oxygen from the air. This air/fuel "cocktail" finds its way into the engine's combustion chamber. The chemical reaction we call "combustion," produces heat which pushes down on the piston. Piston movement is what rotates the crankshaft and... you know the rest of the story.

If you didn't get all that, don't worry. The only thing you need to know is, in the old days, the oxygen in the air/fuel cocktail used to come only from one source, the air -- that is until fuel companies started oxygenating gasoline.

You are probably saying to yourself: I just want to cut trees. Why do I have to know this? For one, it explains why your saw may require more frequent carburetor adjustments. The chemical makeup of fuel (amount of oxygen in it) can change even if you always buy the same fuel from the same source. Atmospheric conditions also still make a difference. Changes in temperature, pressure, and humidity continue to play a part. So does the elevation where you are working.

Have you ever hiked in the mountains and noticed that you got "winded" more easily? Compared to air at sea level, air at high elevation is less dense and has less oxygen in it. Each time you take a breath, your lungs absorb less oxygen than they would if you were breathing at sea level. Your saw notices this, too. When it is set for sea-level operation and you move to a job at a higher elevation, it will run rich.

Oxygenated fuel adds another dimension. Usually, without making any changes, except fuel, the saw may run lean. Unless the carburetor is reset to put more fuel in the mixture, extra oxygen in it makes the saw run lean. When the combustion charge is too lean, the engine may run hot and even detonate... the result is piston scuffing or seizure. Over-speeding can also occur. This can damage or destroy the rod bearing. These are all costly repairs.

Adjusting The Carburetor For Different Fuel

Today's new saws all have limiting caps on their adjustment needles. These caps prohibit the carburetor from being set overly "rich, which would cause the saw engine to produce excess emissions. The range of adjustment these caps permit are preset at the factory. Usually, proper carburetor adjustment is within this range, but not always. Sometimes, it is impossible to correctly adjust the carburetor because the caps prohibit the needles from going to a position where they need to be -- usually a richer adjustment. When this occurs, it is necessary to bring the saw into the shop. We have special tools that allow us to remove the limiters, reset the range, and reinstall them.

Adjusting Carb

You are already thinking: I'll just take those pesky limiting caps off and leave them off. This isn't a good idea because the limiting caps also keep the needles from vibrating out of adjustment during use. You may recall, earlier adjustment needles were fitted with springs that held tension on the threads. These kept adjustment needles in place. Today's carburetors don't have these, so if the limit caps are removed and not reinstalled, your carburetor will not stay in adjustment.

Frequent Carb Adjustments

If you are finding it necessary to make frequent carburetor adjustments, this is normal. Since the amount of oxygenation in fuel can vary from tank to tank, keeping the saw adjusted properly is an ongoing challenge. This is compounded by the fact that today's saws already run very lean. This makes them less tolerant of being out of adjustment -- especially on the lean side.

More About Octane

Another octane related item involves mix oil. When oil is mixed with gasoline, the end result is a mixture with lower octane than the gasoline alone. This is one reason it is important to mix according to the manufacturer's specifications. Some people add extra oil figuring they are doing their saw a favor. They are not. This can lower the octane of the mix, plus cause carbon to build up in the combustion chamber and exhaust system. Our advice is to mix according to the specs -- and do it accurately.

While on the subject of mix oil, it is important to know that some of today's fuel doesn't stay blended once it is mixed with oil. As mixed fuel ages (and gets exposure to moisture) it may separate.When this occurs, even shaking the container does not re blend it. The result is a portion of fuel inside a saw's tank with no oil in it.

Mix Oil

Stihl Ultra Mix

Both Stihl and Husqvarna have done extensive testing to come up with their mix oil formulations -- and they are both very good. Since we get an opportunity to look inside pro saw engines on daily basis, we've observed that both brands of oil work fine in today's pro saw engines. They both provide good lubrication and neither causes excessive carbon build up. This is not the case for all mix oils. Our advice is pretty simple: run either Stihl or Husqvarna brand oil.

Mix oil has gone through an evolution of its own. Saw manufacturers have developed special additives that improve its ability to blend with and stabilize today's fuel. Some of this new oil also uses synthetic base stock. If you want to run what we think is the best mix oil for pro saws, try Stihl's Ultra oil. It is 100% synthetic. From the internals of saws we see in our shop that have had a steady diet of this oil, it looks very good. In fact, it is the oil that most of us who work at Madsen's use in our personal power equipment. Husky offers a similar "premium" mix oil that is made from a blend of conventional and synthetic oil.

It is also important to realize how little oil there is in a 50 :1 mixture. Only 1/50 of the mix in a saw's fuel tank is lubricant. With this little oil in the mix, it needs to be good stuff!

Why Mix Oil For Boat Motors Doesn't Work

Two-cycle oil blended for an outboard won't work well in your pro chain saw. The reason is, water cooled two-cycles (boat motors) run at lower temperatures than air cooled two-cycles (pro chain saws). What this means is, boat motor oil is formulated to work at the wrong temperature for a pro saw. Boat motors don't work well with chain saw oil either. These are different engines and have different lubrication needs.

50 Fuel
How to Store Mixed Fuel

We suggest that any fuel that has been in a container for over six weeks not be used. Part of the reason for this is the fuel's octane rating degrades. Fuel can also be contaminated by moisture or condensation. Running fuel whose phases have separated can lead to a catastrophic engine failure.

How to Store Saw Engines

Any time a saw engine is not going to be used for more that a month, the fuel should be removed. This means dumping the tank and running the engine so there is no fuel left in the carburetor. An even better way to store a saw engine is to remove the fuel as described, and then partially fill the tank with a product like 50 Fuel. Then restart the engine so the 50 Fuel is in the carburetor. 50 Fuel is stabilized unoxygenated fuel that will not go bad for a year or more.

Future Pro Saws

Sometimes the future is difficult to see, but in this case, it is clear. It is just a matter of time before electronic engine management finds its way into pro saw engines. Today's carburetors will soon be replaced with what the automotive world calls electronic fuel injection. These systems monitor a number of engine characteristics and adjust for perfect fuel delivery and ignition timing. Like automotive engines, this technology will allow pro saws to deliver even more performance and less emissions.


Husky Autotune Carburetor

This is a the new Husqvarna "Autotune" carburetor. This is a first step in electronic fuel management that is available today on a few models of pro saws. As electronic components become less expensive, lighter weight, and more durable, we believe saw manufacturers will employ more of this technology to make tomorrow's pro saws both cleaner running and more powerful.


What started on high-performance sports cars has already been miniaturized and applied to motorcycles, snowmobiles, and large outboard motors.  The cost, size, and durability of the components are evolving. Hopefully before too long, they will be incorporated into the design of all new pro saw engines.

Our Advice

Got questions about fuel for pro chain saws? Call or stop in.