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Chainsaws

What Fuel Mix Ratio for Two-Stroke Tools (50:1, 40:1, …)?

Most two-stroke outdoor power equipment such as chainsaws and leaf blowers require fuel that is a mixture of gasoline and oil. For best operation, the oil must be mixed in at a specified mix ratio relative to the gas, with typical ratios ranging from 100:1 to 15:1.

The proper fuel mix ratio is a hotly debated topic, which I learned when trying to find out the best mix ratio for chainsaws a while back. After investing considerable time into studying the arguments for different ratios, I came to a conclusion that is quite simple.

In this article, I will tell you what you need to know about fuel mix ratios. In short, a fuel mix ratio of 50:1 is the best choice for most air-cooled 2-cycle tools when using modern 2-cycle oil. This ratio provides the best compromise between lubrication and clean combustion, has been universally adopted by 2-stroke tool manufacturers, and is also the only ratio pre-mixed gas is typically available in. While not optimal, other mix ratios may often also be used, and may in rare cases be still preferable over 50:1.

Next, we will unpack this, starting with how the industry has moved from thicker mixes to 50:1, and then going deeper into the compromise around oil content in 2-cycle fuel. We will also take a quick look into oil types, brand-specific recommendations, old engines and the consequences of using a 40:1 mix.

50:1 is usually best

The recommended fuel-oil mix ratio for most small 2-cycle (2-stroke) engines in handheld tools is 50:1 when using synthetic 2-cycle oil.

You can use the 50:1 ratio with any synthetic, semi-synthetic or additive oil and all two-stroke handheld power tools unless the manufacturer specifically recommends otherwise.

There are some exceptions to this rule:

  • If you are using old mineral two-stroke oil without any additives, you may prefer thicker mix ratios of 25:1 or 33:1 to ensure adequate lubrication.
  • Husqvarna recommends 33:1 for 75cc and bigger engines in high-load applications

It’s about the oil

The 50:1 mix ratio is thinner, i.e. has less oil, than those used in the past. Older tools had thicker recommended mix ratios of 40:1, 33:1, 25:1 or even 16:1.

According to my research, the reason for the shift from 33:1 or 25:1 to the thin 50:1 mix seems to be that newer engine oils lubricate better than old. New engine oils are synthetic, semi-synthetic or have additives that improve their performance so that less oil is needed for the same lubrication effect.

There are claims that the 50:1 mix ratio was forced on the 2-stroke tool manufacturers by EPA and EU regulations to reduce emissions. I was not able to find backing to these claims, and the advances in oil technology seem a more plausible explanation.

What does 50:1 mean?

A mix ratio of 50:1 or “50 to 1” means a mixture where one (1) part of oil is mixed into 50 parts of gasoline.

The mixing ratio expression is handy since it is a mixing guide: for a 50:1 fuel, add one fluid ounce of oil into 50 fluid ounces of gas.

An alternative way to express mix ratios is oil content percentage. A mix ratio of 50:1 corresponds roughly to a 2% oil content in the fuel.

All other mix ratios work the same: 40:1 means 1 part of oil in 40 parts of gas, 32:1 means 1 part of oil in 32 parts of gas, and so on.

Common mix ratios

The table below lists some common fuel mix ratios for 2-cycle tools (chainsaws, leaf blowers, weed eaters).

Mix ratioOil content
[%]
Notes
75:11.3%very thin
60:11.6%thin
50:12.0%standard w.
synthetic oil
43:12.3%3 fl.oz. per gal
40:12.4%thicker option,
also pre-mixed
33:12.9%Husqvarna
75cc and larger

Why does fuel mix ratio matter?

The ratio of oil and gasoline in two-stroke fuel is important as it affects engine lubrication, fouling and emissions. It also has a small effect on engine power.

Lubrication

Conventional two-stroke engines absolutely need some oil in the gasoline to lubricate the cylinder and the crankcase – without oil, the piston will seize in the cylinder, resulting in permanent damage.

The more oil in gas, the better the lubrication. As oil has some downsides, the amount of oil is usually set to provide adequate lubrication. The oil content required for adequate lubrication depends on the oil type and additives, but with synthetic oils 50:1 should be enough.

Fouling

While oil is necessary, it does promote engine fouling. Most engine oils burn only partially and not very clean in the engine, leaving a residue on the engine parts. This residue or deposits can cause engine issues.

The more oil in gas, the worse the fouling. The amount of fouling also depends on the oil composition: synthetic oils and some additives promote purer combustion and detergent additives also reduce the stickiness of the residue.

Emissions

The incomplete and poor burning of engine oil increases the smoke in the engine exhaust. This smoke consists of small soot particles and constitutes particulate emissions.

The more oil in the gas, the higher the emissions. As with fouling, the cleanest results call for as little oil as possible. The amount of smoking depends also on oil composition and engine operation point.

Mix ratio is a compromise

As you see, the choice of gasoline–oil mix ratio is a compromise between lubrication and clean combustion: there should be enough oil to prevent engine seizing and premature wear, but no more to keep the fouling and emissions down.

Note that the consequence of too little oil and inadequate lubrication (engine failure) is much worse than too much oil. Using a thick, oil-rich mix ratio like 25:1 or 33:1 is playing it safe, and was necessary with older oils and engines.

However, most modern oils and engines will be adequately lubricated with today’s oils at 50:1 ratio.

Which oil to use?

With all small two-stroke power equipment (chainsaws, leaf blowers, weed eaters), prefer engine oil intended for air-cooled 2-cycle (2-stroke) engines. Popular brand and products are Echo’s Power Blend and Red Armor, PowerCare’s 50:1 and Husqvarna’s XP+ and HP oils.

Some oil types to specifically not to use with such equipment:

  • oil intended for water-cooled 2-cycle engines (e.g. outboard motors, TCW rated): these are for lower service temperatures
  • regular motor oil for 4-cycle engines
  • castor oil (great performance but heavy residue)

What mix ratio for each tool?

The 50:1 mix ratio works almost universally for power equipment with air-cooled 2-cycle engines when using modern 2-cycle oil. The table below lists outdoor tool brands and recommended mix ratios that I was able to find.

BrandModelsMix ratio
Echoall air-cooled
2-cycle
50:1
Husqvarnaunder 75cc
over 75cc
50:1
33:1
Jonsered(probably all
new 2-cycle)
50:1
Stihlall 2-cycle
equipment
50:1

The only general exception from 50:1 is Husqvarna, which states that models above 75cc may be run on a mix of 33:1 in certain heavy use applications. Note that the wording (kind of) suggests that 50:1 would actually work there too.

What mix ratio for old equipment?

Many older 2-cycle equipment (2000’s, 1990’s, and earlier) typically have a thicker fuel mix such as 33:1 or 25:1 specified in the instructions.

Note that in most cases these tools should also be run with a 50:1 mix when using modern oils. The oil-rich mixtures were originally specified for the tools because of the poorer lubricity of the mineral oils available at the time.

40:1 vs 50:1?

The second-most popular fuel mix ratio of recent years is 40:1, and there is lots of debate of the relative merits of 40:1 vs 50:1. So are there cases when 40:1 should be preferred? Does the difference even matter?

My conclusion of the debate is that 50:1 should be preferred over 40:1 with modern 2-cycle oils, but there is probably no big difference between the two ratios. The 40:1 mix will potentially lubricate a bit better, but also leave a bit more residue – you will probably not even notice the difference.

While most pre-mixed fuels are available only in 50:1, TruFuel still offers 40:1 as an alternative (green can).