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Post Info TOPIC: The Pro's & Con's of Fuel Mixtures


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The Pro's & Con's of Fuel Mixtures

Hello people,

There's always been some debate about fuel mixtures, & how a "rich" mixture will provide you with more power, & how a "lean" mixture will provide you better mpg... But what is the best way to go, & are there any compromises ? What does a Rich & Lean mixture even mean ? Is it worth playing around with the mixtures ? How do I alter these settings ? Is it possible to better the standard settings ?

All of the above questions are valid & warrant further investigation & explanation, but we need to understand the needs of our engines first, & then we can see how our car-manufacturers actually do try their best to give us the optimum air / fuel ratios to satisfy all the mandatory requirements, including the best compromise of power, economy & meeting "Smog"- legalities.



An engines throttle controls the amount of air an engine takes in. The main function of any fuel delivery system is to mix the fuel with the in-comming air in the proper proportion, or Ratio...Small variations in air - fuel ratios can have a dramatic effect on Power-output, fuel consumption, & exhaust emissions.



Laws of Physics tell us that combustion of any substance requires a sufficient ratio of Surface Area to Mass - ( small enough particles ), and the correct amount of oxygen - ( in proportion to the amount of fuel ). In internal combustion engines these conditions are met by atomizing the fuel into tiny droplets, and by metering the fuel in the correct proportion to the intake air. In terms of "numbers", approx. 14.7 parts of air are required to support complete combustion of 1 part of fuel. eg 14.7 : 1 ratio by mass.

NB. Interestingly, by volume, it takes approx. 11,500 parts of air to support complete combustion of 1 part of fuel...



These two terms, "rich" & "lean" are used to describe mixtures which deviate from the theoretically perfect air-fuel ratio & burn less efficiently.

A Rich-mixture is one that has a lower air-fuel ratio ; there is insufficient air (oxygen) to support complete combustion of the fuel. Rich-mixtures increase fuel consumption & emissions of hydrocarbons ( HC ) & Carbon Monoxide ( CO) - the products of incompletely burned gasoline. They tend to reduce power, increase carbon deposits &, in extreme cases, foul spark-plugs & dilute your engines lube-oil.

A Lean-mixture is one with a higher air-fuel ratio ; there is more air than necessary for complete combustion of the fuel. The fuel will burn completely, but more slowly & at a higher combustion temperature. Lean-mixtures reduce power, elevate engine temperature, &  increase emissions of Oxides of Nitrogen - (NOx), - a product of combustion at excessively high temps. They also cause drive-ability problems. In extreme situations, the high temps from lean-combustion will cause pre-ignition...- Violent, un-timed combustion of the mixture that has the potential to cause serious engine damage.

As mentioned earlier, the "Best" air fuel ratio, ( also known as the Stoichiometric Ratio ) is 14.7 : 1. This mixture is neither Rich or Lean. The Stoichiometric ratio can also be described in terms of the air requirements of the engine. BOSCH calls this the "excess-air-factor", & represents it by the Greek letter ( I don't have the symbol here LOL ) known to us as LAMBDA. So, when at the stoichiometric ratio, of when the amount of air equals the amount required for complete combustion of the fuel, & there is no excess-air,


When there is excess-air, - ( Lean mixture ), Lambda is GREATER than 1.

When there is a shortage of air, - ( Rich mixture ), Lambda is LESS than 1.



Maximum Power, the air-fuel ratio = 12.6 : 1              LAMBDA = 0.86.

Maximum Economy, the air-fuel ratio = 15.4 : 1          LAMBDA = 1.05.

The ideal setting, the air-fuel ratio = 14.7 : 1             LAMBDA = 1


I do hope that the above gives some insight & help to help everyone out for their own needs, & gives insight as to what goes on and why it's important to stay within reasonable limits of your fuel-settings. The above optimum fuel mixture variations ARE the limits of where you can move to before any positive gains become negative !






"Only an alert & knowledgeable citizenry can compel the proper meshing of the huge industrial & military machinery of defense with our peaceful methods & goals, so that security & liberty may prosper together".    Dwight D.Eisenhower.

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