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by John Comeskey of SPS and James Walker of scR motorsports

While most enthusiasts clearly understand the need to increase airflow in order to get more power, few understand the role of fuel and how fuel delivery is controlled in a modern motor. This article will focus on fuel delivery with a specific look at proper ratios, the meaning of "stoichiometric," how fuel flow is controlled and measured, and ways that an enthusiast can modify fuel delivery to match other motor enhancements.

Why Does Fuel Mixture Matter?

As you know, energy is only released when atoms combine to create new molecules. The fuel mixture – or combination of input ingredients – are important factors determining the efficiency of the chemical reaction. Poor mixtures result in excess atoms of certain elements that are unable to mate to other atoms to create new molecules. As a result, the excess atoms take up valuable space within the combustion chamber and interfere with the reaction process of other atoms. This is costly in terms of efficiency and lost power.

In theory, one would want every bit of the combustion space to be filled only with atoms that will eventually be used to create energy from the exothermic reaction. And one should wish to avoid any "wasted" atoms - and hence "wasted" space - within the combustion chamber.

What are Fuel Ratios?

Fuel ratios are simply the relationship between the number of oxygen molecules relative to the number of hydrocarbon molecules. This is traditionally measured in terms of oxygen molecules per one hydrocarbon molecule – or "X" parts of air per "1" part of fuel. For example, "13.0:1" (read as, "thirteen to one") reflects 13 parts of air per one part of fuel.

What is "Stoichiometric?"

The term "stoichiometric" describes the mathematically "correct" combination of hydrocarbons and oxygen within a theoretical mixture for the internal combustion process. A stoichiometric mixture combines "just enough" oxygen with "just enough" carbon and hydrogen to ensure that all atoms have a mate. This "perfect" ratio has been calculated as 14.7 parts air to one part fuel – or 14.7:1.

In theory, no leftovers result from the reaction if combustion conditions are efficient and if the input mixtures are stoichiometric. In fact, water and carbon dioxide would be the only emissions produced from a stoichiometric fuel ratio in a completely efficient reaction environment.

However, even though a stoichiometric ratio is theoretically the best combination of oxygen and hydrocarbons for a perfect combustion process, in practice, it is not always the best for a specific purpose. One reason for this is that the calculation of the stoichiometric ratio assumes a "perfect" combustion environment in which every available atom is able to find every available mate. Because combustion environments are not completely efficient, there are cases in which fuel ratios will vary from stoichiometric in order to compensate for the inefficiencies.

Fuel Efficiency Versus Power

Even though stoichiometric is "mathematically" correct, it is neither the most fuel-efficient nor the most powerful mixture.

For better fuel efficiency, it is possible - and desirable - to "lean" the mixture, or reduce the amount of fuel relative to the amount of air. In fact, maximum thermal efficiency occurs at ratios between 16-18:1. And some experimental high-efficiency motors will run in "lean" mode while cruising. Such mixtures on these motors may become as lean as 20.0:1

This obviously contributes to excellent fuel efficiency. However, such lean ratios also result in very hot temperatures and relatively unstable mixtures. This can lead to detonation under load and is not best for producing power.

For more power, it is actually best to use ratios that are "richer" than stoichiometric – or ratios that use more fuel relative to a given volume of air. It is important to remember that because combustion environments are not perfectly efficient, it is sometimes difficult for every available atom to match up to a mate. This is especially true under high loads. To help ensure that every oxygen atom is used in the combustion process – and thus results in the release of energy – slight amounts of extra fuel should be added into the mixture. This increases the chance of creating the desired exothermic reaction. And it reduces temperatures, which may prevent detonation.

For these reasons, it is generally accepted that ratios for producing optimal power in any motor should be between 12-13:1. And based upon data that SPS and scR motorsports have collected on the dyno, it appears that maximum power from the DOHC Saturn motor occurs between 12.7:1 and 13.0:1.