Race Fuel Jet Sizing: Does Ethnicity Affect Performance?

The relationship between jet size and fuel economy is a complex one, with many variables at play. While it may seem intuitive that decreasing jet size would lead to improved fuel economy, particularly in city driving where part-throttle operation is common, the reality is that jet size can impact fuel flow at both low and high engine loads.

The main jets in a carburettor are not always the limiting factor for fuel flow. Other factors, such as the angle channels and tubes that carry the fuel mixture, can also restrict fuel flow. As such, simply changing jet size may not have the desired effect on fuel economy and can even lead to engine damage if not done correctly, as seen in the case of using oxygenated race gas without proper carburettor adjustments.

To optimise fuel economy, it is essential to consider the entire carburettor setup, including jet size, air bleeds, accelerator pump linkages, and more. Spark plug reading and exhaust gas temperature sensors can also provide valuable insights into the engine's air-fuel ratio and help fine-tune the carburettor for optimal performance and efficiency.

In summary, while jet size can influence fuel economy, it is just one piece of the puzzle, and changes should be made carefully and in conjunction with other carburettor adjustments to avoid unintended consequences.

Carburettor jet size affects fuel flow

Carburettors are devices that mix air and fuel together and are often used in car engines. They feature a venturi – a constricted neck – through which air flows on its way to the engine. The partial vacuum caused by increased air speed through the venturi sucks fuel through a jet, and this jet size affects fuel flow.

The jet size is important as it regulates fuel flow. There must be several jets of different sizes to provide the different amounts of fuel needed at any moment. For example, when the engine is idling, very little fuel is required. On the other hand, when the throttle is opened, the air flow suddenly speeds up, and more fuel is required.

Changing the jet size can affect fuel flow and, consequently, engine performance. For instance, a leaner jet setting can result in a leaner air-fuel mixture, which may improve fuel efficiency. However, it is important to note that there are many factors that influence fuel flow and engine performance, and jet size is just one of them. Other factors include air flow, throttle position, engine speed, and more.

It is also worth mentioning that carburettors are not as simple as just changing the jet size. The main jets and metering rods work in concert, so adjusting the jet size may require changes to other components as well. Additionally, different carburettors may have different tuning requirements, so it is always recommended to refer to the specific tuning manual for the carburettor in question.

Oxygenated race gas requires more fuel flow

Oxygenated race gas contains additives that create oxygen molecules when the compound is broken down. This extra oxygen in the combustion chamber means more fuel can be added, and when the two are burned together, the result is additional energy.

However, to make more power, more fuel needs to be burned. Everett Bradham, the shop foreman for Willy's Carburetor and Dyno Shop, says that "you've got to get more fuel in there... the first thing a lot of guys will think of is just to go up on the jet sizes until you get what you need." Bradham continues that while this can work, "as you go up on the jet sizes you start losing a little bit of driveability—the carburetor won't operate as cleanly."

The tech guys at BLP Racing explain that the main jet in a Holley carb is not always what limits the fuel flow. There are other factors, such as the angle channels that carry the fuel mixture up to the boosters, and the tubes inside the boosters that carry fuel from the metering block to the discharge ports. If these passages after the main jet are not large enough, then a larger main jet will not flow additional fuel.

In some cases, a three-circuit carburetor may be required to add enough extra fuel to keep the air/fuel ratio safe.

Jet size is not the only factor affecting fuel flow

While changing the jet size can affect fuel flow, it is not the only factor that determines fuel flow. There are several other factors that can impact the amount of fuel that flows through a carburettor.

For example, the source of the fuel, whether it be pump gas or race gas, can impact the stoichiometric ratio and, consequently, the amount of fuel that needs to flow through the carburettor. Additionally, the design of the carburettor itself, including the size of the angle channels, tubes, and metering blocks, can restrict fuel flow and prevent a larger main jet from flowing additional fuel.

Furthermore, factors such as engine compression, air temperature, and ignition timing can also influence fuel flow and power output. Changes in these factors may require adjustments to the carburettor, such as changing the jets, metering blocks, or air bleeds, to optimise fuel flow and engine performance.

It is also important to note that jet size may not significantly affect fuel flow at part throttle or during city driving, as the carburettor may not be flowing at full capacity. In such cases, other factors, such as spark plug reading, ignition timing, and vacuum advance, may have a greater impact on fuel economy.

Therefore, while jet size can be a factor in fuel flow, it is important to consider the broader context of the engine setup, carburettor design, and driving conditions when assessing the impact on fuel flow.

Spark plug reading is a useful tuning tool

Spark plugs can also be used to check ignition timing by observing the colour of the ground strap and the position of the "blue line" on the strap. If there is no blue line, there is not enough timing, and if the blue line disappears, there is too much timing. Ideally, the blue line should be just below the sharp bend in the ground strap.

Other things to look out for when reading spark plugs include sooty threads, which can indicate that the plug is not tightened enough, and spotting on the porcelain, which can indicate detonation.

Tuning for best power also tunes for best mileage

Tuning for the best power and mileage involves optimising the air-fuel ratio, which is the ratio of air to fuel that will result in perfect combustion. The stoichiometric air-fuel ratio is 14.7:1, meaning that for every kilogram of fuel the engine burns, it consumes 14.7 kilograms of air. This ratio is important because it is the point at which the minimum amount of pollutants are emitted, and the factory-fitted catalytic converter can do its job best.

However, running a slightly leaner air-fuel ratio can improve fuel economy. A leaner ratio means that there is more air than fuel in the mixture. My own testing has proven that using an air-fuel ratio of around 15.5:1 will show a small but noticeable improvement in fuel economy. Direct injection technology can also show some worthwhile improvements in fuel economy as they have excellent control over when the fuel is added and how it is mixed with the air.

To further improve fuel economy, you can minimise pumping losses. Pumping losses occur when the throttle body is closed during cruising, creating a vacuum in the intake manifold as the engine works harder to pull air past the throttle. This reduces the efficiency of the engine at low throttle openings.

In addition, a better exhaust design or a less restrictive intake can allow the engine to breathe easier, improving volumetric efficiency and aiding economy. When getting your engine tuned, you can also consider lean cruise options which can provide reasonable gains in economy.

It is worth noting that while a leaner air-fuel ratio can improve fuel economy, there is a limit to how lean the mixture can be. With conventional engines that use fuel injectors in the intake ports, a very lean mixture may cause the engine to misfire as the combustion charge is too lean for the spark plug to properly ignite. Therefore, it is important to strike a balance when tuning for the best power and mileage.

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