top of page
  • Writer's pictureTechTune Performance

Understanding Air-Fuel Ratios

Updated: Oct 21, 2023

The air-fuel ratio (AFR) is a critical component of any internal combustion engine's performance. For diesel engines in particular, understanding and optimizing the AFR during dyno tuning can greatly influence the engine's power, efficiency, and emissions. This article explores the importance of AFR in diesel vehicles undergoing dyno tuning.

Understanding Air-Fuel Ratios

In simple terms, the air-fuel ratio refers to the proportion of air to fuel in the combustion chamber. In diesel engines, this can vary significantly due to the nature of diesel combustion. Unlike gasoline engines, which operate at a stoichiometric AFR (around 14.7 parts air : 1 part fuel), diesel engines typically operate lean, meaning they use more air than is theoretically required for complete combustion. With a turbo diesel, it begins to smoke/soot at an air-fuel ratio of 16-15:1. In order for a turbo diesel to control its exhaust gas temperatures (EGTs) it needs to run lean, where a turbo gasoline vehicle needs to run slightly richer in higher RPM and under higher loads to quench EGT's to run cooler, protect the turbo and prevent knock.

The Importance of AFR in Dyno Tuning

1. Performance Enhancement: Properly adjusted AFR can lead to improved engine performance. Leaner mixtures, with more air, diesel engines have their version called "diesel knock." It's a result of the premature ignition of the fuel-air mixture. Proper AFR, along with injection timing, plays a crucial role in preventing this phenomenon. 2. Fuel Efficiency: A leaner AFR is typically more fuel-efficient, as it allows for a more complete combustion of the fuel. By optimising the AFR during dyno tuning, one can potentially improve the vehicle's miles per gallon (MPG) figures.

1. Emissions Control: A well-optimised AFR can reduce harmful emissions. An overly rich mixture can increase particulate emissions, while an overly lean mixture can increase NOx emissions. Finding the right balance is crucial for environmental considerations.

4. Engine Health and Longevity: Proper AFRs ensure that the diesel fuel is burnt effectively, preventing incomplete combustion. Incomplete combustion can lead to the formation of soot and carbon deposits in the combustion chamber, exhaust system, and diesel particulate filters, which can adversely affect engine health and longevity.

Challenges in Diesel AFR Tuning

  • Variability in Operating Conditions: Diesel engines can operate under a wide range of conditions, from idle to full load. The optimal AFR can vary significantly across these conditions.

  • Turbocharging: Many modern diesel engines are turbocharged. The addition of forced induction introduces more variables into the AFR equation, making the tuning process more complex.

  • Emission Regulations: With stricter emission norms coming into play globally, tuners must ensure that modifications made during the dyno tuning process don't make the vehicle non-compliant with these regulations.

The Role of Dyno Tuning

Dyno tuning provides a controlled environment to test and optimise a diesel vehicle's AFR. By simulating various operating conditions, tuners can fine-tune the engine's performance across its entire operating range. Modern dynos, combined with advanced tuning software, allow for real-time data logging and feedback, ensuring that the engine's AFR is optimised for both performance and compliance.


Understanding and optimising the air-fuel ratio is crucial for the performance, efficiency, and environmental impact of diesel vehicles. Dyno tuning offers a precision tool for achieving this optimisation. As with any engine modification, it's essential to work with experienced professionals who can balance the demands of performance, efficiency, and regulatory compliance. Remember, diesels don't have to soot to make good safe power.

Note: The information provided in this article is for informational purposes only. Before making any modifications to your vehicle, always consult with professionals to ensure safety and compliance with local regulations.

Disclaimer: The information provided in this article is for general informational purposes only. While we endeavor to keep the information up-to-date and accurate, there may be some variations or changes in the actual process or outcomes due to various factors. TechTune Performance is not responsible for any decisions made based on this information.

252 views0 comments

Recent Posts

See All


Commenting has been turned off.
bottom of page