… by Experience & Gut Feel
Throughout the racing community, experience and gut feel are the most common methods for the selection of nozzle and jet size numbers for setup and tuning of mechanical fuel injection. It is a form of iteration or ‘connecting the dots’. Given enough time and expense, experience from good and bad runs will reveal a tune-up. The experienced tuner knows nozzle and jet size numbers for specific air densities. The experienced tuner also knows the spark plug color, engine temperature, and performance level result. However, it is often difficult to connect the dots using iteration from these past setups to determine a new setup. I’ve seen many experienced tuners struggle with the selection of nozzles and jets for a new engine size, a new blower overdrive, or a new location with a different air density. Engine backfires, melted pistons, or heads are a frequent consequence of a new setup from competitors at racing events around the world. In the absence of experience or a limited amount of experience, engine damage and competition losses are an even more frequent consequence of a new setup.
… with Accurate Numerical Control
However setting up numerical control over the selection of nozzle and jet sizes based on an optimum air to fuel ratio value can be done instead, regardless of whether you are experienced or not. It can make a new setup run really well the first time out. It also saves the cost and risk of numerous runs with nozzle and jet size ‘hits-and-misses’.
A. In a simple mechanical fuel injection system, the air to fuel ratio of a tune-up is a phenomenal measurement of that tune-up.
B. In a more complex mechanical fuel injection system with more than one bypass or enrichment nozzle, an air to fuel ratio of each segment of the fuel curve can be determined. With numerical control each of those air to fuel ratios can be maintained for a variety of air density values. And with numerical control each of those air to fuel ratios can be fine-tuned in response to run result data. That can be done completely independent of the other air to fuel ratios and their time increments.
AFR Historic World Standard for Engine Design
The air to fuel ratio (AFR) number is a simple numerical value. It has been the standard for over 100 years throughout combustion engineering development of engines all around the world. These projects involved thousands of combustion engineering man hours of analysis with engine tune-ups well rated from one development to another by simple air to fuel ratio numbers. All major production vehicle & vessel engines are air to fuel ratio measured, analyzed, and regulated.
Air to fuel ratio marries most all sources of engine fuel determination:
- from chemistry to field installation testing
- from one engine design to another
- from one engine size to another
- with supercharged engines, from one boost level to another.
AFR Historic World Standard for Air Density Corrections
In addition, air to fuel ratio is the most stable baseline target for fuel corrections to air density changes from:
- one weather front to another
- one altitude to another
- one season to another
- one location to another.
Our Racing, Publications, & Pro-Calc Use the Same AFR Historic World Standard
In running our drag race blown V-8 on different fuels, we developed air to fuel ratio numbers that best controlled our engine. Comparing an AFR number from one setup with numbers from different setups from prior races brought a stable tuning baseline for all subsequent races.
Simple Math for Precise Numerical Control of AFR
Throughout our manuals, simple math is presented to calculate nozzle & jet sizes to deliver a predetermined amount of fuel necessary for the amount of air intake. That is done instead of ‘guestimates’ of those numbers from experience (or a shot in the dark from lack of experience). That is done instead of iteration from previous runs (or ‘hit & miss’ iteration from a lack of previous run experience).
Reference Index of Math & AFR Info in Our Books
Fuel Injection Racing Secrets
- normally aspirated & blown V-8 illustrations to determine nozzles, main bypass, and high speed bypass sizes
- numerical example using blower displacement in supercharged engines to determine the weight of air; to determine the weight of fuel; to determine nozzles and jetting
- theoretical AFRs for different racing fuels
- actual examples of AFRs from various racers with different fuels.
- numerical example to compute boost in order to determine the amount of air and to determine the amount of fuel.
- numerical examples to determine air density and grains of water from weather correction. This helps to determine the amount of air correction and to determine the amount of fuel.
- flame speed & AFR
- the relationship of AFR to lambda for fuel control used in electronic fuel injection
- the relationship of AFR to equivalence ratio used in combustion engineering
- extensive analysis of AFRs used in racing with methanol
- various nitromethane methanol mixture AFRs.
Jetting for Racing Mechanical Fuel Injection – Small Block
- math to determine air flow illustrated for small blocks
- math to determine fuel flow illustrated for small blocks
- math for AFR illustrated for small blocks
- small block V8 illustrations used to determine nozzles, main bypass, and high speed bypass.
Jetting for Racing Mechanical Fuel Injection – Big Block
- math to determine air flow illustrated for big blocks
- math to determine fuel flow illustrated for big blocks
- math for AFR illustrated for big blocks
- big block V8 illustrations used to determine nozzles, main bypass, and high speed bypass.
Reference Index of AFR Info from our On-line Nozzle & Jetting Calculator
Pro-Calc fuel injection on-line calculator
- provides an easy to use data entry, calculations, and readout to determine nozzles and jet size, low & high speed AFR, fuel pressure, fuel pump size, air density values, and air density corrections
- AFR can be calculated from nozzle and jet sizes
- Nozzle and jet sizes can be calculated from AFR
- AFRs can be determined with and without a high speed bypass.
Reference Index of Free AFR Info on our Web Site
- free articles: Racing & Highway Fuels, Ultimate Tuning of Mechanical Fuel Injection, Air Density Effects
- free news letters: 03/20/2012: Fuel Injection Tuning & Tax Time, 11/01/2011: Different Racing Fuels.
About the Math
This technical information applies to most any normally aspirated engine size, turbocharged engine size, or forced induction supercharger size if so equipped.
Fuel injection nozzle and jet sizes can be calculated ahead of time from an appropriate air to fuel ratio number. This can be done to determine what nozzles and jets to buy for a new setup; what spares to buy; what jets and nozzles to start out with for a flow bench setup, a dynamometer setup, initial test outing, or initial race.
Recently a new FI engine was set up for a weekend dynamometer test. A ‘guestimate’ was made of nozzles, jets, and spares for the test. The engine was too rich, and target performance could not be achieved. The expensive dynamometer test could not be completed. Subsequent analysis revealed AFRs from the nozzles, jets, and spares to be way off. All the time on race weekends, I see a tuner picking through a tool box for a best fit nozzle or jet that is not quite the one that is needed because of a shortage of the right spares. The expense of the test outing is incurred only to identify that another test outing is needed with another ‘guestimate’ of the right parts. The alternative for us instead was using AFR to determine proper nozzle and jetting needs ahead of time. We were able to experience a reliable set up, most often right out of the trailer, for most any track or course, elevation, and weather that we raced in. It freed us to concentrate on other demands such as vehicle or vessel setup, qualifying position, driving, and of course the picnic.