Throughout our publications, air intake and fuel delivery values are converted to a ‘uniform standard of units’. That is “pounds per revolution”. The tuning demands for weight of air and weight of fuel become easy to understand, record, and analyze with that numerical value. And easy to change in fine tuning increments with this simple uniform standard.
Racing Lingo Often Not in Convenient Standard of Units
Racers often speak of differing standards such as metric units of cubic centimeters per rev, English units of cubic feet per minute, pounds per hour, and others. Racers become used to tuning within their combination of units. However, it can be difficult to tune precisely with a mixture of air cubic feet per minute to fuel pounds per hour for example. A vehicle or vessel that has to accelerate is not at any one fuel or air demand level for a minute let alone an hour. Air & fuel amounts are constantly changing. In another example, English units of pounds per hour differ greatly from metric units of cubic centimeters per revolution. It is more difficult to determine the needed changes in fuel amounts from changes in air amounts with differing units and time intervals. Trial and error iteration from a ‘guestimate’ becomes the practice. The outcome is often a backfire or melted engine component. The tuner has to remember the amount of air as well as the amount of fuel. With air to fuel ratio, the tuner has to remember only one value.
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We were able to avoid the dilemma of differing units when we developed our first performance manual, “Fuel Injection Racing Secrets”. The problem of how to present ratios of air to fuel combustion values with differing units was solved with a conversion to a uniform standard:
– pounds of air per engine revolution
– pounds of fuel per engine revolution.
AFR Standard Is Well Proven and Easy to Apply
We determined air to fuel ratio numbers with this uniform standard for a thousand run results and hundreds of race engines. We verified AFR values with those from many combustion engineering project reports. Air to fuel ratio with this uniform standard proved to be absolute and repeatable for tuning. It was also a lot easier to write about air and fuel tuning using this uniform standard and a lot easier to illustrate with a math analysis. Pro-Calc, our FI nozzle & jetting calculator, is the latest result that provides easy-to-do numerical control of AFR for accurate tuning of naturally aspirated or forced induction engines.
In our previous high compression blown Hemi V-8 engine, we ran an air to fuel ratio of 3.4 to 1. That was 3.4 pounds of air to 1 pound of fuel. How we determined that numerical control is illustrated in detail throughout our manuals, “Fuel Injection Racing Secrets” & “5000 Horsepower on Methanol”. In many runs at different blower overdrives and air densities, we were able to precisely determine the fuel system jetting & nozzle sizes ahead of time. Earlier, we did it ‘long-hand’ using the math that is reported in our books. Later we did it ‘short-hand’ using our Pro-Calc calculator. For differing race conditions and weather, we were able to precisely adjust jetting and nozzles to maintain the following values, for all runs:
- constant low speed air to fuel ratio (3.2 to 1) with the high speed bypass closed that provided a consistent and best launch
- constant high speed air to fuel ratio (3.4 to 1) with the high speed bypass open that provided the best power and consistent optimum spark plug readings on all cylinders.
With appropriate adjustment to nozzles & jet size to maintain these AFRs, the engine started the same, warmed up the same, and colored the spark plugs the same regardless of changes from air density conditions as well as various supercharger overdrives. The air to fuel ratio number remained a constant for the life of this engine. It was a repeatable value. Air to fuel ratio tuning remained exact for all of our runs with no piston, ring, or valve damage from any overheating ever. No backfiring, bog, or dead rich condition ever occurred with the correct AFR number.
We repeated the practice of AFR tuning with a different engine size, engine compression, and various low percentages of nitro fuel. Although the baseline air to fuel ratio numbers changed uniquely with differing combinations, each unique AFR remained exact and repeatable for each combination, also with no damage or problems. A new air to fuel ratio went with each new combination. Appropriate AFR values for differing fuels are reported throughout our manuals, “Fuel Injection Racing Secrets” & “5000 Horsepower on Methanol”, as well.
Low & High Speed AFRs
In the first step of fine tuning, a high speed bypass is added to mechanical fuel injection. There is an air to fuel ratio with the high speed bypass closed and another value with the high speed open. The behavior of the racing engine is dependent on the difference between those air to fuel ratios. If the difference is too great, the engine will be in one of the following behaviors:
- too rich with the high speed bypass closed and just right with it open
- just right with the high speed bypass closed, and too lean with it open.
A common mistake with mechanical fuel injection is a high speed bypass jet that is too large that causes one of these conditions. Without knowing the AFR before and after the high speed bypass function, there is a lot of guessing to tuning. In our methanol setup with precise AFR control, any high speed jet larger than 0.040 inches was too large. That is a lot smaller size than what I saw in many other racers.