Air/Fuel Ratios

When you are talking about internal combustion engines, you'll always refer to air/fuel ratios. It doesn't matter if it's an EFI or carburetor metering the fuel; engines reat to A/F ratio. Simply defined, it's the ratio of air to fuel in pounds. For example, if 12 pounds of air are mixed with 1 pound of fuel, the A/F ratio is 12:1. Varying the A/F ratio has a direct effect on engine performance.

How can the A/F ratio be measured? Obviously, on an engine dyno, the ratio is printed out and it's an important number that engine builders analyze. On a Dynojmet chassis dynamometer, if it's so equipped, can take A/F ratio readings of the exhaust, using a Horiba meter. As for in-car, you need to inspect the spark plugs. If the porcelain unsulator is black, then the A/F is too rich. If it's colorless, or white, then the mixture is too lean.

Edelbrock also has a device to help in tuning to achieve the desired air/fuel ratio. It's called an Air/Fuel Monitor (PN 6593), which consists of an oxygen sensor and a calbirated elctronic display. Like the oxygen sensor in an EFI application, this measures oxygen content in the exhaust and based on that, the air/fuel ratio can be displayed. Now you can visually see whether your engine is lean or rich!

The sensor is fitted into the exhaust pipe, according to instructions. Then when the engine runs, the monitor measures between 12 to 15:1. Like OE applications, this sensor can only be used with unleaded fuel. Now, with the engine running, and driving, the A/F ratio can be monitored. Form that information, the carb adjustement and jetting can be modified, along with the ignition spark curve. Based upon A/F characteristics, you can tune your engine accurately. You can also install a second sensor, so there's one for each bank of cylinders, and use either dual monitors or switch between each cylinder bank.

If you drag race, you can also use it to assess fuel system performance. During a pass, glancing at the display, you can see how the engine maintains the desired ratio. It may be a little lean at first, due to fuel being pushed back in the tank and float bowls, but down the track it should recover. If the monitor still indicates a lean condition at the finish line, that's an indication that the fuel system doesn't keep up with the engine demand.

Edelbrock has compiled a list of A/R ratio descriptions and how they affect engine performance.

5:1: Rich burn limit due to excess fuel. Combustion is weak or erratic

6-9:1: Extremely rich mixture. Engine will produce black smoke and low power.

10-11:1: Very rich. Some supercharged engines run in this range at full power to control detonation.

12-13:1: Rich. This slightly rich mixture produces the best power for unsupercharged engines. When you read engine dyno charts for performance engines particularly, maximum power will usually be found in this A/F range.

14-15:1: A 14.6:1 A/F ratio is considered stoichiometric, which is chemically ideal. Theoretically, there's no excess fuel or oxygen remaining after combustion. This is the A/F ratio that the ECU in an EFI equipped car is trying to maintain. Overall, this range is best for part throttle cruise.

16-17:1: This lean mixture is the best A/F ratio for economy. It can be borderline for part throttle, since this lean mixture is unstable and prone to detonation. THis ratio is worse if EGR is used.

18-19:1: Very lean ratio. It's considered the upper limit of drivelability. If the ratio is any leaner, detonation will occur, if it hasn't already!!