Tuning With A Digital Air/Fuel Ratio Meter


Computer or no computer, developing a fuel curve for a motor is a difficult process. There are many points to calibrate in a "digitized" system, which when totaled, comprise the base fuel map. This mapping process must be approached systematically with an understanding of the engine's fuel needs. This manual was written primarily to aid in the calibration of our RSR Fuel Injection Systems but the use of the RSR Digital Air/Fuel Ratio Meter has equal validity in carburetor jetting. Whether you are tapping computer keys or changing needles and jets you are after the same thing...the correct air/fuel ratio.


The mixture of the air and fuel can be expressed in three ways all based on a common point called Stoichiometry. Stoichiometry is the chemically “correct” point at which the most complete combustion takes place which is 14.7 parts of air to one part of fuel by weight. Expressed In three ratios we offer the following correlations:

9.80:1 .1020 1.5
10.50:1 .0952 1.4
11.30:1 .0885 1.3
12.25:1 .0816 1.2
13.36:1 .0748 1.1
14.70:1 .0680 1.0
16.33:1 .0612 .9
18.37:1 .0544 .8
21.00:1 .0476 .7
24.50:1 .0408 .6


The proper air/fuel ratio for each particular set of operating conditions is most conveniently broken down into the two categories steady state running and transient operation. Steady state running is taken to mean continuous operation at a given speed and power output with normal engine temperatures. Transient operation includes starting, warming up, and the process of changing from one speed or load to another.


IDLE: Due to the low port velocity and frictional losses Idle mixtures are typically set at a fuel ratio of 1.2 or, expressed differently, 12.25:1 air/fuel ratio. Your motor will idle at stoichiometry (14.7:1) or less than stoichiometry but this is very near misfiring and If operating temperatures are not stabilized at a high level the motor will die. For example, operational fluid temperatures can vary from 150 degrees Fahrenheit to 250 degrees Fahrenheit and Inlet air temperatures can easily vary 100 degrees Fahrenheit. These variations in temperature all necessitate different mixture requirements so it is far better to keep the fuel ratio in the 1.2 region to preserve idle quality and off-idle responsiveness.

STEADY STATE THROTTLE: At a given RPM under steady state load conditions your mixture strength should be a 1.1 fuel ratio or 13.2:1 air/fuel ratio. At this point you will have your peak cylinder or Brake Mean Effective Pressure (BMEP) figures. The development of a base fuel map is generally done around this figure as It is 11% richer than stoichiometry but within the correction range of a closed-loop oxygen sensing system. This allows the base map to be calibrated for maximum power without excessive fuel consumption and still allows the closed-loop operation to self-adjust to 14.7:1 for normal steady state operation.


The principal transient conditions are starting, warming up, acceleration (increase of load), and decelleration (decrease of load).

STARTING AND WARMING-UP: Abnormally or very rich mixtures are required to start a very cold engine. The air/fuel ratios must be progressively reduced from this point during the warm-up period until the engine will run satisfactorily with the normal steady-running air/fuel ratios. Starting or cranking fuel is also a temperature dependent variable with more cranking fuel required for lower temperatures. Air/Fuel ratios on initial start-up In cold weather can easily be 50% greater than stoichiometry i.e. In the 11.0:1 to 10.3:1 air/fuel ratio range.

ACCELERATION: When the throttle is opened for acceleration, thus increasing the manifold pressure, additional fuel must be supplied to prevent misfiring, backfiring, or even complete stopping of the engine. Injection of this acceleration fueling must take place simultaneously with the opening of the throttle. The optimum amount of acceleration fueling is that which will result In the best power air/fuel ratio in the cylinders.

In general this varies with the engine speed and with the throttle position at the start of acceleration, as well as fuel volatility, mixture temperature, and rate of throttle opening. Since partial or slow opening of the throttle requires less than the full acceleration fueling, the amount of extra fuel is usually made roughly proportional to the throttle opening and the angle through which the throttle moves. Mixture strength under these conditions may be as rich as 12.7:1 on warm engines and perhaps as rich as 12.1:1 on cold engines. When an engine reaches normal operating temperature we should not see acceleration fueling richer than 12.7:1.

DECELLERATION: Under closed throttle decelleration fuel must be controlled to prevent rich conditions or lean Induced backfires. This controlled fuel shut-off can be monitored with the digital air/fuel ratio meter. Decellertion fueling should be no leaner than 17.1:1.


The RSR Air/Fuel Ratio Meter displays digitally via 10 L.E.D.s your exact air/fuel ratio. The ten lights are divided into four color divisions: three green, four yellow, two orange and two red L.E.D.s. The air/fuel ratio Is as follows (G=green, Y=yellow, 0=orange, R=red). Ambient Light Sensor 0

17.1 16.5 16.0 15.4 14.9 14.4 13.8 13.8 12.7 12.1

To the upper right of the display Is a small window which houses an ambient light sensor. This automatically dims the display at night. Using this display you will be able to adjust your base fuel map under conditions of:

1.Steady State Running
2. Starting and Warm-up Cycles
3. Acceleration Fueling
4. Decelleration Fueling
5. Full Throttle Operation


You should be aware of the following limitations:

1. Your 0-Sensor must reach 600 degrees Fahrenheit for the display to become active so it will not "read" any exhaust gases until it reaches this temperature. This takes one to two minutes.

2. Your 0-Sensor will become contaminated if you run leaded racing gas and the display will not give true readings. Using unleaded pump gas the 0-Sensor Is easily reliable for 50,000 miles.

3. Nitrous oxide will confuse the sensor because the Lambda 0-Sensor reads free oxygen content and the nitrous will add oxygen molecules to the system giving false readings on your meter.

There is no magical, absolute, digitized answer to developing your applications fuel requirements. You must drive your machine and a considerable amount of judgement must be placed in such areas as cold start, warm-up, acceleration fueling, idle quality, and driveability. Each motor is different and the calibration will be only as good as the effort you put into it. Much subjective decision making will take place and the air/fuel ratio meter will not provide you all the answers. Stop watches, elapsed times, lap times and your own opinions as a tester are as equally valid.


Basically your 10 L.E.D. readout can be used on a dyno or when you actually drive your machine. The lights can go "out.' if the mixture is way too lean but the last red light will stay lighted no matter how rich the mixture is beyond 12:1:1.

If the lights go green under anything except hard decelleratlon or during active 0 Sensing you are too lean. If the last red light goes on you are too rich to the point where the vehicle will "stumble".

You should concentrate your development so that the operating range centers around the two orange lights.

This is approximately 13.8 to 13:2:1 air/fuel ratio or about 11% richer than stoichiometry.

Acceleration fueling will need to be richer than thls...to the first red light while under load.

Full load steady state maximum power will be made in the second orange light or about 13.2:1 air/fuel ratio.

Light load steady state operations can occur in the yellow lights but not below Stoichiometry. On carbureted vehicles or EFI systems that do not feature closed loop operations maximum economy will occur at Stoichiometry or 14.7:1. This Is the middle of the display.

Idle can vary from the last yellow (14.4:1) to the first red light (12.7:1) depending on how cold the engine is on starting. Once again, judgement Is a major factor in idle quality and there are numerous variables such as warm-up curves, after start enrichment, and temperature corrections to deal with. Idle quality is as much a subjective decision as anything.

Important: You should be aware that on fuel injected vehicles your electrical system's health affects your injector fueling. A poor charging system that cannot adequately support the Injection system's demands will cause the Injectors to "lean-out". As battery voltage drops below the systems built-in voltage correction factors the injectors won't stay open to their preprogrammed values and the system will slowly lean-out as voltage drops.


STEADY/FULL LOAD: To do different steady state load readings at the same RPM use different gears and vary your test route by using level ground and by going up and down constant grades.

Full load readings can be done In the placing the vehicle in a higher gear. test RPM and then go to full throttle lower RPM ranges by Stabilize at the (load).

To avoid warp velocities you can get high RPM full-load readings by using your lower gears. Just stabilize at the test RPM and then Instantly go to full throttle (load).

During your full load or during any transitional load shifts you should look at your acceleration fueling. If the meter goes momentarily lean increase your accelerator fueling. If you see a rich stumble decrease your accelerator fueling.

On the RSR Fuel Injection systems you can adjust both your timed (synchronous) and your untimed (asynchronous) acceleration fueling to meet these requirements.

DECELLERATION FUELING: Adjusting the closed throttle fueling at various decelleration RPMs is important for three reasons.

1. Prevent an over-rich condition.
2. Prevent lean back-fires.
3. Meter the correct amount of fuel so there are no hesitations when you re-open the throttle.

Using your air/fuel ratio meter you want to keep the lights out of the red or orange lights under any decelleration. You also want to avoid having the light go "off" or out of the green (lean) range where backfiring can occur. The decelleration fueling requirements in each RPM range will have to be evaluated to provide instantaneous throttle response when suddenly getting back on the gas". Increase the fueling in these ranges if the vehicle hesitates.


The development of the fuel curve or map for your engine is not as simple as some firms will have you believe. Using the RSR Digital Air/Fuel Ratio Meter you can shorten your development time considerably. The meter will instantly show you if you are outside the envelope by being too rich or too lean.

To summarize you should try, once normal operating temperatures are reached to keep the steady state running in the two orange lights and full power to include acceleration fueling not past the first red light.

One final point concerns the mounting or wiring of the digital air/fuel ratio meter... It is imperative that the black wire of the meter be grounded at the negative terminal of the vehicle's battery. The other two wires, the red and the green are attached to switched + and the 0-Sensor output respectively. If you ground the black wire of the sensor to any other location you are in danger of picking up minute electrical signals from other electrical grounds. These other signals will cause false 0-Sensor readings and can cause the display as well as the fuel Injection to malfunction. Once again...ground the black wire of the air/fuel ratio meter to the negative terminal of the battery.

There is a direct relationship between Carbon Monoxide (CO) exhaust gas percentages and your engine's air/fuel ratio. Your RSR Air/Fuel Ratio Meter is not designed to set CO levels and should not be used for these purposes. The following chart gives the relationship of the percentage of CO to your air/fuel ratio.

0.1 = 14.71 0.2 = 14.53 0.3 = 14.41 0.4 = 14.33
0.5 = 14.27 0.6 = 14.22 0.7 = 14.20 0.8 = 14.16
0.9 = 14.14 1.0 = 14.10 1.1 = 14.08 1.2 = 14.03
1.3 = 14.00 1.4 = 13.97 1.5 = 13.93 1.6 = 13.89
1.7 = 13.85 1.8 = 13.81 1.9 = 13.79 2.0 = 13.76
2.1 = 13.72 2.2 = 13.68 2.3 = 13.62 2.4 = 13.58
2.5 = 13.55 2.6 = 13.53 2.7 = 13.48 2.8 = 13.44
2.9 = 13.40 3.0 = 13.37 3.1 = 13.33 3.2 = 13.30
3.3 = 13.26 3.4 = 13.23 3.5 = 13.19 3.6 = 13.14
3.7 = 13.11 3.8 = 13.07 3.9 = 13.02 4.0 = 12.99
4.1 = 12.95 4.2 = 12.92 4.3 = 12.89 4.4 = 12.85
4.5 = 12.82 4.6 = 12.79 4.7 = 12.74 4.8 = 12.69
4.9 = 12.66 5.0 = 12.63 5.1 = 12.58 5.2 = 12.53
5.3 = 12.50 5.4 = 12.45 5.5 = 12.42 5.6 = 12.39
5.7 = 12.36 5.8 = 12.32 5.9 = 12.29 6.0 = 12.24
6.1 = 12.21 6.2 = 12.17 6.3 = 12.12 6.4 = 12.09
6.5 = 12.06 6.6 = 12.02 6.7 = 11.99 6.8 = 11.95
6.9 = 11.92 7.0 = 11.88 7.1 = 11.85 7.2 = 11.81
7.3 = 11.78 7.4 = 11.75 7.5 = 11.71 7.6 = 11.68
7.7 = 11.64 7.8 = 11.60 7.9 = 11.57 8.0 = 11.53
8.1 = 11.49 8.2 = 11.45 8.3 = 11.42 8.4 = 11.39
8.5 = 11.36 8.6 = 11.31 8.7 = 11.27 8.8 = 11.24
8.9 = 11.20 9.0 = 11.15 9.1 = 11.11 9.1 = 11.07
9.3 = 11.04 9.4 = 11.00 9.5 = 10.96 9.6 = 10.93
9.7 = 10.89 9.8 = 10.85 9.9 = 10.81 10.0 = 10.78


Your RSR Air/Fuel Ratio Meter can be used with Alcohol or Methanol or Propane. A visual short-hand version of your 10 L.E.D.s corresponding Air/Fuel Ratio for these two fuels Is as follows:

ALCOHAL 7.6 7.3 7.1 6.8 6.6 6.3 6.1 5.8 5.6 5.3
PROPANE 17.9 17.3 16.8 16.3 15.7 15.2 14.6 14.0 13.5 13.0

Tuning with these fuels is exactly the same as with unleaded gasoline. The tuning instructions on pages 1-7 apply equally to Alcohol/Propane.