Aftermarket Fuel Tuning - Air Fuel Meter

Here we will discuss Air Fuel Gauges and their operation.

The o2 sensor and meter introduction

Our cars come equipped with an oxygen sensor. The sensor reads the amount of oxygen in the exhaust path and reports to the ECU. The o2 sensor puts out a voltage from 0-1volt to correspond with a particular air/fuel ratio. The a/f meter (gauge) reads the voltage from the o2 sensor and depending on the manufacturer estimates an air/fuel ratio from that voltage which it displays on it's screen.

Open Loop vs Closed Loop

You might notice upon a/f gauge installation that the o2 readings are erratic at anything but a wide open throttle blast. We will explain this. Under ordinary conditions the car's ECU will read data from the o2 sensor in order to determine how much fuel to put into the engine to achieve the volts the ECU will automatically adjust the fueling to try and reach that optimal voltage. This happens very rapidly, several times per second. The readouts from the o2 sensor can drop as far down as 0.20 volts or as high as 0.80 volts. So when you look at the a/f gauge, the lights are bouncing up and down. This continuous feed back process is called closed loop operation.

For open loop operation to happen the ECU has to detect a predefined throttle position (usually around 80%). Under open loop operation the ECU will ignore the o2 sensor output and simply work from a predefined code in the ECU. In the case of a stock ECU this will give you quite a rich readout on the a/f gauge. This is where your power is found. If you can use aftermarket tuning devices such as SAFC to richen or lean what information the ECU gets fed, you can change the open loop fuel curve. Unfortunately most a/f gauges and o2 sensors are not of a high enough resolution to make accurate and repeatable readings for maximum horsepower.

Varying o2 Output

Note that o2 sensor output may vary. This is why tuning via o2 sensor only can provide invalid results.

Here is an example to show how the o2 sensor voltage differs under varying exhaust temperatures.

While having lower a/f ratio results displayed under high temperature is definitely safer, it is not the

best way to find maximum horsepower.

These are actual test results:

Temperature O2 Voltage
500c 0.83
750c 0.74
900c 0.66

Optimal o2 Output - a/f ratio

The optimal a/f ratio for non turbo cars is in fact 14:7. This corresponds to a 0.50 o2 voltage output.

However turbo cars require a much more rich mixture for proper and reliable operation. Depending on who you talk to the optimal a/f ratio under boost varies. Also how much boost you are running can have an effect on what ratio you would want. For general street conditions you should shoot for something in the neighborhood of 12.5:1. This is a generalization and you should do your own research to find out the best ratio for your purpose. Also if you are reading o2 voltage, he best figure we have

been able to attain is 0.82 volts under Wide Open Throttle while boosting. You can see by the gauges listed below it would be rather difficult to find .82 volts OR 12.5:1 a/f ratio. Being that the o2 sensor outputs different voltages at different temperatures, an a/f gauge should not be used solely for tuning.

The Meters

GREDDY

Good - Very close to actual wide range o2 sensor, displays actual a/f ratio, peak hold, playback and warning

Bad - Just expensive, requires detailed break in procedure for accurate operation, requires o2 install

Notes:

Price ~$300

Includes primary o2 sensor

Tomei makes similar unit, but information unknown

K&N

Good - Quality unit, a/f ratio correspondence printed on board

Bad - Much more pricely than the copy cat versions

Light

1

2

3

4

5

6

7

8

9

10

Fuel Ratio

17.1

16.5

16.0

15.4

14.9

14.4

13.8

13.2

12.7

12.1

ITEM

PART NUMBER

PRICE*

Rectangle Complete Kit w/ o2 sensor

85-2437

$212.99

Round Complete Kit w/ o2 sensor

85-2441

$233.99

Rectangle Monitor only

85-2439

$138.99

Round Monitor only

85-2442

$148.99

Oxygen sensor only w/ fitting

85-2438

$79.99

Weld-on fitting

85-21688

$5.99

O2 sensor plug

85-21686

$5.99

Under-dash mount gauge bezel, for round monitor

85-2443

$12.99

O2 sensor bushing and plug

85-2440

$13.99

*Carparts.com prices, can be discounted up to 20% by going to the discounts page.

HALMETER

AF30

Good - Highly visible during driving, highest resolution 30 output LED, 1 or 2 lights at a time

Bad - Large size translates into difficult mounting in any other location than stereo area.

Notes:

Price ~$130

Document #1

Document #2

White Wire is for o2 signal

SPLIT SECOND

Good - Compact unit makes installation very simple, 1 light at a time for easy night driving

Bad - Seems to be calibrated more for an even A/F ration (stoich) for non turbo cars

Notes:

Price ~$100

Light

1

2

3

4

5

6

7

8

9

10

Fuel Ratio

17.0

16.0

15.5

15.0

14.7

14.6

14.5

14.2

13.2

12.5

Voltage

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

INTELLITRONIX/CYBERDYNE

Good - Very inexpensive, LEDs can be removed from case and placed in dash for stealth installation

Bad - Not much of a quality unit, readings sometimes vary between units, many lights displayed at one time

Notes:

Price ~$39

Light

1

2

3

4

5

6

7

8

9

10

Fuel Ratio

17.1

16.5

16.0

15.4

14.9

14.4

13.8

13.2

12.7

12.1

Voltage

0.10

0.20

0.30

0.39

0.49

0.59

0.69

0.78

0.88

0.97

AUTO METER

Good - Relatively inexpensive, 20 LED readout for higher resolution

Bad - A nice unit, but the multiple lighting plus the circular motion can give you a headache at night

Notes:

Price ~$59

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

Voltage

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0.55

0.6

0.65

0.7

0.75

0.8

0.85

0.9

0.95

1.0

EFI SYSTEMS

http://www.efisystems.com

Good - Integrated injection pulse monitor, good price for 2 in 1 unit

Bad - 2 displays in one may make for difficult reading at a glance

Notes:

Price ~$99

White Lead: Injection Puls

Green Lead: O2 sensor connection

JUMPTRONIX

http://www.efisystems.com

Good - Very accurate display voltage in .001 increments, large easy to read screen

Bad - You must have a preset knowledge of what the voltages mean.

Notes:

Price ~$99

Displays actual voltage numbers

WIDE BAND A/F METER

A typical o2 sensor has a vary limited range of measurement. Most of it's calibration is around the 14:7 fuel

ratio while the rest of the spectrum has a very narrow range of accuracy. A true wide band o2 sensor will

measure from 10:1 to 20:1 air fuel ratio in real time. They operate on a 0-5v scale instead of the standard

0-1v. In addition they have circuitry and programming in the measuring devices to accurately display the

proper air fuel ratio. You can source a 0-5v o2 sensor for under $200, but without a device to properly read

the information, it is just as inaccurate as the stock o2 sensor.

FJO Wide Band Measuring devices

FJO offers a host of products to accurately measure and display the air/fuel ratio. The precision

measuring device is calibrated for the more accurate wide band o2 sensor. It also has nifty features like

analog and digital output, with a host of partner products that can aid in tuning:

FJO Wide Band Measuring Unit

FJO Digital Air/Fuel Ratio Display Box

FJO Wide Band o2 Sensor

For more detailed information, CLICK HERE

DIY Wide Band Air Fuel Meter

Enclosed in this zip file are schematics for the device. It assumes you already are familiar with what

you are going to build. It uses a wide band o2 sensor and contains circuitry that you must fabricate

using the schematics. If built correctly it can also display 10:1 - 20:1 air/fuel ratio as well.

A typical OEM switching type O2 sensor will not work with this project. This project requires a 2 cell wide band O2 sensor that was used on a lean burn Honda Civic. It is available from Honda dealers, and from www.hparts.com.

Application/part number: 92-95 Honda Civic 1.5 VTEC 36531-P07-003

Zip File : DIYo2.zip

BOSCH Wide Band o2 Sensor

Use at your own risk. This is the o2 sensor used by many aftermarket ECU and measuring systems for

wide band tuning.

Bosch # ~$199

ALTERNATIVES

WOT tuning only a/f gauges??

Some companies sell the above gauges that have been modified for what they call WOT tuning. They change the scale of the readouts on the LED to correspond to a thing range of voltage. Since the A/F gauges are really only helpful at WOT, they have been tuned to work at WOT only. This significantly raises the resolution and accuracy of the gauge. Here are some vendors that sell "modified" a/f gauges:

0.76-0.96: 10 or 20 LED Increments (Intelletronix or Autometer)

http://store.yahoo.com/machv/cybairgaug.html

0.78-0.98: 10 or 20 LED Increments (Intelletronix or Autometer)

http://www.gadgetseller.com/gauges/modifiedproducts.htm

Apex'i S-AFC O2 sensor gauge???

You can use the blue wire from the SAFC to display actual o2 voltage in 0.001 increments. See our SAFC page for more information

WIDE BAND A/F METER

DIY A/F GAUGE

Make your own? Here are some links:

http://www.scirocco.org/tech/misc/afgauge/af.html

The contents of this website should not be reprinted or "cut and pasted" into another website. Feel free to link here.


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