|| Procedures ll Data ||

Air Fuel Ratio Meter

The Air Fuel Ratio (AFR) is the ratio of air to fuel present at combustion. It is also commonly called "mixture". The ideal, or stoichiometric (stoich) ratio of fuel to air is 14.7 parts of air to 1 part fuel.

Typically, across the Eunos range, the ECU will attempt to run at stoich. This is accomplished by the ECU reading the output of the Oxygen Sensor (O2) and adjusting fuel input accordingly. However, this only occurs at times of light load / cruising (closed loop) and at other times the ECU will make fuel available based on other inputs such as the Air Flow Meter, Throttle Position Sensor, Revs per minute and Temperature. Once any serious modifications are made, the ECU can get these complex calculations seriously wrong as it no longer receives reliable input. For example, put a turbo on a naturally aspired engine without modiying the ECU and it will not be capable of increasing fuel input to match the increase of air the turbo inducts. The result? An AFR that is too lean and will result in detonation of the engine. This is an expensive repair.

Also, lean or rich mixtures produce less power than does the stoich mixture.

However, the main goal in this instance is to visually monitor the AFR to prevent a lean running condition under load during set up phases. It is not a permanent fixture. (Once the operational ratios are confirmed there is no longer any need to monitor them further; they will not change as the ECU will remain consistent.) The intention is to learn the current AFRs which are known to be good, ie not dangerous regarding detonation. When the ECU arrives back from Japan (fuel map modifications, speed cut removal and boost triggered fuel cut removed), the behaviour of the AFR can be compared against proven operational parameters without risking several thousands of dollars in rebuild costs.

Any variation to the amount of air flow (intake or exhaust) needs to be monitored to ensure the modification doesn't risk the destruction of the engine itself. You've been warned!


The kit from Jaycar:
There's not really much point in describing the construction and calibration of the kit as the instructions provided are sufficient.
I also got a 3 pin connector kit so I could remove the AFR meter from the Cosmo - its only a diagnostic tool and not necessary for normal use.

The total materials required are:
- Jaycar kit number KC5300
- Narva part number 56273BL (optional if tou want it removabe)
- 1 metre .75mm 3 core flex (or use the cable suppied)

- Normal hand tools including piiers, screwdrivers
- Soldering iron

Level of ability - medium.

The completed kit, sitting on the Cosmo ash tray. (Note the screws aren't fastened yet or the light sensor aligned as this was prior to final calibration.
There are only 3 wires to connect to the AFR meter - 12V positive (ignition), negative and O2 sensor input. The first 2 can be picked up somewhere under the dash and the O2 sensor wiring can be either picked up in the engine bay or at the ECU. As this is a "common" thread, I haven't given ECU wiring details at it will vary between vehicles. (If you cannot identify the correct source of the O2 wiring, please forward the details of your vehicle and I will attempt to assist.)  
The connection was made via the 3 pin connector as above. The plug was located adjacent the drivers left calf as shown:
So can be hidden away when not on use.
Once plugged in, the meter can be placed in a convenient viewing location.
This addition is highly recommended if you are looking at any serious performance upgrades. It very well may save an engine rebuild if used correctly.