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Budget A/F ratio meter
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Posted by: Volvord 784VC

To fine tune the Carburetor or EFI system to get the maximum power and throttle response you have to be able to identify the air / fuel ratio during idle, cruise, power, and acceleration is by using an AF meter. You don't have to buy an expensive AF ratio meter, a budget solution is to install a narrow band oxygen sensor in the header collectors and measure the voltage with a DVOM (digital voltmeter).

An Oxygen sensor is a chemical generator. It is constantly making a comparison between the Oxygen inside the exhaust manifold and air outside the engine. If this comparison shows little or no Oxygen in the exhaust manifold, a voltage is generated. The output of the sensor is usually between 0 and 1.1 volts. The ideal mixture for gasoline this is 14.7 parts of air to one part of fuel, but to protect the engine and maximize power and throttle response this level needs to be varied slightly. When the engine has more fuel than needed, all available Oxygen is consumed in the cylinder and gasses leaving through the exhaust contain almost no Oxygen. This sends out a voltage greater than 0.45 volts. If the engine is running lean, all fuel is burned, and the extra Oxygen leaves the cylinder and flows into the exhaust. In this case, the sensor voltage goes lower than 0.45 volts. Usually the output range seen is 0.2 to 0.7 volts.

The sensor does not begin to generate it's full output until it reaches about 600 degrees F. Prior to this time the sensor is not conductive. It is important to locate the sensor close to the engine and exposed to a full exhaust stream. The easiest solution is to install the sensor into the stock location, if your exhaust manifolds or headers have a bung, if not it is easy to have a bung welded into the header collector or front exhaust pipe. The O2 sensor is constantly in a state of transition between high and low voltage. It is important to remember that the O2 sensor is comparing the amount of Oxygen inside and outside the engine. If the outside of the sensor should become blocked, or coated with oil, sound insulation, undercoating or antifreeze, (among other things), this comparison is not possible.

The following reference chart identifies the normal voltage an oxygen sensor generates in relationship to the air/fuel mixture.

Minimum VoltageMaximum VoltageAir / Fuel Ratio

0.0 Volt - 0.1 Volt - 17.0 : 1
0.1 Volt - 0.2 Volt - 16.0 : 1
0.2 Volt - 0.3 Volt - 15.5 : 1
0.3 Volt - 0.4 Volt - 15.0 : 1
0.4 Volt - 0.5 Volt - 14.7 : 1
0.5 Volt - 0.6 Volt - 14.6 : 1
0.6 Volt - 0.7 Volt - 14.5 : 1
0.7 Volt - 0.8 Volt - 14.2 : 1
0.8 Volt - 0.9 Volt - 13.2 : 1
0.9 Volt - 1.0 Volt - 12.5 : 1

Using a DVOM connected to the O2 sensor warm up the engine to operating temperature. Take voltage reading at idle, wide open throttle, part throttle cruise, and part throttle power modes. The ideal voltages you should obtain are:

Circuit - O2 sensor Voltage - Air / Fuel Ratio

Idle - N/A - N/A
Wide Open Throttle - 0.9 - 1.0 V - 12.5 : 1
Part Throttle Acceleration0.7 - 0.8 V - 14.0 : 1
Part Throttle Cruise0.25 - 0.4 V - 15.5 : 1

More info on Edelbrock carb tuning is available on my site in section:
http://www.telusplanet.net/public/g...edelbrock_1.htm

Posted by: VETTKLR

I'd say that's a little cheaper than the Dynojet Wideband Commander I just picked up.

Good info, Volvord

Posted by: cswrench

All I know is a narrow band o2 sensor will bite you in the ass. The narrow band is not nearly accurate enough to tune off of. Especially in Boosted applications where running 11.0-11.9 AFR Is the norm. Just yesterday I had my car at the dyno. My O2's were reading 980's and I was running 12.9:1!!!!!!

At that rate I will melt down the motor in days.

Unfortunately It was a charity dyno day and was not able to tune at all.

I have on the way 2AO unit from www.wbo2.com

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