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3528 Highway B
St. Charles, MO 63301

Phone: 636-723-4996
Fax: 636-723-1277


Technical Information
Air/Fuel Ratio Monitor
(Rich-Lean Indicator)

The air/fuel ratio monitor measures the air/fuel ratio of a running engine. It is made up of two major components: an oxygen sensor and a display unit. The oxygen sensor must be mounted in the exhaust system slightly downstream of where all manifold runners merge. This location gives the highest exhaust temperatures and provides a sampling of all the cylinders on that manifold.

The monitor will work on all types of fuels. It has been used on engines fueled with gasoline, gasohol, methanol and ethanol -- all with good results. The monitor works on all types of engines, from the smallest motorcycle engines to 500+ cubic-inch alcohol engines. The monitor is calibrated to work on any engine to indicate maximum power, not maximum economy.

This air/fuel ratio monitor will allow an engine to be tuned to optimum performance very quickly. This is of particular interest for race and performance vehicles. Changes to the engine will affect the breathing and fueling characteristics of the engine can be made to perform to the optimum without costly dyno time and/or many trial and error attempts. Even changes in ambient air temperature, pressure and humidity can be adjusted to obtain the best performance. This new technology is much more accurate than exhaust gas temperature equipment. And of course, light years better than “reading” plugs.

The oxygen sensor produces a voltage proportional to the lack of oxygen in the exhaust once it is up to operating temperature. What this means is the sensor is tuned to the optimum air/fuel ratio for an engine, that is all the fuel is consumed by all of the available air in the combustion chamber. This is called stoichiometry. This condition produces the most power out of an internal combustion engine. When the air/fuel ratio is lean of stoichiometry the engine has to pump air that produces no useful work in the combustion process. This extra air appears as free oxygen in the exhaust. As the air/fuel ratio goes richer than stoichiometry, the engine again loses power (to a lesser degree than going lean) because the excessive fuel displaces air that could be used to consume fuel. The excessive fuel also tends to cool the combustion, reducing efficiency as well.