More Options Search

You Need To Know Your O2

Air/fuel ratio measurements-easy, quick, affordable

Published online: Jan 05, 2006 Feature Dennis Lawler

Tuning an engine for maximum power previously required long trial and error sessions at the track and/or the dynamometer.

It has long been known that monitoring both EGTs (exhaust gas temperatures) and AFRs (air/fuel ratios) is the quickest and purest way of analyzing combustion efficiency on any engine. With both sources of data available to the tuner, the dial-in clarity is in "High Def."

So what is AFR and why should snowmobilers care about it? For gasoline-driven engines, the theoretically optimal air/fuel ratio is 14.7 lbs. of air for every pound of fuel.  At this ratio, theoretically, all available oxygen in the air combines with all available fuel.  This ratio is called the stoichiometric ratio and the value is 14.7 (it takes 14.7 parts of air to combine with 1 part gasoline).

In simpler terms, if your measured AFR for a regular gasoline-powered engine is below 14.7, you are rich. If you are above 14.7, you are lean. The optimal AFR-the setting that produces the most horsepower and torque-is different for each engine, especially 2-stroke engines. A general guideline is that if your engine is running slightly rich, say between 12.9 to 13.5 AFR, you will be close to getting the most out of your engine. A lower number is rich and a higher number is lean. Many of the big guys have been using this data for some time but soon we'll all be talking like the big guys.


Pretty Pricey

The problem in the past has been that the EGT side of the tuner's dream has been fairly simple and affordable while the AFR side has been prohibitory, with costs more than $1,000 per cylinder.

The problem was in the sensor itself. Most auto OEMs incorporated a narrow band O2 sensor and that produced only rough air/fuel ratio measurements. These sensors produce a rough electrical signal, which causes the sensor to function almost like an on/off switch.  They can detect whether an engine is rich or lean, but not to a fine degree of accuracy.

A few years back, Honda Motor Corp., worked with NGK and Bosch to develop a wide band sensor that would produce a refined analog signal at a much lower cost. The result has produced competing sensors at varying accuracies and cost. As usual, the cream always floats to the top and that cream is the Bosch LSU-4.2, five-wire sensor. These-to date-are the most accurate and durable in both 2-cycle and 4-cycle engines. The cost is less than $80 and in this day and age, that's not bad.

Now that we have a sensor, let's look around for instruments that give reliable readings and can live in the rugged conditions of outdoor motorsports. There seems to be a reasonable selection of analog instruments in the market and a couple of them use the Bosch LSU-4.2 but the shortfall is in the unstable (bouncing, jumping, crashing) world of motorsports. For anyone with analog gauges, you know what I mean. Try to read a moving pointer while blasting down the test or race course at full speed and then try to remember the exact reading five minutes later. Got the picture?


Digital Is It

Digital is the way to go and the only digital in the world is made by Innovate Motorsports and its award winning LM-1 hand held and the new LC-1 panel mount stand alone gauge kit. They offer high quality, very high accuracy Air/Fuel Ratio Meters, Data Acquisition Systems and Software Tuning Tools. The Innovate people have rocked the motorsports world with their breakthrough designs and are now adapting their technology to snowmobiles and ATV tuning at a price we gearheads can afford.

This equipment is also self-calibrating and compensates for changes in temperature, altitude and sensor condition. With the LM-1, precise AFR measurement allows the user to correctly adjust many variables, including carburetor jetting, fuel injection, turbo fuel curves, etc., without long and expensive dyno sessions. Exhaust Gas Technologies is doing extensive field testing and will report the results to you soon.  


(ED-Lawler is owner of EGT.

For more information,

contact 800-348-4678.)