When the engine is being used at a racetrack or anytime you are trying to get the very most from the engine, the air/fuel mixtures should be adjusted correctly for the ever-changing air density. First, the baseline tune-up for the air/fuel mixtures must be set and correct for the conditions of testing. The next step is fine-tuning for changes in the air density.

The readings from a barometer as well as air temperature and humidity gauges can be used to calculate the density altitude or the air density ratio (ADR percentage) change from your baseline jetting. Knowing the density altitude and comparing it to what it was when you established your baseline tune-up will tell you which way you should change your fuel curve for the weather conditions. The suggested change in jet size is to go down one jet size for every 1,500 feet in density altitude you go up. Conversely, for every 1,500 feet you go down in density altitude, go up one jet size. A lot of racers use density altitude as a tuning aid, but ADR percentage is more user friendly. ADR percentage is easier to use because of the following: If the air quality goes up 2 percent, the engine fuel must increase 2 percent to keep the same tune-up. The ADR percentage and density altitude are both air quality indicators. ADR is expressed in percent of standard conditions, and density altitude is expressed in feet above the same standard conditions.

The weather station from Computech Systems offers both the RaceAir Competition Weather Analyzer and the RaceAir Pro. The RaceAir Competition Weather Analyzer supplies a reading for the density altitude or the air density ratio. The RaceAir Pro has many additional features, including a program that can supply you with a suggested jet change that would be correct for the current air density. These changes in jet size are necessary because as the density altitude goes up, the air becomes thinner, thus the engine needs less fuel.

By using the density altitude or the ADR percentage to help calculate which jet size to use, it becomes easy to keep up with the ever-changing weather conditions and stay a step ahead of the competition. This information can also be used to determine which jet you should change to when going to a race at a high-altitude racetrack before you even get there.

A properly tuned fuel and ignition system will allow your race engine to perform up to its potential and will supply you a better running, more reliable race car. Using an extended range oxygen sensor combined with an infrared exhaust gas analyzer and then reading the spark plugs is the best way to know if the air/fuel mixture is correct for your engine. Taking the time to properly tune your race engine's fuel system will not only allow you to unlock all its power, it may help avoid destroying an expensive race engine due to an incorrect fuel system tune-up.

Whether you purchase a new or used infrared exhaust gas analyzer or borrow one from another racer, this method of checking and setting the air/fuel mixture can help you get the most out of your engine. For a more detailed explanation of the use of a five-gas infrared exhaust gas analyzer in fuel system diagnosis, see www.automotiveu.com.

Jetting With An Infrared Gas Analyzer
CO-To-Air/Fuel Conversion
COA/F
Ratio
COA/F
Ratio
COA/F
Ratio
COA/F
Ratio
0.114.712.113.724.112.956.112.21
0.214.532.213.684.212.926.212.17
0.314.412.313.624.312.896.312.12
0.414.332.413.584.412.856.412.09
0.514.272.513.554.512.826.512.06
0.614.222.613.534.612.796.612.01
0.714.22.713.484.712.746.711.99
0.814.162.813.444.812.696.811.95
0.914.142.913.44.912.666.911.92
114.1313.37512.63711.88
1.114.083.113.335.112.587.111.85
1.214.033.213.35.212.537.211.81
1.3143.313.265.312.57.311.78
1.413.973.413.235.412.457.411.75
1.513.933.513.195.512.427.511.71
1.613.893.613.145.612.397.611.68
1.713.853.713.115.712.367.711.64
1.813.813.813.075.812.327.811.6
1.913.793.913.025.912.297.911.57
213.76412.99612.24811.53

When jetting with an infrared exhaust gas analyzer, use this CO-to-air/fuel conversion table for best results.

SOURCE
Barry Grant
Dahlonega
GA
706-864-8544
www.barrygrant.com
OTC/SPX Corporation
www.otctools.com
Computech Systems Inc