Dramatic changes in engine performance can be caused by changes in the weather. The reason for this is that your carburetor (or injector) meters fuel based on a volume of air. But the amount of oxygen in that volume of air changes with the weather, so carbs and injectors must be adjusted to maintain the best oxygen/fuel ratio for your engine.

This article describes how to monitor those changes and calculate the Relative Air Density so you can select the proper metering jets for changing weather conditions.

WHAT’S RELATIVE AIR DENSITY?

In order for an engine to produce maximum horsepower, the proper jet or injector nozzle size must be determined. But, this will change as weather conditions change. In order to correct for weather conditions, it is necessary to know the optimum Relative Air Density (RAD) for the jet or nozzle size.

RAD is the combination of temperature, barometric pressure, and humidity. You will need to measure each of these, but the instruments don’t have to be expensive. A good quality household weather trio is OK. However, it is important to use the same instruments for consistent results.

Once you have determined the optimum jetting for a particular RAD by ET, MPH or RPM testing, you can record it on a log sheet. If other changes such as timing are made, you will want to note that, too, on the log sheet. You will develop a database with the log sheet that will allow you to go back to that tuneup another day. Simply find the RAD on the new day and refer to your notes for the jetting and/or tuneup used previously for that RAD.

Another point of interest is that HolleyTM carburetor jets are calibrated in increments corresponding to a 3% change in fuel flow. In other words, each change in jet number produces a 3% change in fuel flow, so as a rule of thumb a change in RAD of 3% should require a change in jet size of one number. For example, if you were using number 78 jets, and the RAD changed from 100 to 97, you will probably want to change to number 77 jets. Also keep in mind that all HolleyTM carbs are calibrated and jetted at 60 degrees F, 29.92 in.Hg, which yields a RAD of 99 using the method described here.

Caution: Running your engine too lean can cause engine damage! Always experiment on the rich side and gradually go leaner to obtain the best performance. Always monitor plug readings and engine temperatures.

Click here to download the Air Density Calculator. (You will need Microsoft Excel to open this file.) For a paper copy with all the formulas, send an email to: The price is $5.95.

For you bracket racers who wish to take this a step further and be able to predict changes in your ET due to the weather, send a check for $16.50 + $2.50 shipping and handling to:

E.T. Predictor Solutions
c/o Autoworks
P.O. Box 82
Collingswood, NJ 08108