Fuel Mixtures

There are several ways to check the air/fuel ratio. The first is plug color and condition. With cars it's easy -- just pop it out of gear at the finish line or end of the straight-a-way, and click off the ignition at the same time. DON'T DO THIS IF TURNING OFF THE IGNITION LOCKS UP THE STEERING WHEEL. The other method used to check the air/fuel ratio is to monitor the engine performance in relation to weather conditions. If you looked at a mixture VS horsepower curve, you would notice the rich side of maximum horsepower is more horizontal than the lean side which tapers off more rapidly.

Most engines are set on the rich side so they can accommodate a couple thousand feet of better air. This is why during a race, at night; a car will usually run better as the air gets cooler. Cooler air means a lower Density Altitude, and thicker air. If the fuel remains the same, the mixture will be getting leaner. As the mixture gets leaner, the engine will develop more power because it is approaching the maximum horsepower point. It is advisable to stay slightly on the rich side for two reasons. The first is: there would only be a minimal loss of power; the second is: engine damage will usually occur if you operate on the lean side. This engine damage can happen if your engine is barely on the lean side. Minimal damage can be detected by a white plug color with black speckles on it. These speckles are parts of valves, pistons, and combustion chambers. An ideal plug color would be white with no speckles, but that may be cutting it a little close--better stick with a light tan color. Other warning signs would be a substance oozing out between the center electrode and the porcelain, blistering of the porcelain, or excessive heat coloration on the ground electrode. If you monitor the Density Altitude, and noticed the performance of your vehicle increased as the Density altitude increased--your engine is running in a lean condition. If the Density Altitude increases, the air gets thinner. If the fuel stays the same, the mixture gets richer. If the performance increased, that means the engine liked the richer mixture. This means your engine was operating on the lean side, and is now approaching the maximum horsepower point from the lean side. Thinking of it another way would be: If the Density Altitude gets better (lower) as you go into the evening and night, and your vehicle slows down, then your engine is to lean. Once again, this means your engine is operating on the lean side already, and is now going further down on the lean side. Or if your runs, during a whole evening of racing, remained close to one ET, then your are operating just a tad on the rich side, and are trending to a tad on the lean side. This is nice, but is very unpredictable. Instead, if you were running a little more on the rich side, your car's performance would change with the weather, but would be predictable. In either case, it would be advisable to richen your mixture.

When evaluating whether a run is better or not, look at the MPH only. MPH is directly reflecting the horsepower being made. The horsepower comes from the air that is available. ET is merely a side product it is just along for the ride, but it is dependant on traction being available. It is imperative that you stay in one lane because of differences that may exist between the MPH clocks.

For Bracket racing or Super Class racing, the mixture only needs to be touched up a few times a year as the weather swings from season to season. The mixture will also have to be adjusted if you travel from city to city with different elevations, temperature, and humidity. Staying on the rich side will also keep the vehicles performance trending predictably with the weather changes. NEVER CHANGE THE MIXTURE DURING TIME TRIALS OR ELIMINATIONS. ANTICIPATE THE BEST AIR YOU EXPECT TO ENCOUNTER, AND SET THE MIXTURE BEFORE THE FIRST TIME TRIAL RUN. IF YOU MAKE A CHANGE IN THE MIDDLE OF COMPETITION, IT WILL BE UNKNOWN WHAT ET YOU WILL RUN AFTER THE CHANGE.

For Class racers or other forms of racing where the performance of each car is important, the mixture should be changed when the weather dictates a move to the next possible mixture setting. This could be from round to round or from a heat race to the main event. Typically, you don't have to stay on top of it that closely. You should be enough on the rich side to absorb the air getting a little better during the evening.

Instead of letting the weather change which effects a mixture change, you can change the mixture and monitor its effect on performance. You can do this test first, at home, before you go to the track if the neighbors don't mind too much, or at the track if you have to. Pull out any launch chip that you may have. Thoroughly warm the engine to 170 degrees. Set the tranny brake, and jam the throttle down quickly. If the engine sounds bad when you put it in gear, or doesn't come up on the converter quickly, then make a large change in the rich direction of about .010" on the Main By-Pass (smaller) or 3 sizes larger on a Holley jetted in the 80s or 2 sizes if jetted in the 90's. If it liked it (higher stall speed on converter), go another .005" smaller on a main by pass, or a little larger on a Holley Jet. Keep doing this as long as the stall RPM goes higher. When the RPM stays the same as the last Tranny Brake Dyno Pull did, the mixture is very close. Now make an actual run. If going richer made the stall RPM lower, then put in a main by pass that is .005 larger (leaner) than the original one from the manufacturer or go down a size or two on the Holley Jet. Keep doing this Tranny Brake Dyno Pull with progressively larger pills or smaller jets till the RPM drops, then back up to the last one that gave the highest RPM. Now you are ready to make a run.

At the track you can do the same test as above except instead of monitoring the stall speed on the torque converter monitor the MPH instead.