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Air Fuel Ratio Tuning on a Sprint Car Engine

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During the course of a race night, you may hear crew members and drivers talking to each other and mentioning the words “altitude, ADR, correction factor, or water grains”. You may think to yourself “What does all that lingo mean?” Those factors are all pieces of the puzzle that come into play when setting the fuel for your engine. We will talk about fueling a sprint car motor in this article, let’s go!

Before we get into the ins and outs of it all, let me first explain why we need to change the fueling on these motors. In order for the engine to make the most power, the air to fuel ratio in the cylinder needs to be optimum. Because the air to fuel ratio is very specific in these engines, the slightest change in air that is being introduced into the engine could have a dramatic effect on horsepower output in return, which is why we need to constantly tinker with the fueling. In this article I will talk about what changes will need to be made with the different readings from an engine computer. Do not worry, it is not as complicated as it sounds (some people actually think they need to over complicate it), and engine builders will generally even give you a sheet to follow to make sure you have your settings correct!

Making sure the fueling will be right for your engine starts before you even make it to the racetrack. In a previous article, I talked about the weekly maintenance. Part of that weekly maintenance includes cleaning your fuel system. By making sure your filters, nozzles, nozzle lines, etc are clean, you have no obstructions that could prevent the flow of fuel.

Once you actually do make it to the race track, you will want to have your engine computer handy. The top teams are using the RaceAir Pro, which will give you quite a bit of information about the air around you. It will give you a few readings like the density altitude, the ADR, correction factor, water grains, etc. In all reality, most of the engines and engine builders that I have worked with will only follow a couple of these factors when it comes to changes. Always be sure to listen to the motor builders instructions. Once you sample the air, the computer will give you a reading of all of these different values. Let’s take a look at the values to learn what equals worse or better.

Density Altitude: This is a measurement value in feet that relates to the local measured atmospheric conditions. Basically, it is a value based off of a combination of pressure altitude, temperature, and relative humidity. The lower this number is, the better the air is.

ADR: This value is the ratio produced by dividing the calculated density of the air being sampled by the standard day air density. Basic rule of thumb on ADR, is for every 2% the ADR drops, you must reduce the fuel by 2% as well to keep the same air/fuel ratio. Higher ADR means better air, and lower ADR means worse air.

When I was younger, this is how good air vs. bad air was explained to me. Say the optimum air fuel ratio is 2:1, 2 parts air and 1 part fuel. Now, all of a sudden that air value changes to a 4, so you need to keep up with that air change by changing the amount of fuel entering the cylinder to a 2. Air changes to an 8, then you need to change the fuel to a 4, etc. Think of a jump of density altitude from 4000 feet down to 2000 feet, the same concept as a jump from a 2 to a 4. Or, think of an ADR change from 91 to 99 being the same as a 2 to a 4 change in air. It will not be quite that dramatic as we are talking about minimal amounts of both air and fuel, but that is the basic concept.

In reality, these two values are the two that I have been taught to look at, however I still do take notes of all the other values that the computer brings up for me as well, just for something to look back on in the future.

After your engine computer has taken a sample, you will want to go to your chart provided by your engine builder to see which value you should be looking for. For the most part, an engine builders chart will have a setting for every bit of a jump in value. For example, some might say if the density altitude is between 0-1000, to use this setting. Or if it is between 2000-3000, use this setting. Others might be based on the ADR. There are different settings to use if the ADR is 91 or if it is 97, and so on.

Now comes actually changing the settings. There will likely be two things that you will change, the main pill, and the high speed pressure. Of the engines that I have worked with, I have not yet encountered a builder that wants too drastic of a change, most recommend leaving the same high speed pill in the high speed no matter what, only changing the main and the high speed pressure. For this procedure simply follow the chart for the main pull, and put the main pill that it calls for in.

Something a bit more complicated however is setting the pressure. To do this you will need a High Speed Tester. You will hook your “in” side of the high speed to this tester and open the air valve until the high speed pops off. Here is another part where you want to listen to your motor builder. Generally, there will not be one who checks his high speeds the same as the next. Some like to open it up then close it, and whatever the pressure reads once it closes is the “Set Pressure”. Others want the pressure to be set just after it opens, and others after it is open even farther. That will be something to consult with your motor builder about.

At this point you should be all set, and are good to go out and race! I like to check the air as close to the start time of the race, and keep checking before every race. You would be surprised with how much the air can change from hot lap time to feature time, especially if the temperature cools down.

Hopefully this gives you a better understanding on how to fuel your race motor. I understand that some of the terminology and methods can be confusing if you are newer to the sport; I know they were for me! If you have any questions about anything discussed in this article, or general sprint car technical questions, do not hesitate to give us a call at 800-979-0122, or 1-402-323-3200 for our Australian/New Zealand customers. Any one of our experienced technical specialists will be happy to help!

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