Explaining The Effects of Weather on Mechanical Fuel Injection

Have you ever noticed that your racing engine ran great in the morning but was sluggish in the afternoon? Have you ever noticed that if you travel to other tracks, it’s a crapshoot if your car will run well even though it consistently runs great at your home track? These are all influenced by the weather. Racing engines with mechanical fuel injection are quite common throughout motorsports. Kinsler, Hilborn, Enderle, Rons, Engler, and several recent suppliers with carbon-fiber throttle body components are examples of suppliers of mechanical fuel injection systems.

Keeping track of air density, or the amount of air in the atmosphere, can help make jetting decisions for these mechanical fuel injection systems. Essentially that will control the ratio of air and fuel going into the engine. It is this ratio that makes combustion in an engine powerful or peter out.

Air inlet to this stack-type mechanical fuel injection is affected by air temperature, humidity, and air pressure; the combination affects the fuel needed and the power output.

A good tuner maintains records and determines the setup of the racing engine for weather changes. This is something anyone else can do with a little tuning knowledge and experience. Of course, knowing the mechanics of your racing engine is the most important factor but knowing what’s going on in the atmosphere and how it affects your engine tuning is necessary to improve power and consistency.

Barrel valve just below the inlet of an Enderle fuel injection hat must be adjusted for changes in air density to maintain a good air to fuel ratio.

Combustion Process Deconstructed

In an internal combustion engine, fuel fills the cylinder, and a spark plug ignites the fuel. The ignition process requires two things: a fuel source such as gasoline, methanol, ethanol, or nitromethane, and the second component of oxygen. Different fuels have different characteristics but in many categories of racing with mechanical fuel injection, methanol is the most common. Methanol is stable with a wide tuning window and has a high heat of vaporization. As a result, it absorbs heat in the intake system when it vaporizes. This makes it easy to keep the engine from overheating under extended high power. This characteristic also chills the inlet, condensing the air to make more power – typically 10-percent more than racing gasoline when the enrichment is at an optimum ratio.

However, if there is too much fuel, it also chills the explosion, and doesn’t generate enough heat to make very much power. Conversely, if there is not enough fuel to cool the engine, it can create too much heat which can melt the pistons, valves, or burn out a head gasket.

Nick Bastiao’s drag racing Alcohol Funny Car ran record-setting 5.4-second elapsed times and over 269 mph with air to fuel ratios between 3.1 and 3.3.

An easy way to make sure the right amount of fuel is going to an engine is to track the air/fuel ratio or AFR. That is, the ratio of air to fuel to be maintained during a run. Electronic fuel injection also uses this principle.

Different factors influence what makes a good air/fuel ratio. The burn ratio of methanol is 6.45 parts of air to 1 part of fuel. However, racing engines with mechanical fuel injection are run on the rich side which is represented by a smaller numerical ratio.

Examples of optimum air/fuel ratios:

  • Normally aspirated methanol for drag or land speed racing:
    5:1 typical AFR for best power
  • Normally aspirated methanol for Sprint Car racing:
    4:1 typical AFR for sustained power in competition with extra fuel for cooling
  • Roots supercharged methanol Hemi head engine with 14 psi of boost:
    3.5:1 typical AFR for best power with extra fuel to prevent detonation.

Weather and Air Density

Knowing that combustion requires fuel and oxygen, the next step in maintaining the proper air/fuel ratio is tracking the amount of oxygen in the atmosphere. Even in the absence of other factors, oxygen only makes up about 21-percent of the air in the atmosphere. However, how much air is in the atmosphere at any given time fluctuates depending on various factors including temperature, humidity, and barometric pressure (aka atmospheric pressure). This is known as air density.

In basic terms, temperature is not just a measure of how hot or cold the air is. Thermodynamically, temperature is a measure of how quickly atmospheric molecules are moving around. These molecules are oxygen, carbon dioxide, and nitrogen. The more the molecules are moving around, the warmer it gets. The more molecules stagnate, the cooler it gets. The warmer the air is because of this, the more space there is in the atmosphere for other things like moisture. Conversely, the cooler the air gets, the more densely packed the atmospheric molecules become because they are not moving around.

The space left by air molecules gets filled in so that the volume of atmosphere is not just air but also moisture. This is where humidity comes into play. Humidity is a measure from 0-percent to 100-percent relative to the amount of space between other molecules in the air. A value of 100-percent humidity means that given the amount of space that can suspend a vapor that was left by the absence of other molecules, 100-percent of that space is taken up by moisture. It does not mean the atmosphere is completely filled with water vapor with no air. Humidity is constantly fluctuating depending on the temperature. Warmer temperature expands around the humidity, increasing the space. For a given amount of humidity, the percentage will be lower with warmer temperature.

The barometer measures the pressure exerted by the weight of the air. Warmer temperatures tend to have lower pressure because the weight of air is lighter as it is moving around and thinning out. Cooler temperatures tend to have higher pressure because the weight of stagnant air is heavier. Higher elevation has thinner air and lower pressure because there is less air exerting pressure above the ground. An uncorrected barometric value takes all of that into account.

Air Density In Engine Tuning

In closed-loop electronic fuel injection (EFI), if the air/fuel ratio varies from weather changes that affect air density, the engine controller adjusts the fuel amount to compensate. However, for most mechanical fuel injection, there is no automatic compensation for air density changes.

If air density increases in normally aspirated or supercharged engines with mechanical fuel injection, the amount of fuel to the engine must also be increased to compensate, otherwise you risk throwing off your air/fuel ratio and possibly causing engine damage. Generally, if the temperature increases, the air density most likely decreased. If the temperature decreases, the air density most likely increases. If the barometer drops, the air density most likely decreased. If the barometer rises, the air density most likely rises. If the humidity increases, the air density decreases. If the humidity decreases, the air density increases. Paying attention to these changes — especially with the use of a weather station — is critical to understanding how your car will perform from round to round.

Many years ago, among a few in drag racing, humidity was thought to increase power as a form of water injection. This has not been proven to be the case since that time. When there is more humidity, usually there is less power available because there is less air in the atmosphere.

In the photo, spark plug discoloration of porcelain may indicate that the engine is too lean if it is not burning oil from ring blow-by or valve guide leakage. The discolored rim indicates good engine heat, as does the absence of melted corners on the outer electrode indicates good engine heat. With silver colored spark plug plating, discoloration of threads (not visible in this photo view) would indicate engine heat. A good target is about 1 to 2 threads for aluminum cylinder heads with 3 threads as max. For iron cylinder heads, 5 threads is max. Absence of discoloration in outer electrode indicates enrichment. These observations are a sampling of the various indications from spark plug readings. The readings are accumulated over an entire run. Moments of enrichment or leaning in a run or dynamometer test can create indications that may be conflicting. As a result, other data is important for the determination of a tune-up from spark plug readings such as the length or running time or the RPM range.

“In our blown-alcohol Hemi drag car, we tested various AFRs in several engines over the years. In one of the setups, an AFR of 3.1 to 1 would misfire some of the cylinders. An AFR of 3.2 to 1 was down about 100 horsepower. An AFR of 3.4 to 1 ran really well. An AFR of 3.7 to 1 melted piston domes. At a weekend racing event at our local race track, air density could change from a low of 92-percent in the hot afternoon up to 98-percent in the cool evening finals. Without adjustment of jetting, air to fuel ratios could go in and out of our best range,” says Bob Szabo, noted tuner and author of many books on mechanical fuel injection tuning.

Many tuners do not change the fuel amount to the engine, and air to fuel ratio varies with air density. Some experienced tuners will instead vary the engine compression (with the use of different pistons), ignition timing, and/or clutch engagement RPM to compensate. Most nitro tuners have their own program that differs from another, and several tuners are secretive about their combination. Air to fuel ratio tuning is a good alternative for those trying to compete in this sport.

Engine Tuning In The Real World

Let’s take a 305 cubic-inch Sprint Car with mechanical fuel injection. In this example, the crew chief keeps the mechanical fuel injection nozzles to the engine the same size as nozzles in a previous dynamometer test. Most mechanical fuel injection systems use an oversize fuel pump with a main bypass fuel circuit that is used to return excess fuel to the fuel tank.

“The main jet is the most basic adjustment of a constant flow (fuel injection) metering system.  A smaller main jet makes the engine richer, by allowing less fuel to flow back to the tank and forcing more fuel to go to the engine. A larger main jet makes the engine leaner, by allowing more fuel to flow back to the tank which means less fuel flows to the engine,” says Jim Kinsler.

To maintain the AFR, the crew chief changes the main bypass jet size depending on changes in the weather. Below is data from the actual dynamometer test as well as run data from three locations.

The main bypass jet size of 0.066-inch diameter was originally determined from the dynamometer test with an AFR at 4.9 to 1 for best power. When the main bypass jet size is not changed for an air density change such as this, the engine performance varies dramatically. For example, if the 0.066-inch diameter main bypass from the dyno test was instead run in the engine for the 1st location, we calculated the AFR would be 4.5 to 1. That is a lot richer than the best power AFR of 4.9 to 1. Without jetting compensation, this reduces power by about 30 horsepower for that lower air density with poor response out of the turns.

Spreadsheet of Jetting Records

Excel spread sheet of jetting records for a normally aspirated Chevrolet racing engine showing air to fuel ratio changes from temperature variations; AFR values are computed for torque peak engine volumetric efficiency of typically 100-percent at 5,000 rpm, before the high speed bypass opens; and horsepower peak volumetric efficiency of typically 80-percent at 8,000 rpm, after the high speed bypass opens to lean out the engine for the volumetric efficiency reduction.

Record Keeping and Data Analysis

From the information presented above, you might start to see how important it is to keep good records. If you keep track of how your engine performs under different conditions and with different jetting combinations, you will start to see how best to jet your engine when it counts.

Maintaining an Excel spreadsheet or similar program can be as simple as noting the weather, jetting combination, and information about how well the engine ran at each particular event. It can grow from there to include notes on other engine modifications, track conditions, or driver performance.

For instance, if you know your engine runs really well in the early evening at your home track, note the weather and jetting. That is a good baseline for how you would like your racer to run all the time. Using that as an example, you can decide to make mechanical fuel injection jetting changes based on weather fluctuations from that value, or engine upgrades from that baseline.


This is a brief overview of how air density affects mechanical fuel injection engine tuning. Hopefully it is a jumping-off point for greater control in engine tuning down the road. With mechanical fuel injection, tracking weather changes is an important factor in getting maximum performance from your engine.

Written by Jennifer Szabo & Bob Szabo

About the author

Jennifer Szabo

Jennifer Szabo is the creator of AirDensityOnline.com, an online resource that assists racers in tuning their engines. The site aims to help racers get the most power and consistency from their engine setups. Jennifer is a professional web developer and the daughter of drag racer, Bob Szabo. Bob is a retired welding engineer and author of 6 books about mechanical fuel injection engine tuning at RaceCarBook.com. Bob and Jennifer maintain a blog at their websites in addition to co-writing articles for various auto industry resources.
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