Air Density / Density Altitude / Grains of Water for racing engine tuning from ADO
Air density, track MAP LOCATION: https://airdensityonline.com/track-results/Sacramento_Raceway/
Air Density METRIC units, MAP LOCATION: https://airdensityonline.com/track-results-metric/Sacramento_Raceway/
Air density FORECAST: https://airdensityonline.com/track-forecast/Sacramento_Raceway/
Sacramento Raceway (916) 363 2653
Team Boddie website link
Recently Bob and Jennifer contributed content for an online publication. The amount of information they needed was minimal but Bob and Jennifer decided to expand on the content and provide it for anyone who might benefit.
- Key elements to engine tuning
- Optimum Air/Fuel Ratio in drag racing
- Is Ignition Timing the First Thing to Dial In?
- How important are data logging tools?
- How does a racer select the best fuel?
- Is it still important to read spark plugs and what’s the best method of doing so?
- What’s the best way for a racer using a carburetor to optimize engine performance
Numerous sources were used in creating this content. Check them out as well.
- Bob Szabo, Szabo Publishing, Sacramento, CA (916) 419 6649 https://racecarbook.com
- Jennifer Szabo, Jennifer Szabo Designs, Las Vegas, NV https://airdensityonline.com
- Mike Shriver, SpeedSportS, Sacramento, CA
- Don Jackson, DJE, Orange, CA
- Engler Fuel Injection, Princeton, Indiana, http://www.englermachine.com/
- Hilborn Fuel Injection, Aliso Viejo, CA http://hilborninjection.com/
- NGK Spark Plug, Wixom, MI https://www.ngksparkplugs.com
- Champion Spark Plug, http://www.championautoparts.com/
Key elements for drag race engine tuning are having a well-tested fuel system setup, tracking the weather for changes in the amount of oxygen in the atmosphere, and properly adjusting the engine for the current climate. One element specific to mechanical fuel injection tuning is idle adjustment for a good starting line temperature. Engine oil temperature reaching more than 150 deg F and less than 180 deg F are reasonable. This is done with tuning the idle RPM and fuel amount. Good temperature at the starting line with proper amount of fuel at launch provides the best 60 foot time. Proper fuel amount for the best mid-range power is often a bit leaner than the launch. The best high end fuel amounts are with provisions for ram air contributions when a forward facing air scoop is used. Each of these can be very well managed with air to fuel ratio tuning.
Tuning knowledge is key. Tuning knowledge can be ‘experience-based’ and/or ‘data-based’. Experience-based engine tuning is a result of personal and shared knowledge from various sources. However, good tuning knowledge is in short supply in many regions. As a result, racers become dependent on outside sources such as suppliers, online forums, and word-to-mouth. Unfortunately deciphering whether the information is good or bad is challenging. Learning more about the science of your setup and making your own database is often the best.
Data-based tuning can be made up of visual, sound, and/or odor. Additionally, data can come from gauges, on-board instrumentation, a prior dyno test, a prior flow bench test, and/or proper calculations. Good data-based tuning comes from a good selection of data sources. We see some racers with fully instrumented drag racecars, often with little knowledge of the data generated by those instruments. Investing the time to learn about good data analysis should go hand-in-hand with any new data source.
Engine tuning is also dependent on the competition category. Tuning for a bracket engine such as Top Dragster is done for consistency or predictability. For bracket racing a conservative fuel mixture and engine timing are far more important than a lean-burning, max-power, spark plug melting combo. Tuning for unlimited classes such as Top Alcohol and Competition Eliminator is done for the best performance and that may involve a more volatile setup.
In both of these categories, tuning for air density changes is important. Air density percentages define the changes of the weight of the air going into the engine with weather or altitude changes. Numerically controlled tuning for optimum air/fuel ratios with air density changes becomes precise. ‘Guestimate’ tuning is a common alternative for fuel system adjustment and often the cause of engine failures.
Engines with mechanical fuel injection, carburetors, or open loop electronic fuel injection can be tuned for air density changes to maintain an optimum air/fuel ratio. Engines with closed loop, electronic fuel injection automatically adjust to maintain the air/fuel ratio. Racing engines operate best in a relatively narrow air/fuel ratio range.
When air density goes down, the weight of the air is reduced. To stay in an optimum air/fuel ratio range for many engines, the fuel should be reduced. This applies to naturally aspirated engines as well as supercharged engines with overdrive limitations. Top Alcohol and Nostalgia Top Fuel are examples where supercharger overdrive limits are applied and the supercharger overdrive is not changed. Those can benefit from air/fuel ratio tuning with jetting changes. Mechanical fuel injection, common in these classes, benefits from tuning simplicity since the overall air/fuel ratio can be managed by a single bypass jet. A bigger jet is used to lean it. A smaller jet is used to richen it.
In bracket racing with a supercharged engine and without supercharger overdrive limitations, the supercharger overdrive can be increased to compensate for lower air density. For example, we ran our supercharged engine in 7.90 second NHRA bracket races with varying air densities from 100% down to 90%, adjusting our blower overdrive from 5% up to 15% to compensate. We determined the scientific relationship between air density and blower overdrive to reproduce a constant power level. We were able to maintain near 7.90 second ETs throughout a typical event, from a cool morning to hot afternoon to cool evening. No other tuning was necessary. We could also maintain near 7.90 second performance from one local to another with this tuning method.
For normally aspirated engines or supercharged engines with overdrive limits, tuning to maintain the air/fuel ratio is different. It involves adjustment of the fuel amount with the air density. Lower air density needs less fuel. Higher air density needs more fuel. A very reliable scientific relationship exists between the two that makes tuning a simple numerical control task for these engines as well.
Neat spread sheet ProCalc data for Hilborn fuel injected engine with jetting determined to maintain optimum air/fuel ratios for different air densities at torque peak and horsepower peak RPMs (source ProCalc jetting calculator data output; from ProCalc)
In our supercharged alcohol burning Hemi V-8 combination, we found the following:
|air/fuel ratio||spark plug reading*||result||tuning label|
|3.1 to 1||no plating discolor||misfire on launch||rich limit|
|3.4 to 1||discolor 1.5 threads||runs strong||baseline|
|3.6 to 1||discolor 2-3 threads||almost melt pistons||record attempt level|
|3.7 to 1||discolor 3+ threads||melt two pistons||lean limit|
*not always a reliable indicator by itself
For our mechanical fuel injection tuning, air/fuel ratios were determined for the weight of air and the weight of fuel through the engine. Weight of fuel for mechanical fuel injection: The fuel system is made up of the type of fuel, jetting, and the fuel pump.
Weight of air: For normally aspirated engines, the weight of air is determined from the engine size and efficiency. For the supercharged engine, the weight of air is determined from the supercharger size, overdrive, and efficiency. The weight of air is further determined from the air density. Air density is defined by the air temperature, humidity, and barometric pressure. Each of these values has a different mathematical effect on the weight of air. The revised weight of air or air density can be determined from the combination.
Air density data: We provide an air density data source for various NHRA tracks. You can find it at: Race Track Weather. Track location, altitude, and the current weather are accessible through a smart phone or web device.
Example of display of current air density, water grains, and density altitude data for tuning at Pomona Raceway from https://airdensityonline.com/tracks/; this data is provided year around for NHRA tracks
Air density forecast: A most unique three-day forecast is also provided from this source showing air density, water grains, and density altitude. For tuning preparation ahead of time, this is a unique source for that data. It is really good for determining tuning spares that you will need for a weekend before you get to the track.
Printout of air density forecast for Amarillo Dragway for recent NHRA event from Amarillo Dragway weather
Air/fuel ratio tuning in the field: With the expansion of telecommunications, we developed a smart phone calculator, ProCalc, to determine fuel injection jetting for precise tuning with air/fuel ratios. It is accessible from ProCalc to provide portable tuning capability in the shop, travel, pits, and staging lanes.
Precise air/fuel ratio tuning data for a normally aspirated or forced induction engine; for various drag racing fuels; from the racer’s smart phone using the ProCalc program; accessible on multiple devices at the same time; for tuning data communication such as between shop & track, or pits & staging
With ProCalc and the Airdensityonline.com forecast, an entire tuning plan can be constructed ahead of time for mechanical fuel injection to help relieve the pressures of tuning at the track. We would customarily determine our fuel injection jetting for the entire weekend during the previous week, before an event. Air/fuel ratio tuning was so precise that we could follow that tuning plan throughout the weekend, and racing was a whole lot more fun.
Spark plug discolorations after a run are an accumulative indication of engine temperature. The spark plug plating discolorations after a run indicate the portion of the spark plug that experienced higher temperature somewhere on that run. Spark plug readings are most effective when a strong baseline is developed. If spark plugs are not discolored after a run, then either the engine is too rich or way too lean. Excess fuel will cool the engine if it is too rich. Not enough heat in the engine can occur for extreme leanness as well. Both of these can provide the same wet plug reading but with a different tuning need.
When developing a new setup without a good baseline, reading the spark plug color can be misleading. We saw many tuners with new setups who were lost when they tried using only spark plug color as a tuning indicator. If spark plugs are reused for several runs, then heat indications may not be reliable at all, especially if tuning changes were done between those runs. When a good baseline is already established, location and amount of heat indications on the spark plugs can indicate tuning characteristics.
However, those indicators may occur at the beginning, middle, or end of the run. The only way to sort out the difference is to make launch-only test runs and read the plugs. Then make half-passes and read the plugs. Then make full-passes and read the plugs. This would indicate where the spark plug color is occurring. If spark plug heat indications appear in the test sequence, at least one new spark plug should be installed to examine the effect from the next sequence of testing.
Discoloration of threads on plated park plug indicate tuning characteristics. Plating from different suppliers discolors at different temperatures. Note thread color differences between NGK’s on the left and Champion’s on the right. The presence or absence of a spark plug washer affects the reading as well. Photo taken from “Fuel Injection Racing Secrets” by Bob Szabo.
Several brands of spark plugs are plated, however platings from different brands discolor at different temperatures. Heat indications from different brands may be dramatically different for a specific tune-up. For a new setup, sticking with a single brand may simplify tuning from more consistent heat indications. If the spark plug brand is changed, prior spark plug reading indicators may change. A different brand may look colder or hotter than the brand used to develop a baseline.
When the arc in a spark plug ignites, only the oxygen and fuel molecules go through the arc when it is firing. That is an extremely small percentage of the total molecules of oxygen and fuel to combust in the chamber. Combustion is further dependent on a flame front from subsequent oxygen and fuel burning throughout the cylinder. That flame front is completely dependent on the air/fuel ratio. A rich mixture slows down the flame front. A moderate rich to moderate lean mixture speeds up the flame front. An excessively lean mixture slows down the flame front. Rich or lean indications from spark plug readings alone can be confusing. A good air/fuel ratio baseline in addition to spark plug readings can ensure consistency in your setup.
Testing for maximum 60 foot performance should be done first. Jetting, accelerator pump, transition circuits, float level, and/or fuel pressure adjustments should be done for this first phase of drag racing performance. After carburetor adjustments are done to optimize the 60 foot time, then further adjustments can be done for mid-range and high-end performance.
Specific to air/fuel ratio tuning, adjustments for air density changes are effective once a carburetor combination is set up. Those include the appropriate jetting and other adjustments to fuel flow.
Changes to carburetor venturi size or design to optimize engine performance are more involved and can dramatically affect a prior optimum combination. Error’ing on the small venturi size is less risky than error’ing on the large venturi size. Converting from progressive to straight linkage can be troublesome, especially for racecars with manual transmissions or tight converters. Converting from vacuum to mechanical secondary linkage can also be troublesome if appropriate other features such as double pumper provisions are not included or the engine is lugged.
For a new combination, it would not be a bad idea to start out with the most conservative ‘slave’ carburetor setup such as a small vacuum secondary unit or an older unit from a previous running combination to get through the infant mortality of engine break-in and chassis adjustments. Then, after initial sorting, the killer ‘master’ carburetor could be installed to set up for max performance.
For many classes and national events, both the fuel and often the fuel supplier is specified. In that case, there is no fuel choice. However, for many others such as local bracket racing gasoline setups, that is usually not the case. The selection of gasoline is first dependent on leaded or unleaded. Racecars driven to the track are usually unleaded by ‘highway law’. Sole purpose racecars can be set up that require leaded gasoline with higher anti-detonation characteristics.
The next choice for gasoline fuels is the octane rating. Octane need would be dependent on the engine compression. That is static compression or, for forced induction, a combination of static compression and boost. An engine with the minimum octane that is needed for anti-detonation will usually not run better with a higher octane fuel. That fuel simply burns slower. In some cases, a bump in the octane will allow more ignition timing. Sometimes that increases power, but not always.
Gasoline racing engines have an air/fuel ratio window of about 10% between a lean and rich limit. That is relatively narrow and gasoline choice can affect whether you are in or out of that window. Many gasoline racers we speak with do not know the weight of the gasoline they are running. Some gasoline fuels vary in weight with the season, altitude, or location. Jetting needs can change based on variations in gasoline weight. Air/fuel ratio management is a good way to manage weight as a variable. The fuel can be weighed or the specific gravity can be measured to determine that value.
Many NHRA classes can run alcohol fuels. E85, a blend of ethanol and gasoline, is readily available at the highway pump in some locations around the world. It is usually a low cost alternative fuel for high compression or high boost application. E85 can be run 25% rich or more to retard detonation. Air/fuel ratios are richer than gasoline. That is a lower numerical air/fuel ratio number. However the best air/fuel ratio is dependent on the blending ratio. We saw blends varying from 80-20 to 90-10 not too long ago, although they are starting to become more consistent. An adjustment of a couple points of air/fuel ratio is needed for those differences if they vary from one tank full to another.
Methanol is a good choice in many NHRA classes where it is legal. It absorbs a lot of heat when it vaporizes. As a result, it tends to keep an engine cooler. This is an advantage for drag racing where many racecars do not have a cooling system. As long as it is not contaminated, it is 99.9+% methanol with no blend variation issue. Air/fuel ratios are richer than E85 or gasoline. That means it is a lower numerical air/fuel ratio number. Methanol racing engines have an air/fuel ratio window of about 30% between a lean and rich limit. That is a wide window that makes tuning a lot easier.
Nitromethane is the ultimate power making fuel. Nitro carries a lot of oxygen in the fuel, and extra fuel from excess enrichment can run in the monopropellant mode. Certain racing classes limit nitro to certain percentages mixed with methanol. For example, in exhibition nitro classes, different percentages of nitromethane can be added to a methanol engine to make more power especially in cost-effective lower percentages.
In our research, we provide suggested air/fuel ratios for different percentages, developed from consultation with Don Jackson Engineering. Those are reported in several of our publications available through our online store.
Air/fuel ratio control of different nitromethane percentages is the best way to setup and tune a combination. Trial and error from experience or lack of experience is done instead by many, and we saw a lot of engine damage during the infant mortality testing of many new combinations where the wrong air/fuel ratios were tried out.