Fuel Tuning for Motorsports from Air Density Online

High powered dragsters lining up for a bracket race at the Norwalk Raceway in this IHRA national event. The dragster in the background is supercharged while the dragster in the foreground is carbureted. These two different engine setups illustrate the variety of combinations in drag racing that can be a challenge to participants and tuners. In the current Top Dragster class running racecars similar to these, performance is often in the low 6 second time for a quarter mile distance with speeds well over 200 mph. That indicates power levels over 2,000 horsepower. Tuning for the high end becomes important as well as contributions from additional air from ram air. Ram air can add several hundred horsepower. Air density changes can add or subtract several hundred horsepower. These effects alter the performance to an extent that correct tuning is vital.

Engine tuning can be both art and science. While it is beneficial to track data as a reliable indicator of your engine’s performance, being ‘in-tune’ with the details of your engine can help you feel-out your next steps. Having a good working knowledge of overall engine tuning can help you do just that. Key elements for tuning a racing fuel injection system include:

– having a well-tested fuel system baseline
– tracking the weather for changes in the amount of oxygen in the atmosphere and properly adjusting the engine for that change.

If you have those elements under hand, there are different methods of tuning knowledge to help you make decisions on race day.

TUNING KNOWLEDGE

Tuning knowledge can be experience-based, observation-based, data-based, or a combination. Knowing how to utilize each type is key.

EXPERIENCE-BASED TUNING
Experience-based engine tuning is a result of personal and shared knowledge from various sources. However, good tuning knowledge may be in short supply in many regions since what works for one tune-up or one situation may not apply to everyone. As a result, racers become dependent on outside sources such as suppliers, online forums, and word-of-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.

Racing mechanical fuel injection is popular in marine motorsports. Tuning is complicated by high humidity in the air from the water. Fuel systems set up on a dyno, for example, often need adjustment between the dyno location and the water to accommodate the differences.

OBSERVATION BASED TUNING
Observation-based tuning can be made up of visual, sound, exhaust odor, and/or performance achievement: ET, speed, and/or handling. This involves comparing an item or condition to how it was in the past. This may help to determine how the current setup is performing. Noting these conditions can help in building on data-based tuning mentioned below.

DATA-BASED TUNING
Additionally, data can come from gauges, on-board instrumentation, prior dyno test, prior fuel system flow bench test, and proper calculations.

Good data-based tuning comes from a good selection of data sources. Of course, understanding how those data sources are useful to gauge a setup is also necessary. This takes more time and planning to fully utilize but becomes an invaluable tool to track engine performance over the course of different environmental and engine changes.

Sprint car racing at the Stockton, California Dirt Track. Sprint cars are lining up for the start. Racing fuel injection setups are made for good throttle response out of the turns. Additional fuel enrichment is often adjusted to control engine temperatures with cooling system water temperatures running high for maximum power, commonly over 200 deg F.

TUNING DEPENDS ON THE TYPE OF MOTORSPORTS

Engine tuning is also dependent on the competition category.

– Tuning for a drag racing bracket engine such as Top Dragster is done for consistency or predictability, not necessarily for maximum power. 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 drag classes such as Top Alcohol and Competition Eliminator is done for the best performance, and that may involve a more volatile setup that is on the edge.

AIR DENSITY TUNING TO MAINTAIN AIR/FUEL RATIO
In both of these categories, tuning for air density changes is important. Air density percentages are a representation of the weight of the air going into the engine with weather or altitude changes. Careful tuning to maintain optimum air/fuel ratios with air density changes are vital. ‘Guestimate’ tuning is a common alternative for fuel system adjustment and often the cause of engine failures.

High powered dragsters lining up for a bracket race at the Norwalk Raceway in this IHRA national event. The dragster in the background is supercharged while the dragster in the foreground is carbureted. These two different engine setups illustrate the variety of combinations in drag racing that can be a challenge to participants and tuners. In the current Top Dragster class running racecars similar to these, performance is often in the low 6 second time for a quarter mile distance with speeds well over 200 mph. That indicates power levels over 2,000 horsepower. Tuning for the high end becomes important as well as contributions from additional air from ram air. Ram air can add several hundred horsepower. Air density changes can add or subtract several hundred horsepower. These effects alter the performance to an extent that correct tuning is vital.

DRAG RACING IDLE ENGINE TEMPERATURES
One element specific to mechanical fuel injection tuning for drag racing is idle adjustment for a good starting line temperature. Engine oil temperature reaching more than 150 deg F and less than 180 deg F is reasonable. With fuel injection, this is done with tuning the idle RPM and fuel amount.

With drag racing, a good engine temperature at the starting line with a proper amount of fuel at launch provides the quickest 60-foot time from the launch. Proper fuel amount for the best mid-range power is often a bit leaner than the launch. When a forward-facing air scoop is used, the best high end fuel amounts are with provisions for ram air contributions. All of these can be very well managed with air/fuel ratio tuning.

Engler Machine fuel injection popular in sprint car and other motorsports racing. Airflow is controlled by individual throttle plates in a throttle body manifold fed by ram tubes (located under the air cleaner that is top center in the photo) that are feeding each cylinder. When the throttle is mashed out of a turn, the proper fuel amount is vital for engine response.

BRACKET DRAG RACING WITH SUPERCHARGER
In bracket drag racing with a supercharged engine, the supercharger overdrive can be increased to compensate for lower air density. This can be done in classes without rules for supercharger overdrive limits. For example, we ran our supercharged engine in 7.90 second NHRA bracket drag races with varying air densities from 90% in the hot afternoon up to 100% in the cool evening, adjusting our blower overdrive from 15% down to 5% to compensate. We determined the mathematical 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 hot afternoon to cool evening. No other tuning was necessary. We could also maintain near 7.90 second performance from one elevation to another with this tuning method.

For normally aspirated engines or supercharged engines where the overdrive cannot be changed, tuning to maintain the air/fuel ratio for air density changes is different. It involves adjustment of the fuel amount with the air density.

– Lower air density needs less fuel: larger FI main bypass jet.
– Higher air density needs more fuel: smaller FI main bypass jet.

A very reliable scientific relationship exists between air density and fuel amount that makes tuning a simple numerical task for these engines as well.

Spread sheet from fuel injection jetting calculator data for a normally aspirated 383 cubic inch V-8 racing engine. Jetting was determined to obtain an optimum air/fuel ratio for racing. This was done at torque peak RPM (with 100% volumetric efficiency) and at horsepower peak RPM (with 80% volumetric efficiency). Source: ProCalc jetting calculator.

TUNING FOR WEATHER CHANGES

Racing engines operate best in a relatively narrow air/fuel ratio range.

– 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.

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 drag racers are examples where supercharger overdrive limits apply, and the supercharger overdrive is not changed. These can benefit from air/fuel ratio tuning with proper jetting changes for air density 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 bypass jet is used to lean it.
– A smaller bypass jet is used to richen it.

Note that many Nostalgia Top Fuel racers run all the fuel to the engine, without any bypass jet. Adjustments to the percentage of nitro are an alternative tuning tool. Some tuners also reduce blower overdrive for tuning, change spark advance, or change compression ratio. Tuning in these cases is more complex.

AIR/FUEL RATIO RANGE
In our supercharged alcohol burning drag racing Hemi V-8 combination, a lot richer air/fuel ratios were needed in our high compression, high boost setup. The following range of air/fuel ratio settings and their tuning results are shown for one of our supercharged engine combinations:

*Not always a reliable indicator by itself. Once we encountered melted pistons with no indication of leanness on the spark plugs, however the air/fuel ratio calculation from the jetting was at the lean limit.

HOW AIR/FUEL RATIO IS DETERMINED

For our mechanical fuel injection tuning, air/fuel ratios were determined for the calculated weight of air and the calculated weight of fuel through the engine.

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 from the air temperature, humidity, and barometric pressure combination. 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.

RAM AIR FROM FOREWARD MOUNTED AIR SCOOP
For a foreword mounted air scoop, the weight of air is further increased with speed from ram air. At 200 MPH for example, a foreword mounted air scoop can boost the air into the engine by 2 psi or 4 inches HG barometer. Adding 4 inches HG to a barometer reading can be used to indicate the weight of air at 200 MPH.

AIR DENSITY DATA
We provide an air density data source for various motorsports tracks. You can find it for US as well as international tracks at: https://airdensityonline.com. Track location, altitude, current air density, density altitude, and wind direction 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 Air Density Online.

AIR DENSITY, WATER GRAINS, & DENSITY ALTITUDE 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 probably the only source for that data. It is really good for determining tuning jetting spares that you will need for a weekend before you get to the track.

Air density forecast for Amarillo Dragway from Air Density Online

VARIOUS AIR DENSITY VALUES
Note that temperature, humidity and uncorrected barometer are shown by the time of day. These values are used to determine water grains, used by some tuners to track humidity throughout a day. These values are also used to compute air density and density altitude throughout the day of racing competition. Dew point is shown that can be used to indicate the possibility of a slick racing track surface, if it is close to the air temperature. This can also indicate the possibility of rain that may interrupt an event.

WEIGHT OF FUEL
The fuel system is made up of the type of fuel, jetting, and the fuel pump. The combination is used to determine the weight of fuel going to the engine. The weight of fuel can be determined from the fuel system layout computations. For engines with flow meter and fuel pressure instrumentation, these values can be used to determine the weight of fuel.

FUEL INJECTION CALCULATOR FOR TUNING WITH AIR/FUEL RATIO

With the expansion of telecommunications, a smart phone calculator was developed to determine air/fuel ratios from various racing engine combinations using the weight of air and the weight of fuel data inputs. Called ProCalc, optimum fuel injection jetting can be determined for precise tuning with air/fuel ratio goals. It provides portable tuning capability in the shop, travel, pits, and staging lanes.

Precise air/fuel ratio tuning can be done for both normally aspirated and forced induction engines. Provisions are made for various fuels including gasoline, E85, methanol, and nitromethane. The calculator is accessible from the racer’s smart phone. It is also accessible on multiple devices for virtual tuning such as between shop & track, or pits & staging.

With ProCalc and the forecast from Air Density Online, an entire tuning plan can be constructed ahead of time for mechanical fuel injection to help relieve the pressures of tuning at the track. Fuel injection jetting for the entire weekend can be determined during the previous week, before an event. Air/fuel ratio tuning is very precise and tuning plans made ahead of time appear to be very reliable.

SOURCES

Bob Szabo, Szabo Publishing, Sacramento, CA (916) 400 3953 https://racecarbook.com, https://airdensityonline.com
Mike Shriver, SpeedSportS, Ohio
Don Jackson, DJE, Orange, CA
Engler Fuel Injection, Princeton, Indiana
NGK Spark Plug, Wixom, MI
Champion Spark Plug

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