Running your engine on a dyno helps to zero in your engine’s specific characteristics and tuning specifications. You will leave with a better understanding of your engine, however may only provide data specific only to that location, time of day, and season. Taking your vehicle out to the track with a different air density and running with your new engine specs is sometimes a disappointing experience.
The weakest link between the dyno and the track is often the fuel system. Some fuel systems can maintain a standard when running in differing air densities. Examples are common circle racing 305, 360, and 410 MFI engines. Other engines need regular tweaking. A custom setup with a unique engine size or application can be more difficult to transition between the dyno and the racetrack or course.
Problems transitioning to the track
So where is the problem? The dyno is in a controlled environment with a specific temperature, atmospheric pressure, and atmospheric moisture content. In addition, air quality that is characteristic of an indoor environment at an industrial location is also a factor. The dyno test takes place at a particular altitude and season of the year. The results of the dyno test are dependent on these factors. When you go out to the track, the atmosphere can be very different from the altitude of the location and the season.
The following is a mechanical fuel injection setup for a 305 cubic inch methanol V-8 racing engine with a 3.5 GPM fuel pump. It is from an indoors dyno at a location near sea level and then outside during the summer, at a high-altitude racetrack:
Weather combinations are shown for both locations. A density altitude of 1,620 feet occurred for low altitude dyno location on the day and time of dyno testing in-doors. A density altitude of 7,270 feet occurred for the high-altitude track location on the outside day and time of racing.
*Barometer values provided by the local weather report are corrected to provide a standard reading at sea level across all altitudes. Uncorrected values are more accurate values when it comes to engine tuning. These uncorrected values tend to decrease as altitude increases.
Mechanical fuel injection setups
ref. High HP Tuning for MFI for Norm. Asp. Racing Engines, fuel injection tune-up analysis from Air Density Online
Two jetting setups were determined for the mechanical fuel injection. Both had the same, optimum air/fuel ratios. These two setups were determined from the resulting jetting combinations and air density. Both are nearest an optimum target of 5 to 1 air/fuel ratio. That AFR produced the best power and punch on the dyno and on the track out of the turn.
Both had the same fuel pressure at 5,000 RPM that was just over 50 psi. That was considered the minimum pressure for the best punch coming out of a circle race turn – a standard from experienced mechanical fuel injection suppliers. Both nozzle and main bypass jetting changes were necessary to hold those optimum values for the two different locations and density altitudes. The fuel to the engine differed quite a bit. The dyno test would not reveal the optimum jetting for the high-altitude track.
Density altitude and air density percentages were provided instead of actual altitude. Both of these values include effects from the weather and altitude differences. Density altitude and air density are more valid indicator of the effect on power and the tuning needs than altitude alone. Spark plug readings, engine temperature, the feeling of punch out of a turn, and lap times are common tuning indicators in circle racing. They are used to make jetting adjustments for the best outcome. Tuning at this track weather would be difficult to sort out for an engine right out of the dyno shop under different conditions and circumstances.
Jetting changes for different racing locations around the country are a challenge for crew chiefs and tuners once the engine leaves the dyno. Air density at different locations varies from altitude, temperature, humidity, and air pressure. Altogether, that determines a fuel flow target specific to the current conditions. The selection of a dyno service for the dyno-to-track transition could be dependent on added fuel system tuning, either from the dyno shop with experience or a tuner with experience. If not, trial-and-error adjustments can be costly and time consuming.
Additional factors should be considered when transitioning to the track after a dyno test that are discussed next.
Environmental factors at a dyno facility
In addition to atmospheric and altitude differences between the dyno and the track, other factors influence how the engine performs in the different environments. The dyno is stationary, so there are no effects from ram air or from wind resistance at high speed. In addition, the engine is stationary with no effects on the fuel system from acceleration, turning, or deceleration. These changes may alter fuel flow into the engine when at the track. The original fuel tank in the racer also may experience fuel sloshing or circulating bubbles from return fuel entries that may not be demonstrated in a dyno test with a different fuel tank source. These changes can affect the tune-up needs at the track or course.
Dyno calibration can also be an issue. These factors may affect dyno calibration.
- Calibration of the load measuring instrument on the dyno. One of our local dyno service providers consistently reads horsepower and torque values 10% higher than expected values from similar engine builds. It was reported that the dyno operator has not calibrated anything in the test station since installation many years ago. The dyno operator is also the engine builder, so their motivation is to get good power numbers. Most racer customers are not aware of the absence of calibration and the exaggeration of the power values that may occur.
- Calibration of other instruments such as measuring sensors: fuel flow & fuel pressure, air flow, and temperature measures (fuel temp, air temp, oil temp, exhaust temp). Most dyno service providers do not maintain international calibration standards often required in commercial manufacturing or service.
- Dyno setups and procedures may involve different techniques to collect date. Here are some factors to consider:
- dyno program-controlled rate of engine acceleration: slower the acceleration, the higher the power numbers
- use of dyno exhaust headers not necessarily the same as headers in the racing vehicle that can affect engine power and/or fuel system and ignition timing adjustments
- use of mufflers or sound baffles downstream of the exhaust headers that affect engine power and/or fuel system and ignition timing adjustments
- exhaust leaking out of the header connection points into the dyno room contaminates any engine intake air from the dyno chamber. This reduces the oxygen content in the air and the horsepower. Situations like this can especially distort fuel system calibration if it is open loop such as carbs, mechanical fuel injection, and open loop EFI.
Cross checking the different columns of data can be done to check dyno calibration. One example is comparing the values in the break-specific fuel consumption (BSFC) column to values in other columns used in the BSFC determination. Manual computation of the BSFC from those values can often reveal if they are consistent. If they are, that would indicate reasonable dyno calibration.
Environmental factors at the track
Atmospheric changes throughout the day affect the tune-up. Altitude changes from location to location have an impact on the air density. However, these are factors that should be considered for every outing.
Head Wind: Wind resistance can load the engine, changing the engine temperature that may need enrichment for cooling.
Air Scoop Air: Ram air from a head wind can increase air density at speed that needs more fuel. Less ram air from a tail wind can decrease air density, especially at the low end that may need less fuel.
For racing on the pavement, tire traction and track conditions may affect the amount of power that can be put down to the tire. For humid, cool, or dirty track conditions, less power at the launch may be needed to hook up. The power goals for these conditions may not be set up on the stationary dyno driving through a load cell.
Summer vs winter conditions
A 4,000-foot density altitude difference and 12% air density difference occur between winter and summer at this location. Whether that difference is between the seasons or between the dyno and the racetrack, an engine fuel adjustment of over 10% is needed between the two.
Atmospheric changes, aside from obvious temperature changes, moisture, and air pressure will affect air density and the fuel system needs. Less traction may occur with cooler weather. Wind direction may change from hot to cool weather as well as seasonal. For racing on water, the amount of wind and wind direction may be paramount for performance: no wind makes smooth, sticky water gluing the boat hull, slowing it down. Slight chop from slight wind may be optimum for hull aeration and best speed. High wind may be hull breaking from large waves.
Added factors are tire traction, fuel changes from dyno to the track, and engine warm up time and/or idle time. For gasoline blends and nitro methanol mixtures, fuel changes from the dyno to the track or course can affect the amount of power between the two. Changes in weather can make that progression more difficult to tune.
A dyno test will provide relatively quick results to spell out how your engine runs, but it won’t necessarily provide easy answers to how it will run at the track in all cases. Many factors influence how an engine will run and, with experience, you will learn the ins and outs of how your engine performs. In the meantime, adjusting for environmental factors at the track will help you have successful runs from the dyno to the track.