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Understanding Dynamometers for Performance Applications

 
 
A dynamometer or dyno for short is a device for measuring force, moment of force (TQ), or power.
 
 
Text by Dmitriy Orlov. Photos by Corey Davis and courtesy of SuperFlow Dynamometers and Flowbenches.
 
Scales measure mass, rulers measure length, and a dynamometer (dyno) measures the power output of an engine. The dyno is a tool for measurement, and much like other measurement devices there are many standards, variations, and setups.
 
This image depicts an engine dyno, courtesy of SuperFlow Dynamometers and Flowbenches.
This image depicts an engine dyno, courtesy of SuperFlow Dynamometers and Flowbenches.
 
In the automotive industry, dynos are utilized for various purposes. An engine dyno hooks up directly to a motor without any of the drivetrain components and measures the exact output of an engine. OEMs use this to dial in engines for efficiency, optimal performance, and perform stress tests for longevity and operation in various environments. Race and performance companies do the same, focusing on maximum performance and endurance. The power number from such tests is what the manufacturers indicate for the specs of a car and is typically defined as "crank horsepower.”
 
This image depicts a chassis dyno, courtesy of SuperFlow Dynamometers and Flowbenches.
This image depicts a chassis dyno, courtesy of SuperFlow Dynamometers and Flowbenches.
 
A chassis dyno, much like a treadmill for a car, measures the power a car would put down to the road—commonly known as "wheel horsepower.” This test measures the actual power that the car has, taking into consideration power train losses like transmissions, differentials, wheels, and any friction in between.
Dynos measure power at the WHEELS. Car companies quote power at the crank. Sometimes, people get confused when they see lower numbers from the dyno. On our Dynojet dyno, we see a loss of about 15% from crank to wheel horsepower on 2wd cars and about 20% on AWD cars. That is, if we have an AWD car rated at 300 hp from the factory, we’ll expect to see somewhere around 240 hp at the wheels on our dyno.
Many other types of dynos are used for various components of a car. A shock dyno, for example, is used to test and configure shock absorbers for specific applications.
 
This image depicts SuperFlow's ATR_97000 Light Duty Transmission Dyno, meant for passenger car and pickup transmissions. Courtesy of SuperFlow Dynamometers and Flowbenches.
This image depicts SuperFlow's ATR_97000 Light Duty Transmission Dyno, meant for passenger car and pickup transmissions. Courtesy of SuperFlow Dynamometers and Flowbenches.
 
 In this article, we are focusing on performance dynos, in particular the chassis dyno.
 
 
Chassis Dynos
 
The most common chassis dyno is a rolling road, where the vehicle drives up onto the system and the wheels ride on rollers. Some dynos are designed to measure two-wheel drive cars or all-wheel drive cars with multiple sets of rollers. Other dynos hook up directly to the hubs. Typically, the vehicle is jacked up, wheels are swapped for dyno hubs, and then each hub gets its own dyno device. The all-wheel drive dynos vary as well—some have front and rear rollers coupled, and others do not, which is important for those cars that track wheel speed for controlling electronic AWD systems. Coupled dynos ensure that the front and rear wheels are at the same speed all the time.
 
This image depicts a
This image depicts a "rolling road" chassis dyno where the vehicle drives up onto the system.
 

In the aftermarket performance industry, dynos are most commonly used to gauge factory performance of cars and then used to develop products and engine calibrations to improve on OEM performance. It is important to establish a baseline for a car, which is the starting point and basis for comparison and measurement of gains. This should be done on the same dyno for consistency and to minimize variables. With every modification, the vehicle is then tested and gains are determined. These gains are often used for marketing purposes and can be a testament to product quality and engineering (as with marketing in any industry, results can be made more favorable and are not always entirely true).

Enthusiasts will dyno their vehicles to determine power levels and to compare them with other cars, but many misconceptions and variables need to be considered for fair and accurate comparisons.
Load dynos versus inertial dynos. Some dynamometers have a load generator—some kind of brake that provides resistance against the car. This can be useful to make sure an acceleration pass takes a specific amount of time and can be helpful for tuning—in theory, you can hold a car indefinitely at any given RPM and load—although I don’t see them used that way very often. Inertial dynos like our Dynojet have no load generator other than their own mass. You can’t control the rate of acceleration other than changing the gear the car is in. On the other hand, the inertial dyno has near perfect repeatability, since the mass of the roller never changes. We tested one car that came back to us after five years, and it made almost exactly the same dyno curve it had generated five years before, within about 1%. Load dyno accuracy can drift over time. In order to be accurate, they should be periodically calibrated. If you are using a load-based dyno, ask the operator about the calibration procedures and how recently it has been done.– Dan Hurwitz, owner/tuner at Mach V Motorsports. Sterling, VA

 
Results
 

Power ouput is measured by tracking the RPMs of an engine and monitoring the load that the wheels put onto the rollers of a dyno. Load is displayed as torque, and horsepower is calculated from torque and RPMs. In most cases, the vehicle is put in the gear that is closest to a 1:1 ratio to get the most accurate power number without reductions in gearing.

This image depicts a single-run dyno graph with a smoothing correction factor. On the left axis you can see Power (HP), on the right axis you can see Torque (lbs-ft), both compared to RPMs.

This image depicts a single-run dyno graph.
 
This equation explains the relationship between Torque (lbs-ft), Horsepower (HP) and RPM.
This equation explains the relationship between Torque (lbs-ft), Horsepower (HP) and RPM.
 

The graph shows the two values of HP and TQ relative to RPMs and provides a snapshot of how the engine runs and the overall power band. Reading the graph can also reveal issues or various changes in engine dynamics throughout the RPMs (for example, at which point the timing is changed or when a turbocharger fully spools up). The highest number is the end result and the maximum output of the motor.

This graph shows 3 separate runs on a single plot. Notice there's no smoothing correction factor applied. You can also see the Air-Fuel-Ratio plotted against the engine's RPMs at the bottom.
This graph shows 3 separate runs on a single plot. You can also see the Air-Fuel-Ratio plotted against the engine's RPMs at the bottom.
 

When comparing results, graphs are overlaid, which reveals the gains and improvements in the power band.

 
 
Correction factors and standardizations
 

To account for geographic and environmental variations, correction factors for pressure, temperature, and humidity are considered. Many dynos come with small weather sensors to measure the environment at the time of testing and correct results accordingly. Manufacturers can create their own standards for correcting or apply the SAE (Society of Automotive Engineers) corrections. These corrections can help "level” results of tests between ones done in winter or spring. Nonetheless, conditions should be re-created as close as possible at the time of testing (some facilities have climate-controlled rooms with chassis dynos).

Akrapovic’s clean-room dynamometer

Akrapovic’s clean-room dynamometer
Both of these images depict Akrapovic's "clean-room" dynamometer where they can reduce the variables for consistent results.
 
I personally prefer SAE correction—the standard created by the Society of Automotive Engineers. NASA and SCCA racing also require SAE dyno graphs. I often see graphs posted on the Internet using the STD (Standard on DynoJet dynos) correction factor, which results in a higher horsepower figure because it uses a colder day as the standard. Just be aware that if you are comparing figures, you want to be sure you are comparing the same kinds of numbers. Remember that the correction factor is simply a multiplier that’s applied after the fact. It doesn’t change the underlying data, which can always be displayed by setting Correction Factor to UNC (uncorrected).
– Dan Hurwitz, owner/tuner at Mach V Motorsports. Sterling, VA
 
Variables
 
Many variables exist between different dyno types, manufacturers, and even geographic locations. It's important to understand that a dyno is best suited for comparing before and after numbers or for getting a decent estimate of a car's power output as long as all comparisons are done on the same dyno. The dyno is a measurement tool, and dyno testing a vehicle should be treated much like an experiment. Minimizing variables and maintaining consistency are critical in accurately gauging gains and comparing horsepower.
 
If someone says simply, My car made xxx horsepower at the wheels, start asking questions. On what dyno? Under what circumstances? In what gear? Using what correction and smoothing factors? The answers to those questions will give you a more complete picture of what kind of power that car makes. Like almost everything else in life, the more you learn about dyno numbers, the more details are exposed. Around here, I like to say that we learn something every time we make a dyno run.– Dan Hurwitz, owner/tuner at Mach V Motorsports. Sterling, VA
 
A dynamometer (dyno) is a measurement device used to track and record the output of an engine that gauges any changes from upgrades, stressors or calibrations. In the performance tuning world, the dyno is best used as a tool for logging gains before and after modifications and for dialing in engine calibrations. Like many other measurement tools, there are many variations, standards and differences between dyno types and manufacturers. Just as with any science, it is best to take into account the many variables and to maintain consistency as much as possible for the most realistic results. Understanding these dynamics means choosing the right upgrades, working with the right tuner and having a safe and reliable vehicle in the end.
 
Special thanks to SuperFlow Dynamometers and Flowbenches for their help gathering great images. https://superflow.com/
Special thanks also to Mach V Motorsports out of Sterling, VA for their great input. https://www.machv.com/

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