Automotive enthusiasts have been trying since time immemorial to lay claim to having the quickest, fastest, or most powerful car. In order to prove this, favorite areas for bragging rights include 0-60 times, quarter mile times, elapsed time over a measured course, and horsepower and torque output. Unfortunately, testing on the street and in a controlled setting is not only mutually exclusive, but in most cases it is dangerous and illegal. Furthermore, there are just too many independent variables which can adversely influence the tester's ability to accurately measure and record the horsepower/torque output; i.e., reaction time, traction, weather, elevation, etc.
As a result, we rely on performing tests indoors on the next best means at our disposal: a dyno. While it's not a perfect solution, at least the results can be objectively studied, measured, corroborated and easily replicated. But not all dynos are created equally.
Dynamometer 101 - Basics of Dyno Operation
Dyno is short for dynamometer and is a device that measures torque and, therefore, the horsepower of a vehicle. The dyno itself is essentially a "brake" which can apply a known torque (or "load") to the engine. When the engine is holding a steady speed under a given dyno load, then the torque being applied by the dyno is equal to the torque being produced by the engine.
There are various ways in which the dyno load can be applied. Older dynos use a hydraulic system with a rotor inside a water filled cavity - similar to the torque convertor in an automatic gearbox. Modern dynos, however, generate the load with large electric motors. Even a simple friction disk or drum brake will work fine and this is where the name "brake" in Brake Horsepower comes from. The important thing is that the load is measured accurately and that there are no frictional losses in the systems that escape measurement.
The two main types of dynos are an engine dyno and a chassis dyno. An engine dyno tests the engine out of the car and measures the power of the engine alone. The engine is bolted to a cradle and connected to the dyno with a prop shaft which bolts onto the back of the crankshaft (or the flywheel). The intake and coolant are also plumbed to external fixtures that simulate on-road airflow and then the engine is run from idle to redline. This is how nearly all manufacturers rate the output of car engines since it provides the most optimistic readings.
A chassis dyno, on the other hand, is a machine that has two (or four) large rollers which the car's tires rest on. The vehicle is strapped down and actually driven on top of the rollers which then measure the torque of the engine. A chassis dyno, consequently, measures torque at the tires rather than the crank/flywheel. Friction from rubbing gear faces, inertia from heavy shafts, as well as gear lube all conspire to reduce the advertised horsepower reaching the tires and, hence, the dyno and explain why wheel horsepower is always lower than flywheel horsepower. The sum of this drag is commonly referred to as "parasitic loss."
In order for dyno results to be comparable and universally accepted, there are a number of things that need to be closely monitored during the measurement process. These include air temperature, air pressure, and humidity - all of which affect the amount of power an engine produces. Cold dense air, for example, means a greater mass of oxygen per power cycle and thus more power is generated (provided that air/fuel mixture is properly calibrated). So a dyno's hardware typically includes a weather module that monitors real-time atmospheric conditions and its software uses an SAE formula to correct measured power to a uniform "standard day." In theory, this allows comparing runs made on different days or at different locations with reasonable accuracy, at least on the same manufacturer's machines.
Chassis Dynos
Obviously, most of us would rather not want to go through the time and expense - and headache - to remove an engine from our car and have it tested on an engine dyno. For this reason, chassis dynos are commonplace, usually provide the necessary feedback/information we are looking for, and are accurate.
Common to all chassis dynos are the steel rollers, or drums, that are either placed in the floor or up on an elevated stand. What most chassis dynos measure during a full-throttle acceleration test is the force, or kinetic energy, acting at the frictional interface between drive wheels and the knurled surface of the dyno's drum(s). While all have some means of precisely measuring the rotation (speed) of the drums, many have additional hardware to load the dyno beyond the inertial weight of the drums themselves. Referred to as inertia dynos, they are based on the sound scientific principles of accelerating a certain mass with a known moment (distance) over a given time. The rate of acceleration of that mass and moment is a result of the force applied (torque). If the RPM is known, horsepower can be calculated.
Dyno's like the one used at Curry's Auto Service from Mustang Dynamometer uses an eddy-current (loaded) power absorber and load cell (strain gauge) located downstream of the dyno drums. Eddy current dynos generally measure force (torque) directly through a ram or strain gauge so moments of inertia are not important on these in fully loaded tests. The load cell's fixed distance (moment) from the shaft is used to calculate torque being absorbed. A principal advantage of a loaded dyno like the Mustang Dynamometer is its ability to measure power either under acceleration or in the absence of acceleration, such as in a steady-state, no-acceleration condition. This doesn't necessarily make it any better at producing the full-throttle power measurements we so often seek, but it does permit many additional forms of testing (and tuning).
In a perfect world, we'd all have a chassis dyno in the garage - as an analytical tool, it has few equals. But this is not fiscally feasible for most of us since the Mustang chassis dyno at Curry's, for instance, costs over $100,000.
Benefits of Dynos
Around for decades in one form or another, the advent of powerful personal computers and other modern electronics and sensors have thankfully made the chassis dyno more common, accurate, functional, and operator-friendly than ever. Teamed with a knowledgeable operator and the right options, it can be an invaluable tuning tool regardless if the vehicle is equipped with a low-tech carburetor or sophisticated ECU.
Dynos are relied on whenever possible to quantify the results of engine/ECU modifications, and to measure and plot a car's full-throttle, rear-wheel horsepower/torque curves. This is usually either for pure bragging rights or to quantify the results of a change. Chassis dynamometers can be used to examine not only the effects of engine modifications, but also changes to exhausts systems, drivetrain components such as swapping transmissions, or even lubricating fluids. Most chassis dynos can also be optioned with additional sensor packages. One of the most common is a wide-band oxygen sensor which allows constant monitoring of air/fuel ratios. These sensor packages help expand the dyno's function from a power measurement device to an analytical tuning aid.
Thanks to sophisticated software programs, computer-controlled dynos are also capable of simulating real-world factors such as wind and rolling resistance. Because it can be loaded to accurately reflect the test vehicle's weight as well as wind and rolling resistance factors, this type of dyno can also be used to simulate time-to-distance (i.e., quarter-mile) or time-to-speed (i.e., 0- to 60-mph) runs - as well as calibrating a vehicle's speedometer.
There's more than one mathematical path leading to the calculation of horsepower and torque figures, and obviously not all the manufacturers take the same one. A vehicle will almost always generate different power figures when tested on different manufacturer's models. To get accurate, repeatable readings in order to compare before-and-after combinations, Curry's suggests sticking with the same make of dyno from test to test. |
