Dynamometer = Dyno

By Bill Shaw from Curry's Auto Centers

Automotive enthusiasts have been trying since time in memoriam to lay claim to having the quickest, fastest or most powerful car. This is most often substantiated in 0-60 times, quarter-mile times, elapsed time over a measured course, or horsepower/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 this data, 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 converter in an automatic transmission. Modern dynos, however, generate the load with large electric motors.

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. 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. This explains why wheel horsepower is always lower than flywheel horsepower. The sum of this drag is commonly referred to as “parasitic loss” or drive train 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 with reasonable accuracy, at least on the same manufacturer's machine.

Chassis Dynos

Obviously, most of us would 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 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 (Horsepower = (Torque x RPM) / 5252).

Dynos like the one 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. 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 steadystate, 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, every gearhead would have a chassis dyno in their 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 and functional. And thanks to sophisticated software programs, computer-controlled dynos are now capable of simulating real-world factors such as wind and rolling resistance. Teamed with a knowledgeable operator and the right options, it can be an invaluable tuning tool regardless if the vehicle is equipped with a lowtech carburetor or sophisticated ECU. This is why it is absolutely imperative to have a properly trained technician running the dyno.

Dynos are used if someone is interested in or simply wants to obtain a baseline reading, calibrate their speedometer, quantify the results of engine/ECU modifications, measure and plot a car’s full-throttle, rearwheel horsepower/torque curve, or to document changes of different exhausts systems, drive train components, i.e., transmissions, and 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.

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. For instance, the same car dyno’d on a DynaPak, DynoJet or Mustang will invariably have different rear wheel horsepower readings. This is why—and we can’t stress this enough—that in order to get accurate, repeatable readings when comparing before-and-after combinations, Curry’s suggests sticking with the same make of dyno from test to test.