We may have goofed, but we're man enough to admit it. Despite the countless engine dynamometer tests car magazines publish each month, we've rarely-if ever-explained how an engine dyno actually works. And if we don't explain the methods used to ascertain our data, then that data is potentially meaningless. Consequently, some dismiss the veracity of dynos altogether, but such a conclusion isn't entirely justified. With exception of the flow bench, no single tool has advanced the development of engines as significantly as the engine dyno. Without them, Nextel Cup small-blocks wouldn't produce anywhere near 850 hp from 358 ci and Pro Stock motors couldn't dream of belching out 3 hp per cube. In other words, blame the guy using the equipment, not the equipment itself.
For a comprehensive explanation of how dynos work, we sought the expertise of Harold Bettes and Dan Roberts, two of the foremost authorities on engine dynos in the country. After a long tenure with Superflow, Bettes branched off to form his own consulting firm, Power Technology Consultants. Through his decades of service in the industry, Bettes has held dozens of seminars educating industry insiders on the same material we'll discuss in this story. Likewise, Roberts is the tech guru at Dynamic Test Systems, maker of some of the finest dyno equipment on the market. Thanks to them both, we've got answers to some of the most controversial topics surrounding dyno testing, from correction factors to test accuracy to test-cell layout, as well as the basics of how dynos work.
By far, water-brake dynos are the most popular in performance automotive applications. At the heart of the dyno is a power absorption unit, which attaches directly to an engine's crankshaft or flywheel. As an engine is accelerated on the dyno, the absorption unit's rotational element (rotor) spins inside its stationary housing (stator). It functions much like a torque converter but uses water instead of transmission fluid. Consequently, although the rotor and stator are not physically connected, as the rotor starts to spin, the stator tries to rotate with it in the same direction. Attached to the stator housing on one end and the dyno's steel frame on the other, a strain gauge prevents the absorption unit from spinning and measures torque. "What dynos actually measure is torque, not horsepower," explains Roberts. "Since horsepower is a mathematical equation, the dyno measures torque then calculates horsepower."
Eddy-current (EC) dynos operate on a similar principle, but electrical current provides the load in lieu of water. A steel rotating element spins through an electromagnetic field, and load is increased by cranking up the magnetism. The benefit of EC dynos is their precise load control. In steady-state testing, they can control an engine within 1-2 rpm, opposed to the 5-10 range of a water-brake dyno. However, they're not very practical or appropriate for most engine builders. "Although they're very precise, EC dynos are extremely expensive to set up and operate and have less dynamic operating range in terms of measuring horsepower," says Roberts. "That's why they're better suited for R&D labs at the OEs."
The most sophisticated yet least common type of dyno is the alternating-current (AC) dyno. Essentially a large AC motor, in addition to absorbing power it can also power a motor. This allows AC dynos to replicate any on track condition-such as coasting, upshifting, and downshifting-and simulate and entire race in a test cell. Expensive even by Nextel Cup standards, AC dynos are typically reserved for mega-buck racing operations like Formula 1.
Situated between the power...
Situated between the power absorption unit and the dyno's steel cradle, the strain gauge measures engine torque by preventing the stator from rotating.
Using adapter couplings, dynos...
Using adapter couplings, dynos typically attach directly to an engine's crankshaft or flywheel. A dyno's measuring capacity is directly related to the absorption unit's rotor diameter.