No two test-cell designs are identical, and multiple arrangements can be used to achieve good ventilation. The fan should draw fresh air from outside the test cell, but some dyno rooms have separate air ducting to feed the carb. Likewise, some cells route exhaust fumes outside of the room while others dump the exhaust right into it. The latter is fine as long as there is sufficient airflow. Ultimately, properly ventilating a test cell isn't a one-size-fits-all affair.
Accuracy
Factors such as load control, test-cell design, and operator technique all influence the accuracy of a dyno. Fortunately, these variables are relatively easy to control, which yields extremely repeatable results from test to test. Since most contemporary dynos are automated, the potential for operator error is substantially reduced and smooth load application isn't difficult to achieve. "Tests should be repeatable within much less than 1 percent, and within 0.5 percent with a good test cell," explains Bettes. "At NASCAR shops, they're looking for 1hp gains out of 850hp motors, and 1 850 = a lot less than 1 percent. Pro Stock shops are often within 1 hp from pull to pull."
Oil & Water Temperature
To minimize variations between pulls, dynos must closely regulate an engine's oil and water temperature. Typically, this is accomplished by tapping into the dyno's main water supply tank. Water from the tank is routed to a cooling tower in the test cell, which supplies water to the motor. The cooling tower has its own thermostat and, in some instances, its own heat exchanger. To circulate the water, DTS recommends a 10hp centrifugal pump capable of flowing 80 gph at 80-90 psi.
Oil temperature can be regulated by simply allowing sufficient warm-up and cool-down time between pulls. Dyno manufacturers can also add a water-cooled heat exchanger to help regulate oil temps. "Generally, the sweet spot for oil temperature is 190-220 degrees F, and it should be held within 2-5 degrees from pull to pull to maintain consistency," Bettes explains. "Running a motor outside that sweet spot can affect power in either direction by 3 percent, and that's a scary number."
Operator Error
These days, the technique of the dyno operator has very little influence on test results. That's because the vast majority of dynos are automated. "Even if someone has never seen a dyno before, I can teach them how to run one in 10 mintues," says Bettes. "On an automated system, as the operator adds throttle, the computer automatically adds load to the motor. All the operator has to do is set the rpm range of the test and smoothly apply the throttle. The fans can be set up to turn on and off automatically as well." Conversely, manual dynos are more difficult to use, since the operator must control throttle with one hand and manually add load with the other hand. Although an operator can impact test results far more with a manual system than an automated system, they're rare these days.
Warm-Up & Cool-Down
Although different engines have different needs, all dyno tests should allow for sufficient warm-up time before each pull and cool-down time between pulls. To ensure consistency between tests, fluid temperatures must be stabilized. "Opinions vary greatly, but for a street/strip motor, the oil temperature should be at least 190 degrees before going WOT, and coolant temperatures should be held between 160 and 165 degrees+ F," Bettes explains. "Target fluid temperatures with race motors are all over the map because the temperatures a motor will see on the track is what should be duplicated on the dyno."
To ensure repeatable test results, the throttle handle should be pushed in a smooth, gradual motion. | 
Since water brakes produce heat as load is applied, a fresh supply of cool water must circulate through the system to maintain accuracy. Water is stored in a separate supply tank. | 
Dynos don't require much maintenance, but fuel systems should be checked regularly for leaks and clogged filters. DTS recommends greasing the dyno's main bearings after every 30 hours of use in addition to checking the system's torque calibration. Each manufacturer has its own maintenance schedule. |