Correction Factors
Without correction factors, it would be impossible to accurately compare horsepower numbers from one test facility to another. In theory, correction factors produce numbers that reflect an engine running at sea level, but not all are created equal. There are many SAE correction factors, known in the industry as J codes. "The current is J1349, which corrects to 29.23 inHg at 77 degrees F and 0 percent humidity," explains Bettes. "However, in the automotive aftermarket, it's very common to correct to 29.92 inHg at 60 degrees F and 0 percent humidity. The difference between the two in terms of horsepower calculations is about 4 percent."
Furthermore, to gather pertinent atmospheric conditions, a dyno facility's weather station should be located in the test cell itself. While it shouldn't interfere with an engine's airflow, an ideal scenario would have the weather station hanging down from the ceiling, positioned near the carburetor to most precisely measure the air the engine is ingesting.
Acceleration Rate
Dynos regulate how quickly a motor is allowed to accelerate, and that rate affects power output. There is no set standard, but accelerating a motor at 300 rpm per second is common in the industry. "If you gather data at 300 rpm per second in one test and 600 rpm per second in another test, then the numbers from each aren't comparable," explains Bettes. "Slower acceleration rates typically yield higher horsepower figures than faster acceleration rates. Drag racers usually test using faster acceleration rates than the NASCAR guys."
Load
Can dynos sufficiently load a motor to simulate the weight of a 3,000-4,000 pound car? You bet. Skeptics think that because dynos don't load a motor as much as under real-world use, engine builders can get away with running more aggressive timing and fuel curves, therefore boosting horsepower figures to unrealistic levels. Not true. "In most cases, a dyno can put more load on a motor than it would normally see in a car," says Roberts. "In a car, load decreases as speed increases due to inertia, but that isn't the case on a dyno." Bettes concurs, pointing out that dynos can load a motor at far lower rpm than a car. "You can load a motor at WOT at 3,000 rpm on a dyno, but a motor would never see those conditions at the track or on the street, since the tires would spin or the torque converter would stall at a higher rpm. Also, a dyno operates at a 1:1 ratio, so unlike a car, it's always in High gear."
Ventilation
Exhaust fumes displace oxygen, so even minute traces of the exhaust that makes its way back into the carburetor dramatically reduces power output and test consistency. According to Bettes, if a shop is located next to a freeway, exhaust fumes from cars during rush-hour traffic will adversely affect dyno readings. In addition to providing fresh air for the motor to breathe, proper ventilation helps shed radiant heat. "To accomplish both objectives, the average test cell needs to flow 20,000-25,000 cfm of air," Bettes explains. "To put that in perspective, people only breathe 3 cfm. A ventilation system doesn't have to be elaborate, but having the right size fan and a good cell design is critical to ensure consistent test results." But wouldn't going overboard have a supercharging effect on the motor? "It's possible, but not likely, since the weather station's barometer would compensate for it in the correction factor anyway. Even if you could manage to supercharge a cell, it would only equal a few inches of water. Considering 13.6 inches of water is equivalent to 1 inch of mercury, that's not much pressure at all."
With the luxury of a large test cell instead of a cramped engine compartment, the extent of data-acquisition equipment that can be added to a dyno is virtually limitless. Typically gathered data include rpm, horsepower, torque, EGT (exhaust gas temperature),... | 
...and air/ fuel ratio. Without much difficulty, a dyno can be set up to record blow-by, water flow, oil flow, inlet air temperature, and fuel temperature-at sampling rates up to 100 times per second. | 
Test cells vary greatly in size, but DTS has an easy formula for calculating the airflow requirements of a dyno room for proper ventilation. Multiply the air volume of the test cell by 10. Fans can either draw in fresh air from the outdoors or climate-controlled air from within the building. |