Dynos Don't Lie. Or Do They?
By Shiv S. Pathak

"Dyno Proven"-"Dyno-Derived"-" Dyno-Tested"

All of these terms ring a familiar bell in the aftermarket performance industry. A comforting one, of course. After all, these terms imply that the product in question has proved its merit on the completely infallible, 100% dependable and absolutely repeatable dynamometer. Wouldn't you like to own an intake that has been "dyno proven" to yield 15 wheel horsepower gains versus one that has been "dyno proven" to produce a gain of a measly 10 wheel horsepower?

Unfortunately, the reality of the situation isn't so clear-cut. Like just about every other device known to man, the dynamometer is a tool. And like any tool, it can be misused or tweaked to yield the desired results. How is this possible? Read on and check out the following cheating techniques that run rampant in the performance aftermarket industry:

Wimpy Stock Technique

What better way to make a tuned car look more impressive than make the stock car less stout? That's right. It's easy to do. If a stock car dyno tests between 100 and 120 wheel horsepower, it's not unusual for a tuner to assign the lowest value to the stock baseline dyno run. Voila, an instant 20 horsepower gain with no extra work! This technique works exceptionally well on intercooled turbocharged cars as, depending on intake temperatures (which can be tweaked by either the amount of airflow over the intercooler or the cool-down time between each run), dyno results can be all over the place! Not all tuners take the time to stabilize intake, intercooler and coolant temperatures before each successive dyno pull. And those that don't are presented with many opportunities to skew the results in their favor. And even those that do are often presented with an uncomfortably wide range of power outputs that demands careful consideration before picking an accurate and fair baseline. This is especially true of the more modern turbocharged cars that have the ability, through their sophisticated engine management computers, to actively tune and de-tune themselves as it sees fit given the conditions (temperatures, ambient pressures, lunar placement, etc.)

Happy Correction Factor Trick

Just about every engine or chassis dynamometer has the ability to display the results with any number of applied "correction methods." SAE, DIN, STD, EEC, etc. Each correction method represents a way to equate (for the purpose of comparison) different dynamometer results that where taken under different conditions (barometric level, ambient temperature, altitude, etc.,) Even when used properly, these correction techniques don't always represent a realistic picture. This is because different types of engines react to conditions changes differently. In other words, there is no one-size-fits-all correction method.

Altitude

For example, let's consider a dynamometer located at 5000 feet above sea level. At such elevation, most cars suffer terribly due to the lack of air density. As a result, their power outputs fall noticeably compared to identical cars that operate at or near sea level. For this reason, just about every dynamometer applies a hefty altitude correction in the magnitude of 20% (SAE correction, in this case). This means that a car that put down an actual 100 wheel hp is "corrected" up to 120 wheel hp. While this correction amount is reasonably accurate in some cases, it is notoriously optimistic in the case of turbocharged engines. In such engines, power output rarely falls as dramatically in response to air density reduction. This is due to their turbo control systems that combat air density reductions by allowing for higher boost pressures. These increased boost pressures can almost completely offset the ambient pressure reduction and make the "altitude correction" almost completely unnecessary. However, I have yet to see a high-altitude tuner come forth and not apply the positive correction factor when displaying their grossly optimistic dyno results.

Humidity

Similar issues arise with changes in humidity. Standard dynamometer correction techniques apply an increasingly positive correction as humidity rises. The idea behind this is that air density reduces as moisture content increases. In other words, the more humid the day is, the less power the car will make. However, as with altitude, not all cars react to humidity changes the same way. For example, a naturally aspirated car may behave as predicted by the smarty-pants that derived the correction technique. But a heavily turbocharged may behave exactly opposite to the rule. Turbocharged cars, unlike naturally aspirated cars, often operate right up to their knock (also known as detonation) thresholds. When humidity rises, the extra water content in the air charge actually acts as a passive cooler of sorts, lowering in-cylinder temperatures just enough to allow for a few more degrees of ignition advance without the presence of detonation. In other words, whatever engine output is lost through the reduction in oxygen content is gained (and then some) through a significant bump in thermal efficiency (caused by operating with more ignition advance). Voila-another improperly applied correction factor!

Temperature

Unfortunately, that's not the only way to misuse correction factors. Case in point: Temperature correction. As with altitude, increases in ambient air temperatures almost always yield reductions in engine output. Conversely, reductions in ambient temperature just about always yield increases in engine output. One trick that is used by more than a few tuners is strategic placement of the dynamometer's air temperature sensor (which is used for correction factor calculation). When need for lower-than-normal dyno result arises, it's easy to place the air temperature sensor in a slightly colder environment (out of the engine bay, in a cool shadow, on some insulation, etc.). Similarly, when a higher-than-normal result is needed, all one has to do is to place the sensor in a hot environment (near the exhaust header, in a stagnant pocket of air, in direct sunlight, etc.). Complicating the matters further is that, yes, you guess it, not all cars respond to temperature changes the same way. Turbocharged cars may, in fact, make less power when ambient temperatures drop beyond a certain point. This is often caused by lean-run conditions induced by the increase in air density. Running with the leaner air/fuel ratios, a turbocharged car may run into detonation, which will result in spurious knock sensor activity. Before you know, it several degrees of ignition advance is yanked out and power suffers measurably. This situation is not uncommon in cars, like the WRX, that have their intake temperature sensors placed before the turbo (in the Mass Air Flow sensor) and not just before the throttle body. Latter placement provides a much more accurate indication of in-cylinder air temperatures, allowing the engine management computer to respond with proper fuel and timing compensations.

The Result

For this reason, all dynamometer results provided by Vishnu Performance Systems will be actual, as measured and "uncorrected." Since we (and our dynamometers) are located in the San Francisco Bay Area, there is no need for any altitude correction. Furthermore, since the dyno facilities are in-door, testing conditions are almost always just around room temperature. In the case where testing conditions differ dramatically, we will disclose such information. We feel these efforts will go a long way in ensuring that our claimed gains translate well into real-world performance.