In the Sept. '09 issue, the "Dyno Dash-AWD" article illustrated the differences between dyno types, but what do you do with that baseline graph? How do you read the chart and decipher where there is power potential ready to be unleashed? How can the dyno be used to detect problems in the works that hinder power production? These are the golden questions of performance tuning. We have tracked down some dyno charts that tell interesting stories straight from the front lines. In Part 1, we'll get the ball rolling with a basic tech primer and then it's off to the charts.
Anatomy Of A Power Curve
It's pretty common knowledge that the Y-axis of a power chart is home to the output numbers while the X-axis represents either engine speed or vehicle speed, depending how the dyno is set up. But there's more than power and torque being displayed on that printout.
Yellow line: the power curve crossover on a dyno chart. It always intersects at 5150 rpm w
Power Curve Crossover
Many people out there wonder why the power and torque curves crossover at 5150 rpm. It has to do with how the dyno calculates power. The dyno measures torque and uses that measurement to calculate power output. The formula to calculate horsepower is:(torque x rpm) / 5150. This is why torque and horsepower plots always intersect at 5150 rpm on a dyno chart.
Smoothing Factors
Smoothing factors make the power curve more consistent and easier to compare before and after tuning changes. Smoothing consists of an averaging equation within the dyno's software that equalizes peaks and valleys in the readout. There is an impact on the final results as higher smoothing factors produce lower peak horsepower numbers. The key is to pick a setting and stick to it for all future comparisons.
Correction Factors
Ambient temperature, relative humidity and barometric pressure have a good deal of effect on engine performance. These mighty variables are whittled down and equalized using a correction factor that compensates for the environmental conditions, making the results equal and comparable.
The most widely used correction factor is the SAE Standard J1349, which applies the following weather station data: atmospheric pressure 29.23, air temperature 77 degrees Fahrenheit and humidity zero percent. What are we talking about here?
To better understand how much variation is in play and how much misreading of power is at stake, consider the following facts. Atmospheric pressure is a by-product of altitude and the difference in power readouts between sea level and Denver, Colorado, is in the 20 percent range. A 30-degree jump in temperature from 60-90 degrees will produce about a 3 percent variance. Taking the SAE's zero percent humidity and turning it all the way up to 100 percent nets a 7 percent swing.
We decided to let the dyno act as detective to illustrate how dyno charts are read, interpreted and acted upon while providing some chart-reading lessons along the way. The following charts are from real shops, the events depicted are true and no names were changed to protect the innocent.
[1]
The Case Of The Maxed-Out Turbo
The car is a Mitsubishi Lancer Evolution owned by Aaron O'Neal of English Racing. It was used to develop the Camas, Washington-based company's E85 tuning menu. In this dyno chart [1], we are catching the car as it transitions away from its stock turbo that was pushing 31 psi and made 432 awhp on E85. Note the overall flatness of the power curve's shape and manner in which it tails off after 6000 rpm. This indicates the compressor is well off its efficiency island.
By Evan Griffey
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