Keep in mind that when you run your car at the dragstrip, the engine is breathing ambient
So let's say we are testing our engine on the dyno and the actual conditions are 77 degrees of air with high humidity and the pressure is lower at 28.86. If we have a big-block Chevy on the dyno (because I have a dyno test with all these numbers) and at 6,000 rpm we measure 515 lb-ft at 6,000 rpm, this is what engineers call observed torque. Since our ambient air conditions are not as good as the SAE's STP values, there is a set of calculations we can apply to correct the observed numbers. The calculations create a correction factor of 1.057 percent, which we then multiply by the observed torque to come up with a corrected torque of 544 lb-ft. Then we can take that corrected torque number and plug in the standard equation: hp = torque x rpm/5252. Doing the math, we come up with a corrected horsepower number of 621.
We correct the numbers so we can compare this test with any previous or post test so we can evaluate the changes we did to the engine and not worry about the effect of atmospheric changes. This is important since it's possible that you could be testing an engine at sea level and then also test it at SuperFlow's facility in Colorado Springs, which is roughly at 6,000 feet of altitude. Since the air is much thinner (because the atospheric pressure is lower) at altitude, we need to have a correction factor that accurately accounts for this lack of oxygen in comparison with testing an engine right on the beach in Southern California.
SAE has changed its engine power correction factors a few times with the latest being the J1349 standard, which uses 29.23 inches of mercury, 77 degrees F temperature, and dry air (zero humidity). According to numbers I've seen, this represents a 2.6 percent difference in power compared with the older STP standard. This is the standard that all the new car companies apply to advertised horsepower ratings for cars and trucks. This leads us into a whole new area that a giant story could be written about concerning how we do dyno testing. Just to give you a taste, if you are a veteran CC reader, then you know we do much of our engine dyno testing at Westech. Like almost all engine facilities, Westech uses the older J607 standard because the corrected numbers are better. In addition, Westech knows that starting the testing with a hot oil temperature of at least 180 to 200 degrees F is worth some power as is water temperature readings of closer to 140 degrees rather than 180 degrees. The company will also test most often with open exhaust and an electric water pump rather than an accessory drive that runs off the crankshaft. These examples plus a few others are reasons Westech's numbers tend to be somewhat higher than numbers generated at other dyno facilities. These numbers aren't wrong, but they do look really good.
Can an unscrupulous engine builder/dyno operator cheat a dyno to make it read a higher horsepower number? The answer is an unqualified yes. That's why going to a qualified facility you can trust is important. It's actually very easy to cheat a dyno into reading higher. Sometimes this happens inadvertently because the dyno isn't calibrated as often as it should be. Other times, a less-than-reputable operator can easily cheat the inputs by jacking up the carburetor inlet air temperature or lowering the ambient air pressure number to inflate the correction factor and make the power readings appear higher. A favorite trick is to position the carburetor air inlet sensor right above the headers. This jacks up the inlet air temperature sensor reading and inflates the correction factor. As a way of monitoring dyno numbers, a typical correction factor should be between 3 and 8 percent. Sometimes these correction factors are displayed on the dyno sheet, but more often they are not only to allow room for other more important results. But if you ask to see the correction factor, that number is always available. The only time these corrections might be higher is at test facilities at higher elevations that are located in the mountains.
There's tons more information here than we can fit into this column. If you'd like to dive deeper into the effects of atmospheric conditions on internal combustion engines, we did a story in the June in '06 issue (page 50) on the effects of atmospheric conditions on engine performance and a reference tool called density altitude (DA). This actually comes from the aviation industry where the three main factors of temperature, pressure, and humidity are calculated into a single number represented as a relative altitude. For example, a DA of 1,000 feet is better than 5,000 feet since the amount of oxygen available at sea level is far greater than at a greater altitude. While useful, DA also has its drawbacks. There's also a great weather station software program sold by Patrick Hale at Racing Systems Analysis (RSA, the same guy who wrote the Quarter, and Quarter Jr. simulation programs) that can help when dialing in your drag race car. We clearly answered more than you originally asked, but that's the risk when you enter the What's Your Problem zone.
Racing Systems Analysis (Quarter); Oshtemo, MI; quarterjr.com
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