Horsepower In The Air

The Shell Game of Correction Factors

Since internal-combustion engines are directly affected by the constant change in atmospheric conditions, long ago the automotive industry came up with a plan for using correction factors to establish a common ground from which all power numbers could be compared. The original "gross" horsepower correction factor includes the standard of 29.92 inches of Mercury (Hg), which is standard sea-level pressure, combined with a temperature of 60 degrees F with no humidity, or zero vapor pressure. As you can imagine, these are ideal or "gross" horsepower numbers that are not practical in the real world but serve as a common reference point. Through the early '70s, these were the numbers Detroit advertised and the performance industry followed. In 1972, Detroit switched to a net horsepower correction followed by several more changes, the last of which occurred with SAE standard J1349. This current Detroit correction factor uses a lower 29.235 inches of Mercury pressure with a higher air temperature of 77 degrees F and zero vapor pressure. This correction factor reduces the old gross-horsepower output number by roughly 5 percent but is also more realistic. As an example, the new 427ci LS7 Corvette engine rated at 505 hp would probably correct to closer to 530 hp using the performance-industry gross correction factor (C.F.).

To bring this home, let's take a look at a big-block power curve with three different sets of numbers in the chart below. The first column represents the observed numbers generated on a high-pressure day with 30.02 inches of barometric pressure, air temperature of 73 degrees F, and a vapor pressure of 0.35. Using the classic gross correction factor reference, this equates to 1.025 or a 2.5 percent increase over the observed power. The third column indicates power using the SAE J1349 C.F., which uses 29.235 inches of Hg station pressure, 77 degrees F temperature, and zero percent humidity as its reference point. The observed power data came from a dyno operated at very close to sea level. Using the SAE J1349 standard, the correction factor calculated to 0.978, which reduces the observed power by 2.2 percent! This works out to a differential between the gross and SAE J1349 corrections of roughly 5 percent.

So which set of numbers is correct? All three are accurate as long as they are properly identified. We guarantee no one uses observed (uncorrected) dyno numbers unless the density altitude that day is a negative number (which does happen-it's like racing down a mine shaft). But if you were an engine builder, which set of numbers would you give your customer? Armed with this knowledge, what questions should you ask your engine builder when reading a dyno sheet on the engine you just paid for? At the very least, you would want to know the inlet air temperature, uncorrected station pressure, and vapor pressure. If he begins to waffle, consider becoming very suspicious.