Real Ratings

Here's the situation: You're in the market for an engine rated at 1,000 hp. But before committing you wonder, is that rating accurate? The answer is, sort of. The numbers on an engine spec sheet are formulated according to a set of conditions that may or may not reflect those your engine will face. It's your responsbility to understand the caveats that can affect your actual horsepower. Fortunately, by taking a closer look at the details provided on your engine's spec sheet, that's a fairly easy thing to do.

Most spec sheets feature a section titled Rating Conditions, Reference Definitions, or something similar. (If yours doesn't, contact the engine manufacturer—that's information you need to have.) Under that header you'll find a sentence along these lines: "Ratings are based on IS0 8665 conditions of 100 kPa (29.61 in Hg), 25°C (77°F), and 30 percent relative humidity." ISO conditions are a set of standards for engine testing as declared by the International Organization for Standardization. Test conditions change according to which standards your engine's ratings are based on (could be SAE J1228 or ISO 15550:2002, to name two), but each indicates that the engine has been tested and rated in a specific set environment. Such standardization allows manufacturers to provide you with a clear means of accurately comparing one engine's data to another's.

Armed with a basic understanding of these conditions, you can begin to make sense of how an engine will actually perform in your boat. Let's return to our example: an engine that's rated at 1,000 hp based on ISO 8665 conditions, which are a barometric pressure of 100 kPa (29.61 in. Hg), an ambient temperature of 25°C (77°F), and a relative humidity of 30 percent. If you run your boat in an area with the same barometric pressure, temperature, and relative humidity, you should get 1,000 hp. But if you happen to be somewhere like the Bahamas, where average temperatures and humidity levels are almost always higher, you won't. I spoke with Mike Abrahams, an engineer with Cummins, who explained that on a hot or moist day, the air becomes less dense. This reduction in density results in lower amounts of oxygen available for combustion in the engine and therefore causes a reduction in the engine's overall horsepower. How much? Abrahams posited that it would take an ambient-air temperature change of about 45°F to gain or lose one percent of power. So, even if you're dealing with 90°F versus the ISO 8665 standard of 77°F, the change in output should be negligible. But if the ambient temperature in your engine room is 122°F, you'll lose one percent of power, which is not insignificant. What if you're dealing with 60-percent humidity (like in the Bahamas or Florida) versus 30-percent? According to Abrahams, you'd have a loss of between 0.5 and 0.7 percent.

Variations from the fuel temperatures at the bottom of this chart can have a significant negative effect on performance.

Your engine's spec sheet should also provide additional information on parameters that affect actual horsepower output, information that may seem somewhat self-explanatory but that does bear some explanation: A stated power-output parameter of plus or minus three percent, for example, means that the engine's 1,000-hp rating could vary plus or minus 30 hp. These parameters can be based on the aforementioned operating conditions, various manufacturing variations, or even differences in fuel quality and temperature. Abrahams was careful to note that not all fuel systems are created equal, adding that newer systems are often much less sensitive to temperature change. He said that in some cases, a 20°F change in fuel temperature can raise or lower power by one percent.

All this goes to show that before you buy an engine, you need to carefully study its spec sheet to make sure that you know exactly what you're buying. At the end of the day, it comes down to dollars for horsepower, and you should also make sure that you're getting your money's worth.

This article originally appeared in the January 2008 issue of Power & Motoryacht magazine.

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