![]() The force applied to the drums is calculated from acceleration using Newton's 2nd law, Force = Mass * Acceleration. Acceleration is simply the difference in velocity at the surface of the drums from one revolution to the next. It measures force at the surface of the drum by indirectly measuring it's acceleration. It measures velocity directly by measuring the time it takes to rotate two heavy steel drums one turn. Dynojet's inertial dynamometer measures power according to the terms just described. So to measure horsepower, we need to know force (in pounds) and velocity (in feet per second). In real terms, it would take 1 HP to raise a 550 pound weight up 1 foot in 1 second. By definition, one horsepower is equal to applying a 550 pound force through a distance of 1 foot in one second. Power in mechanical terms is the ability to accomplish a specified amount of work in a given amount of time. Since the DynoJet seems to be widely used and numbers quoted are those from a DynoJet, we are going to use them as our inertia dynamometer example.įirst it is important to have an understanding of how DynoJet gets their horsepower numbers. This is where we need to understand how the rear wheel horsepower number was derived. The most often heard discussion is that what factor can be applied to rear wheel horsepower to reflect crankshaft horsepower. A loading dynamometer applies resistance that is measured (using some type of strain gauge.) An inertia dynamometer (such as DynoJet) does not measure torque, but measures acceleration. There are two types of chassis dynamometers on the market, inertia and loading. The 310 horsepower rating of a 1973 Pontiac Trans Am SD 455 may seem quite dismal if you're comparing it to something like the 370 horsepower rating of the 1970 Pontiac Trans Am RAIV 400, but the actual difference in the power output of these two engines is much smaller than the 60 horsepower discrepancy indicates.This discussion revolves around chassis dynamometer's and is intended to be informative and thought provoking. Net horsepower was attained from engine dynos with the accessories, OE air cleaner, and factory exhaust system present on the test subject engine. To make horsepower numbers more accurate to what you would actually see from your engine when installed in your car, for the 1972 model year, American car manufacturers started using "net" horsepower figures. Between the lack of power-robbing accessories and restrictive exhaust systems, the advertised horsepower numbers were quite inflated when compared to today's power figures. Also these power figures were typically attained with a very unrestrictive open exhaust system, along with the lack of restrictive OE air cleaner assemblies. These gross horsepower figures came from engine dyno results in which the tested engine had no real-world accessories attached such as power steering pumps, smog pumps, and or A/C compressors. "Net" HorsepowerĪs for American carmakers, previous to the 1972 model year, the horsepower figures you typically saw in car manufacturer literature and car magazines were "gross" horsepower figures. One horsepower is equal to 745.7 watts, watts being a unit of power that James Watt also came up with, and the same unit you see everyday associated with things like light bulbs and electrical devices. One horsepower is equivalent to the amount of force needed to lift 550 lbs of weight within a period of 1 second. Watt would call this new unit of measurement "horsepower". Back in the 18th century James Watt was developing steam engines and wanted to come up with a unit of measurement that would indicate the amount of workhorses the steam engine would replace. ![]() ![]() Horsepower is a measure of the ability to move a certain amount of weight a certain distance, within a certain amount of time. ![]()
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