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the_preacher197 the_preacher197 is offline
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Join Date: Jul 2010
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From an old German article - link no longer works. Cut into 2 parts for length:

Part 1
There are two types of chassis dynamometers on the market, inertia and loading. An inertia dynamometer (such as DynoJet) does not measure torque, but measures acceleration. A loading dynamometer applies resistance that is measured (using some type of strain gauge.)

The most often heard discussion is that what factor can be applied to rear wheel horsepower to reflect crankshaft horsepower. This is where we need to understand how the rear wheel horsepower number was derived. 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.

First it is important to have an understanding of how DynoJet gets their horsepower numbers. Power in mechanical terms is the ability to accomplish a specified amount of work in a given amount of time. By definition, one horsepower is equal to applying a 550 pound force through a distance of 1 foot in one second. In real terms, it would take 1 HP to raise a 550 pound weight up 1 foot in 1 second. So to measure horsepower, we need to know force (in pounds) and velocity (in feet per second). Dynojet's inertial dynamometer measures power according to the terms just described. It measures velocity directly by measuring the time it takes to rotate two heavy steel drums one turn. It measures force at the surface of the drum by indirectly measuring it's acceleration. Acceleration is simply the difference in velocity at the surface of the drums from one revolution to the next. The force applied to the drums is calculated from acceleration using Newton's 2nd law, Force = Mass * Acceleration. Since the mass of the drums is know and acceleration has been measured, Power (horsepower) can now be calculated. Torque is then calculated using the horsepower number: Torque = Horsepower * 5252 / RPM.

Once they have these numbers a series of correction factors are applied, some made public, some hidden as proprietary secrets. The public correction factor is the SAE correction factor. This formula assumes a mechanical efficiency of 85%. The formula used is: Where: CF= 1.18 * (29.22/Bdo) * ((Square Root(To+460)/537)) 0.18. To = Intake air temperature in degrees F, Bdo = Dry ambient absolute barometric pressure. This correction factor is meant to predict output in varying atmospheric conditions and is a +/- 7%. The proprietary correction factor is supposed to reflect the loss of power from the crankshaft to the rear wheels.

A Loading Dynamometer applies resistance to the dyne's roller(s) , typically using either a water brake or a current eddy brake. In either case, the amount of force is measure using a strain gauge. The measured force is torque which is a real, indisputable measurement of the actual output at the wheel. Horsepower than can be calculated: Hp = Trq * 5252 / RPM.

A Dynamometer can only measure actual power at the output location. Actual power produced AND delivered by an engine will be highest if measured at the crankshaft, lower at the transmission output shaft and even lower, but more meaningful, still, at the rear wheels. The power that you use is the power at the rear wheels. Some Dynamometer companies add to measured rear wheel power readings a factor that is based on ESTIMATED rear wheel power losses (under what power conditions? 3.0 ltr.? 5.0 ltr.? Under coasting conditions? with a 185/70/15 radial tire? a 335/35/18 radial tire? New heavy radial tire vs. worn old, light, racing tire? Who knows?) In short, there is NO meaningful "average" tire to get a correct rear tire power transmission loss measurement for all cars - so obviously, unless they actually measure the power lost in the rear tires, under driven load conditions, NO dyno company should BE ADDING incorrect power figures into the measured power. It's simply wrong. The fact that they add varying amounts of power to the actual, "true" amount of power delivered and measured to the surface of the drive roller creates a situation that makes it an onerous task to compare power figures from different brands of dynamometer systems. On simple inertial dynamometers, some (most) companies use an average for the inertial mass value of the engine, transmission, driveshaft, axles and rear wheels. This is saying that a 4 cylinder, 2.0 ltr. Porsche 914 has the same rotating mass and same rear wheels as a 8 cylinder, 5.0 ltr. Porsche 928 S+4. This simply is not so and wrong.

It's expensive to measure frictional losses in the engine and drivetrain, requiring the dyno to be able to drive the vehicle with engine off. Add the cost of a 50+hp electric motor, controlled power supply, etc. It's just not likely that $20,000 dyno will be equipped with that equipment. It is also common for dynamometer companies to add to the power readings by adding transmission and driveshaft losses back into the measured power readings. Some companies make a concerted effort try to measure frictional losses and, optionally, add the power to the measured readings. Other companies - some that would surprise you - say that it's not important and give a blanket, single factor for frictional losses in every engine. Some simply say that there is a meaningful "average" for every car,( 4 stroke/ 4 cylinder/ 4 speed transmission, 4 stroke/ 8 cylinder/ automatic transmission) and apply it to every car and that it is not a significant difference. Blanket estimates of "average" losses and corrections are, quite simply, incorrect. At the upper levels of the industry, (we are talking about $150,000 - $500,000 AC or DC 4 quadrant dynamometers) it is not tolerated - shouldn't be - and needn't be. There is a dyno company that actually has different versions of software that displays their own identical data files as different amounts of power depending on whether you use the DOS version or the Windows version of their software!!

True, rear wheel horsepower is the standard of measuring the power that is actually delivered to the rear wheels. It is honest, true, fair and duplicable. It is the ONLY standard that can be duplicated by the entire industry - regardless of the dyno manufacturer. From my experience and that of many others, when comparing True, rear wheel horsepower to DJHP you must apply a factor. It appears that this is a sliding scale based on horsepower but the best estimate is 1.05 to 1.21 (maybe higher). What this means is that for those of you trying to calculate what your crankshaft horsepower is based on DJHP, and are adding 15%, the most common number I hear, you are actually doubling (at least) the factor. Why? Because DJHP already has a puff number added into their DJHP. Lets say DJHP shows 200 hp and you add 15%, you get 230 hp crankshaft horsepower. In reality DJ has already added in 15 or 20% to their 200 DJHP number. How does this help us.? It does not, and is fact harmful to the many dynamometer test facilities that report only what the dyno actually measured. I can not tell you of the many discussions that we have had as to why the horsepower numbers we recorded lower than that of DJ. For those manufacturers that use DJHP as proof of their claims, can you imagine the shock your customers get when the horsepower number of a vehicle tested on a load bearing dyno do not come close to their claim.
Old 07-24-2011, 06:14 PM
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