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      12-04-2012, 01:38 PM   #24
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the text below if from Hotrod...

Quote:

How do chassis-dyno readings compare to engine-dyno readings? Is the amount of power lost through the drivetrain predictable with any degree of accuracy?

Many factors that influence test accuracy are common to all dynos, including engine dynos; these include temperature, airflow, barometric pressure, and torque calibration. But on chassis dynos, many additional factors can affect the results, factors much harder to control than those typically encountered on an engine dyno. Drivetrain losses vary according to gear selection (testing should usually be performed in the transmission's 1:1 gear to minimize this factor), fluid temperatures, acceleration/ load factors, drivetrain inertia, brake drag, the vehicle tie-down method, the weight over the axle, and tire selection, growth, and slippage.

The Dynapack's load brake attaches directly to the axle, eliminating the problems of tire

As TMR's Jeff Bert puts it, "An engine dyno is like weighing yourself with no clothes on; a chassis dyno is like trying to weigh yourself wearing clothes-sometimes you'll have shoes, sometimes pants, sometimes wet clothes, sometimes dry clothes." Although some of the latest dyno software adds sophisticated formulas and assumptions in an attempt to correlate rear-wheel numbers with flywheel numbers, there is really no way to measure, predict, or otherwise determine engine flywheel power from a chassis-dyno test with any repeatable certainty, particularly when using a common acceleration test.

People frequently claim that drivetrain loss is about 10 percent with a stick or 15-18 percent with an automatic. But SuperFlow's Harold Bettes says, "That's a percentage of what number? Obviously 15 percent of 400 hp is different than 15 percent of 700 hp." Also, flywheel horsepower minus 15 percent is different than rear-wheel horsepower plus 15 percent. Late-model drivetrains are often more efficient (they suffer fewer losses) than classic musclecar drivetrains. So assigning a fixed number is a guess, at best-and in a sense, it's really irrelevant: "The goal should be to end up going faster," says Mustang Dyno's Michael Caldwell. "That means ending up with more usable force at the wheels in every gear, which means knowing where you are-what works and what doesn't-and how to get to where you want to be. Better tires, more air, more fuel, and more efficient drivetrain parts, gear selection, and tuning will make it faster. Comparing engine power to wheel power is a distraction."

If you must have a comparison, the most accurate, repeatable way to compare results from an engine dyno and a chassis dyno is to conduct a loaded step test on each system, stabilizing the engine at each selected test rpm point. Ideally you'd do both tests in identical atmospheric conditions, making sure coolant and oil temperatures are the same. Any remaining difference would yield a fairly accurate estimate of the parasitic losses through the drivetrain.

...

What methods can be used to fudge or cheat chassis-dyno power numbers? What is a good giveaway that the numbers may be bogus or otherwise unreliable?

There are many ways to produce inaccurate results. Some methods are malicious; some are from poor test procedures. We've discussed how myriad environmental factors can affect the results. Obtaining consistent, accurate results in the first order requires controlling these variables to the extent possible. "If nothing is controlled, it's all bull," says Jeff Burt. "The more things are controlled, the more accurate the test will be." At a minimum, the facility should accurately correct for atmospheric conditions, which is standard practice for engine dyno-testing.

Varying the tie-down method or tension from run to run can significantly alter the results. Other ways to skew results include changing tire pressure, testing the car when the engine is very hot or cold, lying to the software about estimated wheel slip, hacking the software in general, moving the external dyno cooling fans closer or farther from the air inlet, and placing the temperature sensor in unusually cold or hot air so it skews the SAE correction factor.

Mustang and other manufacturers offer an above-ground option that teams the basic dyno mec

Although hydraulic and eddy-current dynos have the potential to be the most accurate, their myriad test regimens make them more vulnerable to the whims of unscrupulous operators. The strain gauges must be correctly calibrated. Vehicle weight, drag coefficients, and other variables must be correctly entered into the software if load testing is combined with vehicle simulation mode. Dynojet claims it's harder to fudge the numbers on a pure inertia dyno: "Since the mass is fixed, the actual measured results will be the same every time. If you looked at a dyno test as an experiment, the dyno would be the control," with the car, vehicle dynamics, and atmospheric conditions the variables. "On our dynos, the atmospheric conditions are sampled automatically, and there aren't any other user inputs that could skew the results one way or the other."

However, repeatability does not necessarily mean accuracy. Some experts maintain that inertia-dyno data may not actually represent the true power and torque produced by the vehicle being tested. For dead-nuts accuracy, the load-bearing dyno remains the standard. Just be suspicious of extremely favorable correction factors, power gains that don't make sense based on empirical evidence, or unusually high numbers. According to SuperFlow's Bettes, "A very good method of evaluation is to have the speed versus time plotted or to graph engine speed versus time. Another indication is the evaluation of how much fuel flow was used, and looking at brake specific fuel consumption (BSFC) numbers. If in doubt on the correction process, one should ask to see the correction factor and its arithmetic components."

Finally, as previously stated, each manufacturer has its own math formulas buried deep within the dyno software. A competent operator will know the vagaries of his machine and how to compensate for them. However, the average hot rodder should realize that the results of chassis-dyno testing should not be used to compare one car against another; they are best suited for evaluating the effects of incremental changes on the same vehicle.

In other words, dynos are for tuning, racetracks are for racing.
but what do i know... i found this on the internet.
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