How is power corrected by the dynamometer?

The SuperFlow dynamometers have barometric pressure, temperature, and humidity correction factors at each test point. The barometer and air temperature are re-calculated for the correction factor each time data is recorded. We provide humidity input through a sensor on SF-7100 models, or by sling psychrometer for SF-901 models. We found that the relative humidity sensors cannot be successfully mounted in the test cell, because they will absorb hydrocarbon vapors from gasoline or oil. Their calibration drifts rapidly, because they are just as susceptible to gasoline vapors as they are to moisture in the air. For this reason, we recommend the air be ducted from outside of the room to the carburetor, and that the relative humidity be measured at the duct entrance outside of the room to protect the sensor from gasoline vapors. Then the relative humidity must be converted to absolute humidity for power correction.

A more important point is that any carbon monoxide in the room will affect the engine power. So exhaust leaks and poor air circulation can lead to substantial power variations, which are not the result of engine differences. If you can smell exhaust gases in the room, when the engine is running, the engine test data will be affected.

A very important difference between SuperFlow dynamometer correction factors and those used by some other dynamometer manufacturers is that the SuperFlow system calculates the power correction based on the power developed in the cylinder, not just the power developed at the flywheel. The reason is that the variations in atmospheric conditions affect the power developed in the cylinder, but you are measuring the power at the flywheel, after the friction power of the engine has been subtracted. This is a very important point, because the cylinder power number is greater than the crankshaft power number. If you multiply the correction factor times only the crank shaft power number, you will get the wrong answer. Furthermore, the actual net correction for engine power changes with engine speed due to varying friction losses and is not a constant for a given test. This fact is ignored by most simplified correction factor schemes, and consequently, they give a different answer than you would obtain if you actually tested the engine under standard conditions.

The SuperFlow system uses an empirical data base to calculate the actual internal losses in the engine based on piston speed and engine displacement. This data has been gathered by numerous tests run by motoring dynamometers that actually measure the friction power losses. Included in this data base are a number of tests of NASCAR and drag race type engines. We found that our calculations scheme predicts the actual friction losses in the engine to a high degree of accuracy and greatly improves the correction factor accuracy. The ultimate test of the correction factor is, will you get the predicted power? Our system provides the best approach yet developed for ensuring that the answer to this question is yes.

If the friction horsepower is ignored during the calculations, the error in the correction factor will be approximately 20% at peak torque, and 33% at peak horsepower. The actual net corrected difference due to correction factors varies with piston speed. The SuperFlow system compensates for this change.

On motoring dynamometers, the standard procedure is to actually measure the friction power at each speed, and then to add this power to the flywheel power. Then the correction factor is multiplied by the total power to obtain the corrected power. Last, the friction power is subtracted for the estimated power at the flywheel. The SuperFlow system uses this same approach, except that we calculate the friction power based on the summary of all the measurement tests of friction power. It is likely that some of the other dynamometer manufacturers will adopt this scheme as an option in their systems, because it provides a more accurate and theoretically correct answer. For additional details, see the Test Tips Section of the SuperFlow SF-901 Instruction Manual.

_______________________________________________________________________________ DTS Click Here

DTS uses straight SAE BHP at the flywheel.

This results in lower publish power output numbers compared to Superflow.

Example

RPM     DTS-TQ    DTS-BHP       SF-TQ    SF-BHP

3000     463          265               490        280

4000     487          371               515       393

5000     532          507               564        537

6000     497          568               528        603

Conversion Factor 94.5%