Dynojet WinPEP 7 - Manual (Page 19)

D Y N O A N D W I N P E P B A S I C S
How Your Dyno Works
Version 2 WinPEP 7 User Guide
2-3
point of rotation to where the force is being applied. Torque is the product of the
force and the moment arm. For example think about trying to spin a drum by
wrapping a rope around the drum and then pulling on the rope. If the rope is
wrapped around a drum of one foot radius and pulled with 550 pounds of force, the
resulting torque is 550 foot-pounds.
The torque on the dyno’s drum can be calculated by multiplying the force applied by
the drum’s radius. However, engine torque is not equal to the dyno’s drum torque
because the gearing through the drive train changes the moment arm. The change in
the moment arm is proportional to the ratio of
engine speed to drum speed.
Therefore, tachometer readings are necessary to calculate and display engine torque.
C ORRECTION F ACTORS
The calculation of horsepower, or the accuracy of a Dynojet dynamometer, is not
dependent on the location or conditions during the measurement. The performance
of the internal combustion engine however, is sensitive to atmospheric conditions,
especially air density and air temperature. To compare power measurements taken at
different times or places, it is necessary to compensate for differing atmospheric
conditions.
Correction Factors are used to compensate for different operating conditions while
measuring engine horsepower. The typical correction factor (CF) is calculated based
on the absolute barometric pressure, air temperature and the water content of the air
used for combustion by the engine under test. The correction factor attempts to
predict the engine horsepower if the engine were tested at sea level under standard
pressure and temperature conditions.
Absolute barometric pressure is a measure of how hard the air molecules are being
pushed closer to one another. The unit of measurement is typically inches of mercury
(inches Hg). The more pressure, the more molecules there are in a liter of air and the
more air the engine “gobbles up” during the intake stroke. Absolute barometric
pressure is equal to relative barometric pressure only at sea level. Relative barometric
pressure is reported at airports and by weather barometers. A good approximation for
converting relative barometric pressure to absolute barometric pressure is:
AbsHg = RelHg - (Elev/1000)
Where:
AbsHg is Absolute barometric pressure.
RelHg is Relative barometric pressure.
Elev is test location elevation in feet above sea level.
Humidity is the percentage of a volume of air that is occupied by water vapor. Water
vapor displaces oxygen and reduces the amount of combustion air ingested during
the intake stroke.
Air temperature is the temperature of the air entering the intake system of the engine
under test. In some cases this is ambient air temperature, but in other cases the intake
air is significantly heated by the engine and is different than ambient air. Heat tends to
spread air molecules apart. So as temperature increases, there are less molecules in a
liter of air and less air is swallowed during the intake stroke.
Dynojet’s WinPEP 7 software uses the SAE’s latest correction formula (June 1990).
This formula assumes a mechanical efficiency of 85% and is much more accurate than
earlier formulas at extreme conditions.