While getting my bike dyno'd today (after some engine work) in 100 degree weather I began to wonder of the complications of tuning in such heat. Below is a brief explanation off the net of propper calibration however I don't know if my tuner's dyno uses this (any) method as they are closed now. My question is, do all tuners/dyno's use a formula to compensate for atmospheric conditions? If not, what are the ramifications (other than altitude change) for not doing so? Thanks, Roger
Correction Factor
The calculation of horsepower or the accuracy of our dynamometer is not dependent on the location or conditions during the measurement. The performance of the internal combustion engine is, however, 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 engine horsepower measurements for differences in operating conditions during engine testing. The typical correction factor (CF) is calculated based on the absolute barometric pressure, air temperature and water content of the air used for combustion by the engine under test. It attempts to predict the horsepower that would be developed 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 = Absolute barometric pressure
RelHg = Relative barometric pressure
Elev = test location elevation in feet above sea level
Water content is calculated from the ambient wet and dry bulb temperatures. Dry bulb temperature is normal room temperature. Wet bulb temperature is always less than or equal to dry bulb temperature. As air is blown over the wet bulb thermometer the water evaporates and cools the thermometer. The dryer the air, the cooler the wet thermometer indicates. If the ambient air is saturated (humidity = 100%), very little water evaporates and the wet bulb temperature is equal to the dry bulb temperature. These measurements are then converted to partial pressure in inches of mercury and used in the correction formula. 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 (Performance Evaluation Program for Windows 95) 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.
The formula used is:
CF= 1.18 x (29.22/Bdo) x To+460 / 537) - 0.18
Where:
Bdo = Dry ambient absolute barometric pressure
To = Intake air temperature in degrees F
Understanding your motor .
The simplest way to look at the internal combustion engine and understand what is done to increase performance is to see it as an air pump. "The more air it is capable of pumping at any given time, the more power it will make" { To make it plan and simple} (expressed as Volumetric Efficiency or VE). To get the most energy out of that air relies on accurate metering of fuel to that air (expressed as stichomythic, an AFR of 14.7:1 is the theoretical ratio that results in the greatest release of available energy in gasoline). Real world inefficiencies result in an air/fuel ratio (AFR) of 12.8:1 to 13.3:1, for best power and 14.5:1, for best mileage.
The VE, AFR and tuning requirements constantly vary as determined by the mechanical and dynamic properties of the motor. The main mechanical parts are typically engine size (volume), compression ratio/cylinder pressure (static, corrected and dynamic), cam timing/lift, port/valve flow capacity (cfm, including the induction system), exhaust and NOS/super/turbo charging.
The exhaust system is the largest, single, variable influence on how the motor processes air, the shape of the VE graph and tuning requirements. The dynamic parts are typically RPM, throttle position and load (including wind, tire pressure, terrain, gearing and luggage).
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Topic: Tuning compensation for atmospheric conditions (Read 1281 times)