Ok, so Johannesburg at 1700m has 825hPa of air pressure.
Sea level has an average 1013hPa of pressure so that dicates that in Johannesburg you have 825/1013=0.8144 or 81.44% of sea level pressure. Following me here?
This drop in pressure means a NA engine in Johannesburg will have ~81.44% of the power it has at sea level.
A 125kw NA car will have no more than 101.77kw at the flywheel whilst in Johannesburg. A difference of 18.56%.
Now obviously these figures are for an NA engine not FI. FI ECUs will compensate for the thinner
air by increasing the boost to a degree but cannot cope with the absolute pressure completely so
will still be a couple % lower but not as much as the 18.58% for an NA engine.
This begs my question, why the 30% correction on the dyno?
I quote from Borg Warner
"The high-altitude performance of a turbocharged engine is significantly better. Because of the lower air pressure at high altitudes, the power loss of a naturally aspirated engine is considerable. In contrast, the performance of the turbine improves at altitude as a result of the greater pressure difference between the virtually constant pressure upstream of the turbine and the lower ambient pressure at outlet. The lower air density at the compressor inlet is largely equalized. Hence, the engine has barely any power loss."
Moving on:
As ambient correction entails the following :
1) Ambient Pressure
2) Ambient Temperature
3) Relative Humidity
And we know that Pressure correction should not be more than ~18% up here for NA, that leaves Temperature and Humidity.
Temperature needs to be adjusted to 25 degrees Celcius and will have an effect on performance but not nearly as much as Pressure.
Considering the mild Johannesburg climate recently to meet the SAE standard you'd almost have to be lowering the Temperature correction. In this regard a 10 degree celcius change with have ~3% influence on power. So let's say an average Temperature pf 25 degress celcius in Johannesburg, that equates to 0% ambient correction. 30 degrees? A 1% difference.
Humidity is the least effective on power, an increase in 20% relative humidity will equate to less than 1% power difference. The humidity in Joburg ranges from 47% to 60% a correction difference of
~1%. The SAE Standard dicates correction to 0% Humidity, so let's do that. Today's Humidity is 56%, that equates to 2% Ambient Correction.
So Today in Johannesburg any Dyno doing a power run should have adjusted to 20.58% Ambient Correction for NA engined cars. This is a Correction Factor of 1.267.
Ambient Correction for a Dyno needs to be adjusted constantly to get consistent results.
Now, as I understand it, and I'm open to correction, for Turbo applications we need to add the absolute manifold pressure into the equation to cater for Ambient Correction. Remember the higher the boost the lower the effect of altitude. Assuming no efficiency changes in the Turbo due to altitude, which we know is not the case, let's add 20PSI in the case of a Turbo. 20PSI is 1380hPa.
So 1380hPa + 1013hPa becomes 2393hPa absolute manifold pressure for Sea Level.
And 1380hPa + 825hPa becomes 2205hPa AMP for Joburg.
That becomes a Correction Factor of 2393/2205 = 1.085
Therefore the NA CF of 1.236/1.085 = 1.160 or a 16% OVER READ of the power using a realistic NA AC of 20.58%.
By my calculations the Ambient Correction for the Turbo 320i @20PSI should be 8%
Where does the 30% set Ambient Correction come from? Because today if you did a power run at 30% that Dyno would be over reading by 31.7%
Have I missed something here?
