replied to the thread HOWTO - COBB ATR discussions, maps
" The reason why BMW uses load is because as the ambient air density changes (due to temperature and pressure), they want the car to perform the same. They want the engine to make 300 HP in Jacksonville and in Denver.
If you look at the formula above (which I suspect if for an NA engine - see below), you see the calculated load is expressed as the ratio of air currently moving into the engine over the air that could be moving into the engine at at a given RPM, so it's a percent which has been referenced to air flow at STP (standard temperature and pressure). STP is usually referenced to 0C. The STP referred to above is actually SATP(Standard Ambient Temperature and Pressure), but I digress.
The point is in an NA engine load will be 100% (1.00) when the current air flow is equal to the maximum possible airflow through the engine based on the air density and the engine's VE (airflow as a function of RPM as expressed above). For an FI engine load can be above 100% because the peak airflow is referenced to 29.92 in Hg absolute pressure (14.7 psi or 1 atm). The whole point of an FI engine is to raise the charge air pressure above 1 atm.
Lets put some numbers into the formula above. In Jacksonville, lets say the ambient pressure is 29.92 in Hg and the ambient temp is 25°C. Also for the sake of this example, lest say the maximum air volume is 100 cfm at red line. If we evaluate the bottom of the equation, we get a maximum possible airflow at peak RPM of 100 cfm. So if we move 100 cfm through this engine at peak RPM, the load will be 1 (100%).
Let say we are in Denver at 25C and 28.92 in Hg. Evaluating the denominator give us a maximum possible air flow of 96.7 cfm at red line (assuming 100 cfm at red line at SATP) . So if our current airflow is 96.7 and the engine is at red line, the load is still 1 because the ambient pressure and thus the air density has dropped. "
Today, 10:20 AM