Originally Posted by feets
Woody, these engines have an air flow meter but I didn't see a manifold pressure sensor when I was looking through the engine electronics at work today. They're only on the supercharged engines by what I can tell.
There may be a barometric pressure sensor mounted in the engine compartment. That would not measure engine vacuum.
The air flow meter will measure air movement only, not density. It's nothing more than a heated wire with known resistance values. The faster the air moves across it, the more it cools off. That changes the resistance and the computer responds accordingly.
Low throttle opening and low air movement equals reduced fuel. High throttle opening means increasing air flow and more fuel.
***Maybe that will make sense to those not familiar with engine electronics.
I think you probably misunderstood my point. Yes, the system's primary method of metering is the airflow meter but that is taking density into account by measuring the mass of air passed, not just the volume - you can have a massive volume of air that has low mass which will require less fuel than a smaller volume of higher mass. Combined with the MAF is the IAT which is further compensating for density due to temperature.
The N/A cars do have a MAP sensor (see the pic of mine below) for determining engine load since throttle position is not necessarily an indication of airflow. At low revs you can open the throttle wide open but you won't have high airflow but you will have low MAP indicating high load. Similarly at cruising speed you may have moderate airflow but a small throttle opening and higher MAP indicating a low load condition. This is more important for ignition advance control than fuelling but the ECU does take it into account.
The map is the device with the green writing on it.
