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The Mass Air Flow meter is designed to measure the amount of air coming into the engine. It is among the most important sensors on the engine.

Before we begin you must note that the ONLY way to properly change your MAF or your injectors to support more power is to have a computer that's properly calibrated for the output of your new meter and the flow characteristics of your injectors.

Using a table known as the "MAF transfer function", the computer knows that X voltage out of the MAF means Y amount of air and it adjusts fuel and spark delivery accordingly. The critical part is the relationship between X and Y. The most interesting part is that this relationship is not necessarily linear.

When you add more air without increasing voltage (e.g. by putting your sensor in a bigger body), you completely screw up the relationship between the two. If you add air without increasing voltage, the MAF transfer function programmed into the PCM no longer reflects the reality of how much air is actually entering the engine. As a result, you get a lean condition due to the extra air coming in without the PCM "knowing" about it or adding the extra needed fuel. If you increase voltage without airflow, you get the opposite. Eventually you will get a lean (or rich) code because the oxygen sensors are able to detect that the mixture is not what is expected while driving, and the PCM adjusts fuel delivery based on this O2 sensor feedback. This doesn't apply for WOT (where accurate air:fuel mixtures are most critical) because the PCM runs in "open loop" mode without O2 sensor feedback at full throttle.

There are lots of companies out there who claim that their meter is "calibrated" for a certain injector size. This is basically horse hockey. What they've done is pair a set of fuel injectors with a MAF and housing that flow more fuel and report less air for the same injector duty cycle and MAF voltage, respectively. This means the PCM thinks less air is entering the engine than there actually is (running lean) but actually delivering more fuel than it thinks (running rich). Because the percentage of airflow under-reporting is the same as the percentage of fuel over-delivery, the fuel mixture stays about the same.

The problem is that spark delivery also depends on an accurate airflow calculation via the MAF transfer function. If there is more air actually entering the engine than the PCM computes via the MAF voltage and transfer function, it thinks cylinder pressures are lower than they actually are and is giving the engine too much spark advance - which causes detonation.

How do you know if you MAF is working correctly?

The voltage at terminals A and B with engine on should be 10.5V
The voltage between C and D should be .34 - 1.96 V at idle.
 
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