Updated: May 23, 2020
Hot wire sensors, also referred to as MAF (mass air flow) sensors are arguably one of the best load sensors available, when set up correctly.
The problem and strength of the MAF is that they are very sensitive. Much like inlet ports in your cylinder head, the air that goes through your air filter and then swirls and deflects through your air box then, in turn, flows through your MAF in a certain manner depending on a whole lot of factors. When dyno tuned, the tune is calibrated to the readings from your MAF via a VQ map, but if you then change anything forward of your MAF e.g air box, intake duct, air filter, this will in turn change the flow characteristics through the MAF which will offset your tune.
Within ECU software there is a MAP called the VQ (Voltage Quantifier) map. The function of this map is to correlate the MAF output voltage to the 'mass' of air flowing through the sensor. Why not just use the MAF voltage; because it's not scaled right. The MAF voltage needs to be scaled so that the 'mass' variable (a number) in the software is scaled lineally as a mass whereas the MAF voltage is more arbitrary e.g. not linear to mass e.g. 2v output in not double the air mass of 1v output.
The Nissan enthusiasts have, for 10 years+ now upgraded their MAF sensors to larger diameter higher volume MAFs e.g. the 300zx ones. When doing this the swap the VQ maps in the Nissan ECU's. This is/was sometimes essential as the smaller/lower volume MAF meters often ran out of range which meant the vehicle could not be reliably tuned at the top end. To put it another way, a MAF meter has a mass flow ceiling. This may be equivalent to 400hp of air flow mass, so if you tune beyond 400hp, you will run out of scaling meaning you will not know how far beyond 400hp of air mass you are flowing. So it's like running 3 bar of boost using a 2 bar MAP sensor.
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