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Why tuners need good O2 sensors and injectors with quality data.

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Your mass air transfer function is basically the amount of voltage (or frequency) the meter is outputting based off its measurements of the MASS of air that's flowing through it. In a perfect world you can flow your mass air transfer function and plug it in and go but we live in a not near perfect world. There are variables slight air leaks, meter drift, dirty meters that make your transfer function technically wrong, so basically your transfer function is never ever perfect. The amount of fuel the engine needs is directly connected the amount of air entering that engine; engine displacement has ZERO effect on the amount fuel needed for a certain mass of air.

When we tune in open loop the computer has a set of "look up" tables that says this is the air fuel I want or if in closed loop it uses the O2 sensors to trim the fuel to stoich (lambda 1. or roughly 14.7:1 air fuel in pure gas) so in simple terms the ecm is always targeting a particular air fuel value.

It takes this targeted value and based on the amount of air, calculates the amount of fuel it needs to inject. The amount of fuel needed calculation depends on good injector data. (You didn't buy no name used injectors without any way to know their flow rates both high and low slopes did you? See how this is all interconnected yet?)

So in essence there are only 2 things controlling the fueling, the injector size and mass air meter.

But did you ever wonder why tuners NEED good O2 sensor data if it doesn't control the fuel? We know that amount of fuel to inject is basically a calculation based off 2 things, targeted a/f and the mass of air being ingested. How do we measure the outcome of the calculation...the O2 data!

If we have good injector data (from quality known injectors), good O2 sensor data (from new O2 sensors), the only thing that can really be off is the amount of air the computer thinks is entering the engine, therefore we adjust the transfer function in the computer to make the fueling correct which in turn corrects the transfer function that was obviously off because we weren't getting the correct measurement on the O2's.

Now that is why we adjust the mass air transfer function from that point on technically your transfer function should be correct for that intake tract regardless of cams or whatever, but there is way more to tuning than dialing the transfer function.