Our car uses a Speed Density fueling method. There are three basic electronic fueling methods: Alpha-N, speed density, and mass air.
Here's a nice writeup:
EFI 101: Mass Air vs. Speed Density — What They Are & Which is Best for Your Ride - OnAllCylinders
Alpha-N just relies on the current RPM and the throttle opening, looks that up in a fixed table that tells it how much fuel to shoot. Simple, crude, but works fine most of the time. Importantly it's simple, so the processors in the late 70's-80's could handle it.
Speed density was the next that was developed. That adds in the density of the air, so the ECU has a better idea of precisely how much air was actually drawn into the engine. The VE - volumetric efficiency - tables tell the ECU how efficient the engine is as drawing in air under certain conditions. Combine that with the density of the air and you know how much oxygen is available to burn. Figure out your target AFR and you know how much fuel to shoot. The problem here is we're indirectly measuring the amount of air ingested. But, it's simple, and a MAP sensor is just a basic pressure sensor, so it's cheap.
MAF directly measures the amount of air - specifically, the mass of air - ingested. MAF sensors took a while to develop. There's a bit of magic in those little things. Now the ECU knows specifically how much air was drawn in and can make it go boom. This is more precise than speed density.
The problem with MAF is
all of the air has to go through the MAF. Have two air intakes? Need two MAFs. On a forced induction car this gets really complicated because of the bypasses. That air came into the intake track, but didn't go into the engine. A ton more work is now needed for the ECU to understand what's actually in the cylinder.
A speed density system doesn't care about the bypass - well, more specifically, the way it works automatically includes the bypass in its calculations, so it all works out just fine. Technically speaking it's less precise, but if your VE table is good enough, there's no practical difference. Developing a good VE table is a major PITA, which is why aftermarket EFI is such a crap shoot - all aftermarket EFI systems are speed density.
All speed density forced induction cars have two MAP sensors (or, at least,
should). The one forward of the throttle body tells the ECU the condition of the outside air (what altitude we're at) and what the turbos are churning out. The one behind the throttle body tells the ECU the actual pressure in the plenum, thus how much air is actually available to go into the cylinders. When you close the throttle, there's a sudden swing - the manifold goes into a vacuum and the pressure in the intake track spikes. That's when the bypasses open so the turbos don't chew themselves up. You also need an intake air temp sensor somewhere near or in the plenum because air temperature dramatically impacts air density. e.g., one cubit foot of air at 1 bar absolute at 100degrees F has significantly more oxygen atoms than one cubit foot of air at 1 bar absolute at 150degrees F.
