Actually, that's not entirely correct.

A MAP sensor is nothing but a pressure sensor. It can do nothing but measure manifold pressure. An ECU that relies solely on a MAP sensor (speed density) assumes that a given manifold pressure equates to a given air mass that is fueled via a user-defined map. If a significant change is made to something that changes the VE (e.g. exhaust system), the ECU's assumptions will become incorrect, and the system will need to be recalibrated because the ECU has no means of 'knowing' about the change without user defined input.

A MAF sensor measures the volume of air, nothing more. This reading is converted to actual air mass with the help of IAT and BAP. This allows the ECU to detect and adjust for changes in VE without the need for user input. This is a more expensive system, but it is used because it adapts to changes that are anticipated for an engine that is expected to run efficiently and cleanly over the course of several years and 150-200k+ miles.

 

He explained it more technically for you guys

Point is, they are both used to do the same thing. You don't need them both.

 

Just to be picky that's true for the Evo because what we've been calling a MAF is really a VAF (volume air flow) meter. A hot-wire MAF really measures mass flow.

Dave

 

so ... coming from the ideal gas law of PV = nRT and we ultimatley want to know nR (mass of the gas) ... we have a sensor for P (MAP sensor for barimetric pressure), V (MAF sensor), and T (intake air temp. sensor) we can find nR?

Does that about sum it up?

j.

 

I think the reason he assumes MAP is still needed is because there is no ECU sensor measuring the boost pressure AFTER the compressor outlet -- which is a little counterintuitive -- it begs the question, how does the ECU know what the boost is if the ECU is not even looking at the post-compressor boost pressure?

What must be realized is that there is a direct relationship between boost pressure AFTER the compressor and volume of air being drawn in BY the compressor. The mathematical relationship is predictable and well-known and so the stock ECU can effectively measure boost by looking just at this latter air volume.

To be technical it's not really measuring boost but just calculating the value of a load variable which is then used as an index into fuel & ignition maps.

EDIT: jcnel gave a better explanation above, sorry I missed it.

 

'R' is a constant, so it's always known and never changes.

'n' is the number of moles (air molecules) present, and is ultimately what you need to know if you're going to accurately meter fuel. Therefore, with volume from the MAF, temperature from the IAT, and pressure from a BAP or MAP, you can solve the equation necessary to run a mass air system.

 

So that pretty much closes the loop concerning how to calculate the mass of air needed to run our EVO ...

The MAP sensor does not need to be high speed, nor probably as accurate as others on the market ... since the MAF meter is running outside of the compressor, and in ambiet air pressure, the MAP really is just looking for an ambiet air pressure reading.

jcnel.