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Mass Flow vs Air Flow - Confusion after watching the video tutorial

EFI Tuning Fundamentals

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I'm hoping you can clear something up for me as I have now watched the video on Mass Flow vs Air Flow twice however I am still confused and I need to make sure I get this right so I can understand the course in full

You start by saying most factory cars measure Air Flow using a Mass Air flow meter. That bit I understand and have no issue with.

However you then go on to say that a stand alone ECU calculates Mass air flow using the speed density principle. For this equation though you use the CFM from the previous VE tutorial where the actual CFM was 572. But this CFM was measured using a air flow meter in the VE tutorial. So where does the CFM come from on a stand alone ECU which generally use a MAP sensor and not a air flow meter?

I am hoping that someone from HP Academy can answer this as when signing up to the course it was advertised that questions to the course would be answered through the forum however on my last questions that was generated from a tutorial video no one from HP Academy replied. Although several users replied with some great useful information there was no direct response to my question.

Thanks in advance,


Hi Ian, I simply used the volume airflow as an example to show how mass flow can be calculated. In the case of a speed-density based ECU we aren't measuring this at all, but rather the mass airflow is the result of the ECU's calculation. Also note that 572 CFM is the volume flow, not mass flow. This can be calculated theoretically by taking into account the engine capacity and engine rpm. You'll see a more thorough work through of how the speed density principle works further through the course.

I may be misinterpreting this but...

A conventional air flow meter does two things, estimate the airflow velocity through a known orifice area and the temperature to estimate the actual volume and mass of air into the manifold and hence the fuelling required.

A MAP uses the engine rpm (demand), absolute pressure in the manifold, and temperature to estimate the mass of air entering the manifold through the restriction of the throttle.

Is part of your problem reconciling CFM against the mass of the air, as it will vary with temperature?

Rather than thinking of CFM (or cubic meters/minute or second) we simple think of pounds or kilograms per minute, or second? It may also help if one thinks simply of the efficiency of cylinder filling in mass terms rather than 'volumetric efficiency' as, IMO, that's bastardisation term that should be reserved for comparing the cylinder filling percentage compared to the potential directly available to it in volumetric, rather than mass, terms - it seems to be confusing a lot of people who have trouble understanding the pressure-temperature relationship for the air mass filling the cylinder.

I may be on my own there, though ;-)

OK so think I got the wrong end of the stick and thought you were actually showing how the ECU works out the air via the speed density but maybe you were not. I'll re watch the video and then move forwards as like you say if its explained further in the tutorials then I'm sure more will become clear then.

Thanks both for the response.

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