MAF to MAP conversion

A MAF measures the mass of the incoming air directly and to replace a MAF system you need to substitute with a Speed Density system which must estimate the mass air flow based on MAP and IAT.

Speed Density uses RPM, MAP, and IAT to estimate air flow.  

So who is good at physics?

If I want to calculate MASS of air flow then what calculation do I do?

I have PRESSURE, TEMERATURE and RPM as available inputs to the equation. I guess we need the MASS of an air molecule and the volume of the engine?

Ummm... getting woolier by the minute..... then I guess every rotation of the engine we have shifted a volume of 3lt (assuming 100% efficiency). Knowing the "gas laws" we can work out what the MASS of the air entering the engine was given the pressure at the manifold and "inlet" temperature.

Is it then just a case of scaling this figure to match the frequency or voltage range of the original MAF?

Regards

Pete

Key:-----------------------

IAT: intake air temperature, which measures the temperature of the air the engine is about to inhale

SD: Speed Density

MAF: Mass Air Flow

MAP: Manifold Air Pressure

RPM: Revolutions Per Minute

The IAT is also used to control ignition advance.  

** Cold inlet air ***

helps the engine resist detonation, so the engine can tolerate a bit more advance (the cold air tends to condense the fuel mist from the injectors back into liquid fuel, which takes longer to burn).  

*** Very hot inlet air ***

is more likely to cause the engine to detonate, so less advance must be run to avoid that destructive detonation

For a normally aspirated 4-valves per cylinder motor, you assume a VE (Volumetric Efficiency of 80%).

You can pick your units of measurement but I like to think of flow as XX cubic feet per minute, i.e. CFM. Which means first you have to convert the engine displacement into cubic feet.

Which is: ( CC x 0.061 ) / 1728

For a turbo engine you simply multiply the flow figure by the pressure ratio after adjusting for compressor and intercooler efficiency.

Basic N/A formula assuming 100% VE  is:

displacement in cubic feet * RPM divided by 2 (as it's a 4-stroke engine)

Then times by 0.8 to give figure at 80% VE

Punch the relevant formulas into a computer and you should easily be able to produce a graph of air-flow - versus - RPM.

Yours,

J

Bingo!

Cheers

Pete

You can probably see now that by knowing your boost curve; the efficiency of your turbos and intercooler; the capacity and VE of your engine; and the RPM... you can gauge how many CFM the engine is flowing.

That's all a MAF sensor does, i.e. gauge how many CFM's of air the motor is flowing. So to replace it, you need a piece of gubbins that takes in manifold pressure and RPM and processes these signals to produce a composite "air flow" signal that you then feed into the ECU instead of the MAF signal.

You can estimate it by assuming turbo and intercooler efficiency of 70%. Now stick it all into a computer and plot a graph of flow - versus - manifold pressure and Bob's your uncle.

A good way for the MKIV would be to data-log the MAF, Manifold Pressure & RPM of a UK car. All you need then is to program your box to produce the same output voltage as the MAF sensor under the same conditions of manifold pressure and RPM. You'd no-doubt have to have a bit of in-built adjustability if you were trying to make a "universal fit" thingy.

Also, I think temperature is something you will not have to consider as that should already be taken care of. Unless, of course, you were thinking of designing a complete ECU.

Yours,

J  

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So to replace it, you need a piece of gubbins that takes in manifold pressure and RPM and processes these signals to produce a composite "air flow" signal that you then feed into the ECU instead of the MAF signal.

You can estimate it by assuming turbo and intercooler efficiency of 70%. Now stick it all into a computer and plot a graph of flow - versus - manifold pressure and Bob's your uncle.

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Precisely what I needed to know. The data logging idea is also a good check of the theory.

I think this is what the HKS VPC thing does isn't it?

Thanks

Pete

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Also, I think temperature is something you will not have to consider as that should already be taken care of.

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Isn't the standard air temp sensor in the MAF housing so I would have to take care of it wouldn't I?

Pete

I don't know where the IAT sensor is situated on the UK car. As for the HKS gubbins, the VPC is some kind of adjustable unit that can be used to replace the MAF sensor on the UK car. But this unit will need mapping and all that jazz. Have never fitted a VPC so I can't give you any info.

I don't like 99% of the commercial bolt-on stuff, particularly when it comes to fuelling. I'm more dedicated to designing my own stuff these days, after having had so many let downs.

Yours,

J

A lot of this could be related to my ECU post. If the J Spec can be converted Pete then a VPC could be used. The VPC can be bought in various specs, including injector sizes, then the SAFC can be used to trim. I know what James is saying about "bolt on " bits but the cost would be significantly less, & the thought of rewiring a Supra ECU.......

If you want a Guinea Pig Pete, I'll order up a VPC?