Most voltage output MAFs are non-linear presumably to increase the resolution around idle of a low resolution ADC in a noisy environment. The quantisation error is pretty high at say 1% of full scale deflection even with a 10 bit ADC.

 

Right, but I was just wondering about the full capabilities of the sensor.

I think the actual response from the hot wire naturally produces that signal though as the cooling rate on the wire is very non-linear.

 

so is it the physical size of the housing maf's sit in thats the problem i.e bore diameter size ?
or far more advanced electronic stuff?
sorry i'm not at all up with tech stuff

 

No, The HPX is not programmable. It is something we have right now, not the project we are working on, and yes, it's a 9th order for the transfer function.

 

100% correct.

 

One thing I have confirmed on my travels through the MAF code recently is that the load is proportional to MAF pulses between CAS interrupts, with some of the load values being compensated by air temp and/or baro also.

What may not be immediately obvious, and has implications, is that load is NOT compensated by the MAF scaling, smoothing or baro/IAT vs RPM tables. These tables only work on the IPW calculation.

The implications are that the load used to lookup your tables is not adjustable, it will land where it lands based on MAF pulses per CAS. If your MAF pulses from your replacement MAF or simulator box are approximately linear with the airflow then your fuel and timing tables will still look OK.

This does also suggest that we can't properly compensate the load used to lookup ignition timing if we mess it up using an induction kit. We can make a global adjustment using MAF size, but we can't change the curve using the MAF scaling table for anything except adjusting IPW. So on the stock MAF using an induction kit we carry on as normal and just remap the ignition timing in the cells it actually hits rather than attempt to correct it to hit the "right" cells.

 

That's good info to have. Thanks for sharing.

When I was doing my testing on the maf scaling table way back, I knew it ws used to adjust the IPW, but never even thought about whether it affected the load or not. As you mentioned, this definitely has implications on tuning load based tables.


Eric

 

Is "MAF Size" the only adjustable variable that affects LOAD then?

 

Yes.

 

You might want to look for some type of displacement scalar. Most other ecm stratagies using mass air use engine displacement to calculate load. Obviously as load is Theroetical engine airflow max/actual engine airflow (maf reading).. plus a few other things.

If this scalar is there then correcting load would be easy as you would simply cut the displacement by the same amount the maf was scaled.

 

"MAF size" would appear be a combination of MAF size and engine displacement. The other multipliers and dividers are usually bit shifts or round values like 0x80 or 0x4000. There isn't anything else surprising in there - I've gone through the entire routines from MAF pulse to load.

 

Bringing this back up because of a value in JCSBanks SD patch and because I just like the idea of an additional MAF sensor setup that everybody can use.

Is there an equation to go from MAF Frequency, Intake Airtemp and Barometric Pressure (along with any lookup tables) to mass airflow rate (lb/min or g/sec)?

 

I think Eric and Aaron worked out something before.

It should be proportional to:
MAF Hz * Baro / absolute IAT * MAF scaling (with offset) * MAF smoothing * baroIATlookup

As soon as you try it on a different induction system (or one other than stock and even then you have to trust Mitsi's MAF calibration) you'll get errors, at low flow they will compounded because mainly the MAF scaling will also be wrong, also the MAF smoothing and the baroIATlookup. There seem to be some quite wide variations between near stock engines and their AFRs which would suggest that this would be an unreliable method, but more a numerical reference you can use on that individual car to compare in future, but then temp and baro comp load does that already.

I would be more inclined to calcuate airflow based on IPW, injector size and WB AFR, although it won't work during acceleration enrichment, knock enrichment, cold or afterstart enrichment.

Other option is compensated (by baro and IAT) load, but it is not proportional to air charge per cylinder event at low flows because the MAF scaling (with offset) * MAF smoothing * baroIATlookup is not applied to load. The effect though is small at high flows, and the main item that affects it is MAF scaling (with offset).

 

jcs, reading your post #81, re the point in the airflow calculation which is used to lookup the fuel and timing maps (only partially compensated) makes me inclined to think we would be better-off using % as load scaling instead of g/s.
ie airflow has not been fully factored/calculated/compensated so the units are a bit arbitary.
Or is that just silly thinking?

 

Yes the units for load are arbitary.