I figured I'd post this here since the question can probably be better answered by some of you guys who have been tinkering with the 165 ECM pretty heavily.

Just how accurately can a MAF estimate air being consumed by an engine (assume stock Bosch MAF w/ screens)? I'm certain it has to have some variability (the algorithms in the ECM would have a large part to do with this I would imagine), but would this variability be linear?

For example, suppose its off by +-5% at 1500 RPM. Would it be off by +-5% at 4000 RPM?

It crossed my mind to use a MAF sensor to measure incoming air, and use that to calculate actual engine VE. I know the VE tables in the 730 aren't necessarily representative of real life VE, but I'm curious to see how far/close the relation between the two would be.

 

The resolution is the lowest at low voltages (idle) due to the 8 bit A/D. Id say around +/- 4% would be typical. The resolution gets much better as the voltage singnal increases.

The VE calculated from the MAF should be fairly accurate as long as the MAF calibration is good . Ford uses something similar to calculate pressure referenced items like the baro just from the MAF readings, and, according to the repair info, it should be exactly what the actual baro is when everything is working properly.

 

The Delphi paper here -
http://delphi.com/pdf/techpapers/2000-01-0546.pdf

discusses factors that affect the accuracy of MAF flow-measurement
designs, including the shape of the transfer function and A/D converter
resolution.

While the relative MAF accuracy is best at high flow, absolute meter
resolution is best at minimum flow.


HTH

 

Ive been pondering this for a long time now but didnt want to sound stupid.

What about doing a 730' MAF/MAP hybrid like your TBI Dimented. Has this been covered?

 

Knowing the air flow is to some degree just academic.
It's load that dictates optimum fuel, and timing.
Hence, the LV8 calculations.

If one was datlogging it, then there might be some interesting data found.

The only real downside to a MAP/MAF, that I see, it that it probably make tuning even more complex. Well, depending on how good of job you actually want to do. If you want to use the over reporting air error for AE, you can, but is that the sort of thing you really want to do?. If you're looking for accuracy, which is the only reason to use both, then IMO, you don't want to have all sorts of fudging in the system.

 

If you want to calculate actual VE I think it would be better to start with a wide band than a MAF sensor. If we assume that a MAF sensor can provide accurate engine airflow you will still have to divide by RPM and numcyl to get single cycle cylinder flow. This means you need a matched pair of MAF and RPM. I think latentcies might drive you nuts.

If you start with a delivered A/F ratio from a wide band you can get closer. You should know what the lbs/hr of your injectors are. Convert this to grams/sec. Multiply grams per second and pulse width in seconds. For batch fire multiply by two since there are two events per cycle. Now you have single cylinder fuel in grams. Multiply fuel in grams by A/F ratio from wide band and get single cylinder air in grams.

To get mass air flow multiply single cylinder air in grams times RPM/2 and numcyl. This will be about as close as you will get to actual airflow.

For VE calculate theoretical single cylinder air using the ideal gas law. Then divide actual by theorectical.

Error sources are wide band A/F ratio, injector constant, and conversion calculations. So the answer you get will probably be a close approximation.

HTH

John

 

I wasn't expecting this many replies... thanks for chiming in.

I went ahead and saved that paper, I skimmed through it for now. Looks like an interesting read, but its going to take me a while to read it, as it seems like it requires some thinking to follow it.

I suppose it would be easier, and less of a hassle to set up a wideband. However, a wideband wouldn't be too useful at idle with a moderate cam. Basically, this was an idea that just sort of developed without a meaningful purpose. More of a curiousity thing than an actual tuning aid, although the thought has crossed my mind. I don't have much confidence in my lower VE tables at or near idle. Seems like I can play with them all day without noticing much of a difference in the plugs or how the engine runs.

I'm going to be converting from SD to MAF in the not too distant future. I want to tinker some with the 165, and my choices would be to either buy an IROC or convert mine. So guess what? I'm converting mine. I just might setup a MAF while I'm still running SD just for kicks and see what happens. I know some of you guys would be interested in seeing that.

I've been having an impossible time getting my header collectors to seal. Collectors sit flush against a straight edge, Y pipe sits flush against the header on both sides, yet I can only get them sealed for a few days at a time (each time I change gaskets). I've tried just about every gasket out there, and recently threw in a ball-joint looking thing (not sure what its called) between the collector and y pipe. Helped, but leak still comes back. I'm going to try going with a thicker y -pipe flange, since I've noticed that the gaskets are getting eaten halfway between bolts, and the flang on the y pipe is slightly bent (I understand this is normal though).

Because of this, I am always worried that the leak is present and I haven't noticed it yet. It would be nice to have something other than O2 voltage to analyze. A wideband wouldn't help much with a leak present.

By the way, is having injectors flow tested the only way to know the actual injector flow rate?

 

The LV8 calcs. are primarily just to provide a pseudo-map for the load based items since there is no map sensor present. It would be undesirable to use it for fueling since its basically just using the MAF to approximate a MAP. Using the actual airfow from the MAF is better, since the end result of both MAF and SD calcs is to get the mass of air in some form. If you already know the mass of the air, then you might as well go ahead and use it. The only difference between SD and MAF is that SD estimates the mass of air in each cylinder while MAF estimates the flowrate and needed injector duty cycle. As long as the MAF table is correct, youll get a fairly accurate report across a wide range of temperatures.