[Diy_efi] pic based afm -> maf conversion

Wed Jan 18 23:46:10 UTC 2006

"very similar info to what a  AFM would" ... but not the same.  If 
"close enough" tuning were OK, the AFM would still be there, right?

The output curves in the test rig described will have a certain 
relationship for one density of air - barometric pressure, temp, 
humidity - and another relationship altogether when the density 
changes.  The same number of CFM that max out the AFM's output could be 
so dense that the MAF is maxed out, too, or (at altitude, on a summer 
day) they could make for a much smaller (raw) reading from that same 
MAF.  That's the primary weakness in vane air flow meters - they measure 
volume, not mass.  Most L-Jet based systems have some way to determine 
barometric pressure and intake air temp - this last usually in the AFM, 
by the way - so they can derive the mass of the air.  Since the sensor 
will be giving a signal that relates to the air mass, a way must be 
found to remove the barometric pressure measurement (the only remaining 
measurement external to the now-absent AFM) from the computer's 
calculations.

A couple of thoughts:

If you remove the baro sensor (by making its value constant,) the 
computer will not - if it were going to - advance or retard the timing 
in response to weather conditions and/or altitude.  This could be no big 
deal - the computer might not do any futzing with the timing anyway 
based on baro readings.  It could be a problem manifested as lack of 
torque (high altitude) or pinging (low altitude.)  Probably nothing, but 
the only way out of it would be to remove the baro component from your 
MAF signal before supplying it to the ECM.  Any takers on this last one?

If the ECM is counting on air door overswing to provide acceleration 
enrichment, the engine may end up with a stumble on sudden throttle 
opening, perhaps necessitating the watching of the MAF or TPS for rapid 
increases.

In most of the systems I've dealt with, air temp has more influence on 
spark control than fuel.  Does your old AFM have the ACT/IAT sensor 
built into it?  IDK about your Bosch sensor, but there are MAFs out 
there that have ACT sensors as part of the package.  If it keeps your 
ECM from getting stupid and going into damage control mode, it would be 
worth the effort to include an air charge temp sensor.

TomV

Daniel R. Nicoson wrote:

> I may have missed something on this thread but I think the MAF vs MAP 
> debate is irrelevant in this case.  Ashley stated the change was from 
> an airflow meter (AFM) to a MAF.  Unless Ashley has control of the 
> code calculating resulting PW etc, MAP isn't an option.  Being a BMW 
> I'd bet he doesn't have control of the calculations inside the ECU.
>  
> The MAF will give the ECU very similar info to what a  AFM would.  
> Airflow moving the vane is a form of mass airflow measurement, just 
> not as precise as a MAF (probably a whole other debate...)  So the 
> signal out of his MAF=>PIC=>ECU will be pretty useful.  He might be 
> off by some temperature input from the airflow meter but he could 
> leave that temp sensor in place.  Honestly, you might not even need a 
> PIC. 
>  
> The reason I say MAP isn't a consideration, if we can't control the 
> ECU function, then the ECU won't make the proper correlation between 
> RPM, MAP & temp etc.  It is expecting a 0-5v signal "representing" 
> mass air flow, which the original sensor provided and now the MAF will 
> provide.  As already discussed the MAP does not provide the same info 
> as any form of mass airflow.
>  
> I think your mapping exercise could be done very simply (easy for me 
> to say).  Configure your AFM and MAF in the same inlet tract so the 
> air going through the MAF then goes through the AFM, log the output of 
> both devices as you drive (might have to do this on your work bench in 
> which case you need a big "sucker" to pull the air).  Now you have the 
> "equivalent" transfer functions.  If two crazy curves result, the PIC 
> can just do table lookups all day to get your ECU input correct.  If 
> the two curves are fairly similar in shape then you possibly could get 
> by with an analog circuit, opamps & resistors. 
>  
> What is the goal of this exercise other than to do it?
>  
> If you are truly running a LOT more air through the engine (30% or 
> more), will the current ECU calibration accept more air?  In other 
> words if you are maxing out the AFM transfer function, it won't matter 
> if the MAF signal represents more air, the "transfer function" in the 
> ECU will still be maxed out.
>  
> Someone suggested the Mustang MAF's.  I tune a 1994 5.0.  I can tell 
> you that those meters will support up to about 300-320 Hp with a 
> 0-5volt output.  These have some nice features in that the actual 
> sensor is in a recessed air passage in the MAF.  That is good because 
> it makes the MAF less sensitive to turbulence in the intake tract.  If 
> your motor is a smaller 4 cylinder and won't start to pull as much air 
> as a 5.0, leave the stock "screen" on this Ford MAF.  That screen 
> forces the air to be smooth going through the MAF for good accurate 
> readings.  If the your engine can only pull enough air for say 200 hp, 
> this screen won't hurt since you can flow almost 300 Hp of air through 
> the MAF w/screen.  In fact I just looked up, my stock 1994 Mustang 5.0 
> MAF give you 932 kg/hr of air at 5.0volts.
>  
> I will be watching how you fare with this project.  I have a 1998 BMW 
> 540 & a 1998 BMW 528.  The BMW world hasn't been hacked as fully as 
> the Ford and GM world.  I'm curious to see how far you can go 
> modifying your BMW.  On my Mustang I have the luxury of full control 
> of the fuel & spark tables, MAF function and a bunch of other stuff to 
> change since I use the TwEECer.  So I can let the Ford ECU do all the 
> work for me as I change parts.  I don't think you have that kind of 
> control on your BMW, makes tuning much more difficult. 
>  
> A couple other things:
>  
> Get over the idea that a MAF has more restriction than MAP.  Not true, 
> actually irrelevant again.  IF the MAF is properly sized to flow 
> enough air in a given application, then it will flow plenty of air.  
> Remember, you sized it for the application, so how could it restrict 
> you?  Yes, a larger MAF will flow more air than a smaller MAF if asked 
> to but if both MAF's flow enough for the given engine, the larger MAF 
> will never be asked to exceed the flow of the smaller.
>  
> The discussion below about reversion from wild cams and MAF 
> sensitivities to position in the intake tract are worth considering.  
> In this case it sounds like the engine is still pretty stock so 
> reversion shouldn't be a real big bear (BMW's generally like high RPM  
> though so there could be some significant cam overlap causing some 
> reversion).  The farther the MAF is from the intake valve, the less 
> likely this will be a problem.  Lots of Mustangs out there with pretty 
> wild cams running pretty good, don't be scared off by this one.
>  
> As far as sensitivity to upstream of the MAF, do it like the mid 
> 1990's Mustangs, use a "cone shaped paper filter" (from a Mustang of 
> course) and have your MAF pull the air in directly.  That way the air 
> is pretty smooth as it enters the MAF.  I use the "big" cone filter 
> from Ford's V-10 pickup engine on my 5.0.  My MAF sucks right out of 
> the filter, works great.
>  
> Cool project, good luck!
>  
> Dan Nicoson
>  
>
>     -----Original Message-----
>     *From:* diy_efi-bounces at diy-efi.org
>     [mailto:diy_efi-bounces at diy-efi.org]*On Behalf Of *Tom Visel
>     *Sent:* Wednesday, January 18, 2006 2:23 PM
>     *To:* diy_efi at diy-efi.org
>     *Subject:* Re: [Diy_efi] pic based afm -> maf conversion
>
>     Manifold pressure vs. RPM (speed-density) gives you a very handy
>     way of giving fine control over fueling in known areas where a MAF
>     interpreter would not.  MAF lets you play without paying as much,
>     but if the playing's over, the keenest edge will be found with a
>     speed density system tuned using a wide-band O2.  Either way, you
>     would have to eliminate or make constant the ECM's barometric
>     pressure input, as this would be compensated for twice otherwise -
>     once in your black box and once when the computer tries to figure
>     it into what it thinks is the VAF (vane air flow or volume air
>     flow) sensor's output.
>
>     BTW, the reason for the ubiquity of MAFs on modern vehicles is for
>     tighter emissions controls: to allow for the measurement of EGR
>     flow, and to allow for a MAP sensor as a redundant system and for
>     rationality checks to see if the MAF's calibration is drifting. 
>     Almost every vehicle has both a MAF and a MAP these days (at least
>     in North America.)
>
>     TomV
>
>> = 
>>
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