Converting vane type airflow meter to MAF

[Mike]

>I just joined this list a couple days ago so I applogize if this is a 
>repeat question.  I am trying to determine the feasibility of 
>converting the vane type airflow meter in my 88 BMW 535i to a MAF 
>system.  My main reasons for doing this are the increased airflow to 
>the engine (more HP) and the reliability (no moving parts).  My 
>questions are:
>
>1) Are my assumptions above correct?
>
>2) How difficult is this conversion to do?  I am an embedded systems 
>   programmer so the software isn't really a problem.  What I really 
>   need to know is how complex is the conversion algorithm?  Do I 
>   need to add a MAP or a trottle position sensor?
>
>3) Are there any books you can recommend for this type of project?
>
>4) Is there is good "hobbyist" MAF out there that will fit well to 
>   this engine (3.5l ~6600 RPM ~220 HP).  I can deal with voltage or 
>   pulse output.  I was thinking of using a sensor from a 5.0 l
>   Mustang since it would be easy to find a replacement if necessary.
>
>Thanks.
>
>Brian Franchuk
>
>

I did a VAF ("volume air flow" which uses a moving-core measureing device
similar to a vane) to MAF conversion on my Probe GT using a PIC16C71, a
12-bit Linear Tech DAC and a Ford 55mm MAF from a 1992 Taurus.

In the case of the 2.5L V6 in my PGT, I can't say I saw a large increase in
power though the effect is noticeable - maybe 5HP. However, 55mm is quite
small and I'll be trying a 70mm or 73mm MAF in the spring. These are
available from Vortech and whatnot for about $190US new. I'm not sure if an
otherwise stock 2.5L V6 of 164HP flows enough air even at 7000RPM to see
55mm as a restriction but 3.5L @ 220HP might.

I didn't add any additional sensors to the PGT though I might add an IAT
correction to the conversion this spring. I characterized the setup at about
20oC ambient but I find the car runs rich when it's cold out and maybe a tad
lean when it's hot. I have a little pot on the controller I can use to
adjust the offset a tad rich or lean but it'd be nice to have this done
automagically by sensing IAT and using that to correct the mass-to-volume
conversion.

Check out my web site at http://www.golden.net/~trinity and find the MAF
conversion page. It's quite cheesy and more of an outline to what I did than
a technical treatise but it does describe how I characterized the setup by
placing both sensors in series, datalogging both simultaneously and creating
a look-up table from the resulting data. I may have to re-do this when I
switch to a 70mm unit.

I'd go with the Ford unit though I'd look into a 70mm Vortech unit first. I
think the garden-variety 5.0L used a 55mm (or so) MAF and you may want
something larger with a motor as you describe. GM sensors aren't too bad
airflow wise but I found their 30-150Hz output too slow for high-speed
conversion using my PIC-box. I wanted the linear output of the Ford MAF for
this reason. I also like the design airflow-wise of the Ford unit over the
GM for a given package size.

I think some late-80s 300ZXs also came with a very-nice looking MAF (I saw
such a MAF in a wreckers once and asked where it came from and that's what
he told me...300ZX). It looked like it'd flow Hurricane Andrew though I know
nothing of the output characteristics.


--
 Mike