[Diy_efi] Q regarding placement of a pressure sensor

[Perry Harrington]

On Mon, Jul 15, 2002 at 10:31:52AM -0700, Bob Moon wrote:
> Hello all,
> 
>   I am contemplating the source location for the
> airflow signal going to the ECU in my Laser.  It
> currently has a Karmann vortex MAF sensor that is
> quite restrictive, and expensive to tune (plus, it's
> more fun to figure out how to do it myself).  There
> are several piggy-back computers that modify the pulse
> rate from the sensor to tune the fuel curve, but the
> devices that remove the MAF sensor altogether are few
> and expensive.

This is not a MAF sensor, it's an ultrasonic VAF sensor.
It's an overly complex way of measuring air flow.  They
pass an ultrasonic signal through a vortex of air and
measure it's effects as a variable frequency signal to the
ECU.  Mitsubishi and Saab have used this tech.

> 
>   I am thinking about converting from the MAF to a MAP
> sensor arrangement.  Typically, the MAP is routed to
> the intake post-throttle body, providing readings from
> -15 psi to +30 psi or so.  However, it seems to me
> that the mass of the air moving through the system
> will be the same before the turbo as after.  

If you're adventurous you might consider a MegaSquirt, it's
MAP based and goes to 21.5PSI absolute with the turbo
sensor.  You can hack it for a 3bar sensor.

> 
>   This leads to my question:  why not run a pressure
> xducer before the turbo (say, in the stock MAF
> location) and just read the vacuum that is created as
> the turbo/engine inhales the air?  This would provide
> a reading from -15 to 0 psi (the further negative, the
> more air is flowing) reading, and I could then convert
> the voltage to a square wave for the ECU to interpret.
>  Is this reading not accurate enough?  The only reason
> I can see would be that the signal wouldn't be
> consistent, but it seems to me that an airflow
> consistent enough to make a Karmann meter work would
> also provide a consistent, repeatable xducer signal.

The method you suggest is how they calibrate MAF sensors.
I don't know the exact equations, but you measure the
vacuum across a port of X size in a pipe of Y diameter.

The drop is directly proportional to flow.  They use this
method because it does not introduce any turbulence in
the process.

> 
> Thanks,
> 
> Bob Moon
> 

--Perry

-- 
Perry Harrington			Data Acquisition & Instrumentation, Inc	
perry at dainst dot com					 http://www.dainst.com/

Those who would give up essential liberty to purchase a little temporary safety
deserve neither liberty or safety. Nor, are they likely to end up with either.
                             -- Benjamin Franklin

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