The relationship between manifold absolute pressure (or vacuum) is not
directly related to airflow. Yu can low air flow and high vacuum, high
airflow and low vacuum, Low vacuum and low flow and low vacuum and high
flow. Systems that use MAP with airflow are using a different approach to
calculating fuel metering and are often known as speed density systems.
Speed density systems often use a throttle position encoder or sensor as the
MAP along with throttle position is needed to determine actual engine load
for fuel and ignition input.

Dennis

-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of
jon
Sent: Saturday, November 04, 2006 1:40 PM
To: vanagon@GERRY.VANAGON.COM
Subject: Re: AFM Voltage Ratios

One way to figure this out is to connect a MAP to the engine with a known
good AFM. Then, compare the values. Since it appears AFMs are linear, and
so are MAPs (at least the ones I looked at), a circuit could adjust a MAP
output to simulate an AFM.

Jon

On Sat, 4 Nov 2006, Geza Polony wrote:

> This is a question I've been asking for a long time, both here and on
> westfalia.org, and no one seems to know. You're right, there must be SOME
> concrete relationship between manifold pressure and AFM output voltage,
but
> no one seems to know what it is. The Bentley doesn't help, either. I took
> mine off and measured the voltage using a hair dryer to simulate the air
> flow, but that doesn't really tell you anything except that the voltage
does
> change, and pretty much evenly.
>
> Without knowing what the voltage should be for a given set of AFM
> conditions, it's impossible to calibrate the AFM. And you can bet that,
with
> most of these AFM's having 20-odd years on them, the voltage output "ain't
> what it used to be." That would mean that at some point in the scale of
air
> flow, the engines are running too rich or too lean.
>
> Unless there's something we're not getting. Why wouldn't the voltage data
be
> available?
>
> ??
>
> Geza
>
> On Fri, 3 Nov 2006 18:22:51 -0800, jon <jon@KENNEKE.COM> wrote:
>
> >Does anyone have data on AFM voltage (out of the resistive divider)
versus
> >manifold pressure? I would think they would directly correlate in some
> >linear fashion.
> >
> >Yes, I think of these types of things on a Friday night. ;)
> >
> >Jon
>