<VV> Re: MAP sensor - utlimately for a Corvair

[burkhard at rochester.rr.com]

Chuck-

The MAP sensor measures absolute pressure, not gauge pressure -- MAP 
means a Manifold Absolute Pressure.

They are all (at least all the variants I'm aware of) 5V sensors.  
There will be 3 wires total on the connector.  The sensor is fed a 
regulated 5.00 V (and it's pretty good) supply voltage from the ECM, 
along with a sensor ground (not directly engine ground, chassis ground, 
or any other ground) and they send back 1 signal to the ECM.  The 
sensor itself is ratiometric (the output varies depending on the input) 
so the pressure is calculated this way by the ECM code as well.  By 
this I mean that a returned value of 4.00 V is not looked up directly, 
but rather the ratio of the sensor signal to the reference voltage 
4.00/5.00 is the basis of the calculation. In this manner, if the 
reference voltage varies slightly (say 5.05V), it doesn't affect the 
ultimate reading.  So, the transfer function will look something like 
this:

Pressure = %Vref * SLOPE + INTERCEPT

Current Delphi MAP sensors use a slope of 126.6328 kPa/%Vref and and 
intercept of 0.0156 kPa. Thus if you are at a standard sea level 
atmospheric pressure of 101.3 kPa and feed the sensor 5.00V, you'll get 
back 4.00 V.  
101.3 = [(4.00/5.00) * 126.6328] + 0.0156.  

Different manufacturers can have different transfer functions (slopes 
and intercepts) though, so you need to see what you've got.  Also, 
turbo sensors need to go to a lot higher pressure.  These are generally 
either 2 bar (200 kPa) or 3 bar (300 kPa) sensors, rather than the 1 
bar sensors used on naturally aspirated applications.  Whatever you 
use, be sure to check the calibration and transfer function before you 
actually use it.

You mentioned applications that mount them off-engine. You can do that, 
but it is critical that the feedline to it be able to gravity drain 
back into the intake manifold and not the sensor. Also, don't allow 
a "U-trap" in the plumbing to the sensor for water to collect.

Hope that helps!

Jim Burkhard

----- Original Message -----
From: Rt66Vairs at aol.com
Date: Thursday, October 25, 2007 11:29 am
Subject: Re: <VV> MAP sensor - utlimately for a Corvair
To: chsadek at comcast.net, virtualvairs at corvair.org

> 
> 
> In a message dated 10/25/2007 8:23:37 A.M. Pacific Daylight Time,  
> chsadek at comcast.net writes:
> 
> Manifold  Air Pressure sensors used on more modern cars sense 
> vacuum in the 
> manifold as  I understand it. Some are mounted on a firewall with 
> a vacuum line 
> to the  manifold. 
> Questions. What voltage is supplied and what voltage range vs.  
> vacuum range 
> do they sense? What is the reference? Absolute or ambient? If  
> ambient 
> pressure, how does the sensor get it? Is there a port on the  sensor?
> 
> Thanks
> 
> Chuck S
> BBRT