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

[BBRT]

Thanks, "Absolutely" helps.

Chuck
----- Original Message ----- 
From: <burkhard at rochester.rr.com>
To: <chsadek at comcast.net>
Cc: <virtualvairs at corvair.org>
Sent: Thursday, October 25, 2007 1:22 PM
Subject: Re: MAP sensor - utlimately for a Corvair


> 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 
>