Since installing my GM 3-bar MAP sensor that I bought at Advance Auto, I noticed that while the motor isn't running, the GM MAP detects around 860 mbar rather than 1000 mbar (which, the atmospheric sensor detects 988 or so, which makes sense as I'm above sea level).

In Crome Gold (which I'm using) and as well Crome (though there's a second option in regular Crome ROMs), the GM 3-bar MAP is said to be 1.76 volts at 1 ATM.

According to (just about) everything I read, the GM 3-bar MAP is actually supposed to be around 1.6 volts at 1 ATM.

A couple of things push me in toward this train of thought.

One, the boost/vacuum readings have been lower than my silly autometer gauge. I expect a difference, since the gauge is cheap, and just an estimate. However, consistently, in boost, it would appear to me that the gauge (and we used 3 different gauges to confirm will confirm that I'm running almost exactly 9psi of boost, and the MAP sensor insists that I'm hardly over 7 psi of boost (corrected for 988mbar=1atm).

A simple line plot will reveal that 860-870mbar (what my MAP sensor reads with the engine not running) should be around 1.6 volts (which is what, as I read, should be 1 ATM for a Gm 3-bar map) and that shifting the graph from 1.6 to 1.76 changes the boost reading from roughly .48 bar to over .6 bar (around 7psi to 9psi, I think).

Should I use 'Custom MAP' and set my MAP sensor to be equal to values that I find on a website like this?: http://www.robietherobot.com/storm/mapsensor.htm

I wonder if the makers of Crome were using data from DIFFERENT 3-bar MAP sensors that were the likes of whatever this guy was using: http://hhscott.com/93tmv/3-bar_map.htm since it appears that HIS 3-bar MAP from GM, in fact, reads in the 1.7's for voltage at 1 ATM...

anyone have this same experience, or input?

Edit: According to the tables I'm looking at, this is just about how things should look... do this, and rebuild the tables? https://i.imgur.com/nmwTA64.jpg

 

If the voltage scalars in Crome do not match the specs for the sensor that you are using, Create a custom scale and input the proper numbers.

 

Came back to report,

After changing from Crome's stock 'GM 3-bar MAP' setup (which seems to correspond with an older/different manufacturer that makes/made GM MAP sensors with 100 kPa/1000mbar at ~1.75 volts) to my own custom setup with a 0v = 36 mbar (which turned itself into 35) and 5v = 3150, it calculated 1 atm at 1.57 volts (which is just about right, considering 100kPa/1000mbar should be at 1.54 volts or so, when I plot it out), the car runs MUCH better.

It comes out to 623 mbar/volt (many people say 628, which is wrong because it assumes 0v=0mbar instead of 0v=36mbar).

I have to say that with Crome's stock GM 3-bar MAP sensor setting, it told me I was running just about 7psi, when I was (in fact) running 9.0 psi.

This is a problem when considering the amount of ignition retard intended for 9psi in my tables was not being used. Rather (slightly) less retard that was implemented closer to 7psi was being used.

That is to say that there was more ignition advance than what we wanted, because the MAP was assuming that there was less boost than there actually was.

I took my tables, copied/pasted columns from the original 'Crome GM 3-bar map' tune into my 'new custom 3-bar map tune' by making myself a chart. I attached the little Excel spreadsheet if anyone needs to this.

Three important columns are here, voltage (which starts at 0 and goes to 5v as you go down), true GM 3-bar pressure, and Crome GM 3-bar pressure. Notice, in all cases, the Crome 3-bar sensor has a lower mbar/kPa reading than a REAL GM 3-bar sensor has.

I looked at my 'new GM 3-bar tune' columns (which I start at 100mbar, and go by increments of 150mbar). I copied columns from the 'Crome GM 3-bar tune' and pasted said columns directly into my 'new GM 3-bar tune' accordingly.

For example, I took the 865 mbar column from my 'Crome GM 3-bar tune' and pasted it into my 1003 mbar column of my 'new GM 3-bar tune'.

I made sure that ignition timing was how I wanted it (but copied/pasted 1000mbar from the Crome 3-bar tune into 1000mbar of my new GM 3-bar tune) by getting the ignition columns how I originally intended, rather than how they were actually working. That is to say that, although I tossed the columns from the Fuel tables from 865mbar in one tune to 1000 mbar in my new tune, I tossed IGNITION columns from the 865mbar in one tune to 865mbar (or whatever number is closest) into my new tune.

My car runs better than ever.

I'm glad it worked out though.

Lesson: Those using a real GM 3-bar Map sensor that is reading 800-900mbar when the car is off (rather than 1000 or so) (assuming you're not 5000 feet above sea level in a mountain town) or 1.6 volts when the engine is off (instead of 1.75ish), you need to use '3 Bar GM MAP Sensor(Test)' in regular Crome/Crome Pro under Advanced Boost Tools+, OR using the Crome Gold ROM, you need to pick 'Custom' and make the minimum 35mbar and maximum 3150mbar.

This will get your MAP sensor's readings MUCH closer to how they should be.

Thanks for your input, GhostAccord.

 

What was the part number of the 3 Bar map sensor? There are 3 different popular 3 Bar map sensors that GM has. All slightly different calibration which is probably why it was off.

GM 12223861 -14.4PSIg (2kPA, 20mbar) to 31.1PSIg (316kPA, 3160mbar)
GM 12219935 -8.3PSIg (44kPA, 440mbar) to 30.8PSIg (314kPA, 3140mbar)
GM 16040749 -13.1PSIg (11kPA, 110mbar) to 29.3PSIg (303.4kPA, 3034mbar)

 

That's a great question. I got mine from EFI Source: http://www.efisource.com/shop/gm-sty...sor-w-pigtail/

"GM style". I have a different one from a year ago (that I never had gotten to work) that behaves exactly the same (but I don't remember the brand.

It shows just under 1.6 volt with the motor not running (but ignition on).

That led me to change calibration. I didn't realize that there were 3 different ones!

 

There is actually more than just these 3, but these are the most used and sold by people