Offset using chart for MAP sensor

[c131frdave]

Below is the chart for my 2.5 Bar MAP sensor. It's a Bosch 0261230042 used in the Redline Saturn cars. I installed a TVS supercharger on my Trail Blazer SS a few years ago, and never got my tune right- and I suspect part of the reason is because I've never had the MAP sensor setup correctly.

The way the chart reads to me is the range is 20-250 kPa. So, does that mean I set it up in the software as:

MAP Sensor Linear: 250 kPa
MAP Sensor Offset: 20 kPa
MAP Sensor Linear B: 83.3kPa


Actual spec file is here: http://almisoft.ru/files/bosch_map.pdf

In the spec file, it shows a chart of P1 and P2, and both points are slightly above and slightly below the actual sensor's endpoints. In other words, the graph shows that sensor will detect pressure below P1 (20kPa) slightly, and above P2 (250kPa) slightly. So do I need to account for that?

It says output voltage at P1 is .4 volts, and at P2 is 4.65V. If VCM Editor is looking for kPa at 5V for the first parameter, and kPa at 0V for offset, how would one figure that out? I could get my vaccum pump out and put a vacuum on the sensor ad test it I suppose, but what a pain that is if I have the data file for the sensor.

Any help would be appreciated.

[c131frdave]

BTW, I had a similar post on the Gen 3 forum, but I realized my engine was Gen 4, so I moved over here...

[DSteck]

0.40v is 20kPa
4.65v is 250kPa

Solve the matrix. One volt is a boost of 54.118kPa. The linear value represents how much range there is over five volts. It is not the value at five volts. So, the linear value is 270.59kpa.

Solving the matrix tells you that the offset (value at zero volts) is -1.65kPa.

[c131frdave]

Awesome, thanks. I'm not completely sure what "solve the matrix" means. The ratio would suggest:

4.65V/250kPa = 5V/X kPa solving for X = 268.8
and
4.65V/250kPa = 0/X kPa solving for X = 0

So obviously I'm missing something. Any chance for a quick lesson on how the matrix works? Thanks so much for the reply!!

[c131frdave]

UPDATE: Rut Roh Shaggy. I can't set my MAP Sensor Linear more than 256.

Does that mean I need to install a 3Bar OS??

[DSteck]

The linear value is easy.

(MaxP - MinP)/(MaxV - MinV) = (250-20)/(4.65-0.4) = 54.118kPa per volt. Per five volts it's 270.59kPa.

The offset is just solving y = mx + b.

As far as matrix solving, that's not something I can teach you in a post.

[c131frdave]

I think I got it. Thanks. You're solving for the slope of the line if I read it right.

So, since my 2Bar OS won't accept 270.58kPa, do I need to install a 3Bar OS?

[DSteck]

It won't accept the negative offset either. A custom OS won't fix that. You need a different MAP sensor.

[c131frdave]

I see you have some in your signature. PM incoming...

[DSteck]

I have none in stock with no ETA on more. I'm so busy, I barely have time to wipe my own butt. Lol.

[c131frdave]

Well poop. Okay, disregard my PM then..

[c131frdave]

Just for my own sanity, check my math please:

Slope = (250-20)/(4.65-.4) = 54.117 kPa per V, so Linear = 54.117 X 5 = 270.59kPa

Zero intercept would be 250 - [270.6 X (4.65/5)] = -1.64kPa

If that's right, you'd think you could scale it somehow. The most it will allow me to put in is 256, which is only 4.6kPa short. And zero on the low side, which is only 1.64 kPa high. Is it possible to scale it somehow? The most boost I will ever make is about 1/2 bar- probably less. Can the Linear B be changed to skew the range to work? (Not sure exactly what Liner B is except that the help note says it should be 1/3 of the Linear value).

If not, anyone know of a 2 bar TMAP sensor with the hold down tab on the side? I could probably make a hold down tab I suppose.. Frustrating..

[c131frdave]

Okay, using the excellent information from DSteck (thanks so much), I think scaling would be fairly simple. The graph shows that the resistance is linear, so it has to be a straight simple variable resister with the "wiper" on some sort of pressure membrane. The pressure measuring device affects resistance linearly, which is perfect.

So, if we raise the voltage from the ECM to 5.5 volts, if I did my math right, the following would occur:

Existing: @ 20kPa, I get .4 volts, or 8% of signal @250kPa I get 4.6 volts, or 93%

So, at 5.5v, @ 20kPa 8% X 5.5 = .44 volts @250kPa, 93% X 5.5 = 5.115 volts (which is over range, but that's okay because my SC only is capable of 1.5 bar)

So, (250-20)/(5.115-.44)= 49.2 kPa/V Linear = 49.2 X 5 = 246kPa

What's interesting is the offset. It remains the same. 250 - [246 X (5.115/5)]= -1.65kPa. But, it's only 1.65kPa, which is what, .2 psi or something if I set the offset to zero? Not sure that would make a tremendous amount of difference. Building a switching power supply to deliver a stable 5.5 volts to the sensor would be fairly simple. Actually. a Buck Voltage Switching Regulator is only five bucks from ebay.

Anyway, I could set the MAP linear to 246 (even though it would really be 270 at 5.5v) which would turn my MAP into a "nearly" 2 bar sensor than a 2.5. Hmm. Thoughts? (I noticed Bosch rates the MAP up to 5.5 volts, so as long as the regulator regulates, I'd be good.) Hmmmm......