I'm having some trouble with the onboard map sensor in my MS3 basic. I believe that the on board sensor should be a MPX4250, but when I calibrate for that sensor, I'm maxing out at 85 kPa with the engine off, I'm showing that we should be at ~101 kPa in SLC from local weather data (unless I'm crazy). I've been running the car in Alpha N (I'm on AE101 ITBs) for a few weeks now, but I'm trying to set up ITB mode with a speed density map for cruise and keep alpha N for high throttle inputs. I was logging TPS and RPM vs MAP and noticed I was never breaking ~83 kPa. Should I just re-calibrate the upper voltage reading base off of the MPX4250 until it matches the local weather readings? I understand the sensor can 'age' since it has a pocket of reference vacuum to base it's readings off of.

 

The 4250 is a 400kpa sensor. I bet you have a 3 bar sensor. Try calibrating for the 3250 or whatever it is.

Oops

 

The 4250 is a 250kPa sensor, not a 400kPa sensor.

Op, what's your current barometric reading?

 

85 kPa with the hose unhooked from the sensor.

 

Yeah, I actually tried all the defaults. The 3 bar sensor calibration had the most sensible reading at ~95 kPa. I didn't want to blindly use that though since it still didn't quite match, and it scales differently than the 2.5 bar sensor.


edit: Please excuse the lame double post. Forgot I could edit. I read much more than I post on forums.

 

Here's a screenshot of my current config and readings. This is key on engine off. Reading from the nearest airport is 30.01 inHg/101.62 kPa. I'm reading 85.5 kPa. If you guys can confirm my MS3 basic should have the 4250, I guess I'll just rescale the value at 5 volts until my reading matches the airport.

 

Barometric readings from weather stations are somewhat ambiguous. Some display the absolute barometric pressure, but some display the barometric pressure compared to the maximum pressure for that altitude. You can tell that when you see a station at 2500ft display 98kPa, lol.

 

SLC is at 4200ft. 85kpa sounds right on the money to me.

My suspicion is that airport barometric pressure readings are sea level corrected. If you look up the barometric readings for DEN (Denver), it's in the 30.08 range. The reading for Telluride, CO, which is ~9700ft above seal level, is 30.26in.hg (102.4kpa). I delivered a car to a customer in Big Bear several years ago at ~6700ft of elevation and key-off MAP readings were in the 76-77kpa range.

e: Note "pressure" vs. "absolute pressure" : http://marvin.byu.edu/Weather/2015/01/05_sum.html (102kpa vs 85kpa)

 

My private pilot friend just verified this for me. All airport barometric readings are corrected to 29.92in.hg. If SLC is having a perfectly normal day with normal pressure for their altitude, they will give 29.92 as their barometric pressure reading, despite an absolute reading in the ~25.7in.hg range.

IOW, your MAP sensor is calibrated correctly

 

Awesome, thank you. Yeah I did the math and realized I'm supposed to be reading ~87 kPa on a normal day and the weather has been really weird. Must be the lack of oxygen up here getting to my head :P. False alarm.

 

Good information to know about airport barometric pressure readings. Thanks.