MegaSquirt 2.89 Barometric Correction.

[TheSilverBuick]

I've gone back and forth about posting this question, but have decided why not.

I thought I had all my settings set to let the VE table adjust for the elevation change between Ely, NV and Fontana, CA, but it isn't so. Pretty much as soon as I start down the mountain the fuel mixture starts leaning out. :-\

I have the mapdaddy second baro installed and I can bring up it's readings in TunerStudio. The fuel settings are set at "Two Independent Sensors" and "Adjust Map for Baro". But there must be more to it.

So under the 2-Point Barometric Corrections I think I need to go (if not to the table). This is what the MegaManual says. Basically near as I can tell it says it's not doing any correcting. *(the default settings of 147 and -47 are in there)

2-Point Barometric Correction - if you have selected two-point corrections, you can set the baro corrections here (if you have selected 'tables. baro correction is set under 'Tables/Barometric Correction'). Correction for barometric effects is performed using the linear function:

Barometric_correction = At_total_vacuum + (Rate * barometer) / 100

At Total Vacuum (%): 'At total vacuum' contains the total correction at a barometer reading of 0 kPa.
Rate (%): 'Rate' contains the percentage per 100 kPa to scale the barometer value.
Using the default values of 147 and -47, we see that for a barometer of 100 kPa, we have 100% correction (i.e., no correction). Barometric_correction = 147 + (-47*100) / 100 = 100%. Note that the values of 47 and 147 are not coincidental - they cancel at 100 kPa to give no baro correction, as you would expect.

I guess I'm confused as to what the variables actually represent, and which way to change them to make them relavant and set the correction properly. Where to I look up or have to test to find the "At Total Vacuum" variable for my application? (7500'-0', 82kPa-103kPa) as well as calculate(?) the rate?

I'm going to go out side now and blindly play with the numbers on an Ely tune, but some guidance would be appreciated


(I didn't signed up for the msefi forum to ask there because their forum doesn't like my e-mail address :-\)

[TheSilverBuick]

Re: MegaSquirt 2.89 Barometric Correction.

Loading up an Ely Tune and bumping the correction to 171 brought the AFR's back into line. I still don't know what that number means or if it'll have to be adjusted heading home.

I'm wondering if the Total Vacuum should be set at 100 and the rate at 15 (~82kPa/103kPa)...

[Matt Cramer]

Re: MegaSquirt 2.89 Barometric Correction.

I'll get you the equation when I'm back at work.

[TheSilverBuick]

Re: MegaSquirt 2.89 Barometric Correction.

Thanks Matt, I'd appreciate it.

[Matt Cramer]

Re: MegaSquirt 2.89 Barometric Correction.

Here's the information I got from Al Grippo:

The baro correction is for exhaust backpressure reduction at altitude. The eq is:

barocor(%) = bcor0 + bcormult x baro (kPa)

The main compensation is MAP which accounts for the thinner air. The total correction is:

barocor_total(%) = MAP(kPa) x barocor(%) / 100(kPa)

At WOT baro = MAP, at leat for an N/A engine.

Putting all this in a table:

baro(kPa) barocor(%) barocor_total(%)
60 118.8 71.3
70 114.1 79.9
80 109.4 87.5
90 104.7 94.2
100 100 100
110 95.3 104.8
120 90.6 108.7

[TheSilverBuick]

Re: MegaSquirt 2.89 Barometric Correction.

Thanks Matt, I will have to chew on that one a bit. I'm going to be leaving So. Cal. here at 97kPa and back to Ely at 83kPa in a bit, so I'll be trying them out.

I'm going to try those values in table format (I expect them needing some trimming up). My table won't let me enter values below 80kPa, but I'm sure I can find the place to modify it in the cfg file.

[TheSilverBuick]

Re: MegaSquirt 2.89 Barometric Correction.

Whoo Hoo, I have Barometric Correction and a rough calibration!

I turned the table on and it was quite easy to set.

I loaded up an Ely tune, set the line node's (six points) at interval's of five, from 105kPa down to 80kPa (I want to go to 70KPa, but it won't allow me yet). Then I set the 100-105kPa to a point that the idle had the same AFR I had in Ely. Then as I climbed the mountain so to speak, at intervals of five kPa I'd adjust the Node's to get the AFR where I want. Simple!

The Line is essentially straight, I'm going to re-tune the afr's to be a little more ragged edge again and on the next trip to CA (Anti-Tour) I'll refine the afr's with the Barometric Correction Table.