BARO correction does not seem to work

[Philip Lochner]

I live at 1550m (4720ft) and tuned my MS at this altitude. Took a trip to the coast this weekend and noticed how the mixitures became leaner as we dropped to lower altitudes.

I've got my baro correction setting on Initial MAP reading, so every now and then I would drop the clutch, turn off ignition (allow engine to coem to complete standstill) and fire everything up again to allow MS to take a new initial baro reading.

Despite this I had to increase required fuel from 20.2 up to 23 (15%) to keep AFRs similar to where I had them at altitude.

On my return trip I noticed again how mixtures got richer again despite switching things off every now and then.

Now I have some questions:
1) Does this mean that my baro correction does not work at all?
2) Does this mean that my baro correction does not compensate as much as needed?
3) If I should fit a second MAP sensor for real time baro correction, will I still experience AFR changes with change in altitude?
4) Or is there something else going on that I'm not aware of.

[Kevin Wood]

> I live at 1550m (4720ft) and tuned my MS at this altitude. Took a trip to
> the coast this weekend and noticed how the mixitures became leaner as we
> dropped to lower altitudes.

Interesting. You're going to laugh at the altitudes here, but here goes. I
live at about 600 ft. above sea level. I recently spent a weekend driving
round Wales where I reached around 1800 feet on a couple of occasions. My
car ran seriously lean at altitude. Above 1500 it was very much down on
power and not coping at all well. Tried restarting the engine at hight to
get another sample to no avail. Thought I'd probably left the correction
turned off but when I got home I found it was on.

I should mention I was on MS1/Extra but assuming the correction is
basically working in both code bases it's interesting that you were
essentially getting too much correction with increasing altitude and me
not anything like enough.

Mine's a 2.0L tuned Zetec on ITB's, by the way.

Kevin

Posted by email.

[grippo]

We are very shortly planning to release v2.683 for MS II. This version has a 6 point table of baro prressure vs a correction change that will allow you to modify the baro correction equation for your particular car, compensating for things like exhaust size. With this you can take O2 vs baro data at various altitudes. From this you can determine the % additional/ less correction you need to get your AFR where you want it. This does require that you add a second MAP sensor for baro. But you should be able to tune the table to give you a very stable AFR at any altitude.

If you only have a single map sensor, the best you can do is to tune for an altitude midway between the extremes you expect to encounter. If you stop and restart the car, make sure you turn the key totally off so the processor is re-powered. If the correction you get after this is not right, you can adjust the offset and slope of the correction so it will change more or less as you change altitude. You can do this with the present v2.36 code. In fact if the correction is linear, this may be all you need to do. But you would still need to stop and restart from time to time - there is no easy way around this. Some OEMs have code that grabs the MAP reading whenever the throttle is wide open and uss that to update the baro correction.

[krisr]

Jerry's MAPDaddy is a thing of beauty. I run one and it appears to work for me. Once fuel comes down to a half reasonsable price and it starts to get warmer over here I might take a trip out to the mountains one weekend.

[Philip Lochner]

you can adjust the offset and slope of the correction so it will change more or less as you change altitude. You can do this with the present v2.36 code

Thanks Mr Grippo!

I was bragging with my MS2 to a friend yesterday when I spotted the Baro correction settings under "sensor calibration" However, unless I missed it I can not find any guidelines on the MS web sites providing detail on what these two % settings ("At total Vacuum" and "Rate") mean or which way to adjust them. Are there some guidelines somewhere?

[svenakela]

This does require that you add a second MAP sensor for baro. But you should be able to tune the table to give you a very stable AFR at any altitude.

Grippo, does this mean that the second map sensor is just needed when tuning or can the pressure table be adjusted "by hand"? Or must there always be a second sensor mounted?

[Philip Lochner]

it's interesting that you were
essentially getting too much correction with increasing altitude and me
not anything like enough.

My gut feel tells me that the settings you have under "Sensor calibration/Baro correction" could be different to mine. I also suspect that you tuned close to sea level whereas I tuned at altitude which together could explain the difference in behaviour.

Seeing just how much AFRs changed served as a wakeup call to me to also get this aspect right - not that it happens often that I go to the coast with this car - but should I do installs for other cars.

[grippo]

[
Grippo, does this mean that the second map sensor is just needed when tuning or can the pressure table be adjusted "by hand"? Or must there always be a second sensor mounted?

The second MAP sensor always need to be in the car so it can tell when the altitude is changing. You could write code and wire the MS to do it by hand, but this would not be the right way to do it.

[grippo]

I was bragging with my MS2 to a friend yesterday when I spotted the Baro correction settings under "sensor calibration" However, unless I missed it I can not find any guidelines on the MS web sites providing detail on what these two % settings ("At total Vacuum" and "Rate") mean or which way to adjust them. Are there some guidelines somewhere?

The eq for the baro correction is:

baro_corr = Tot Vacuum value + Rate x baro

where baro is the reading on the baro map sensor. To get everything right the first time you need to go back down the mountain and record the baro reading, AFR, target AFR at say 5 points along the way down. Using AFR and target AFR you can calculate how much correction you want more or less than what you already have from the above equation.

The above eq using the default values works out as:

baro_corr = 147% - .47 x baro(kPa)

so when baro reads 100 kPa, baro_corr is 100% - that is, NO correction.
Now plot this eq. out using baro values of say 80 to 120 kPa along the horiz axis and baro_cor in % along the vertical axis, and you will get a straight line.

Now, write down the measured baro readings vs the AFR % corrections you need to match the target AFR at each point. As an example, suppose that at a baro of 95 you need to be 5% richer. Then Use the above eq. to calculate the baro_corr for a baro of 95 and add 5% to this number and plot it, preferably with a different color pen. Do this for all of your measured values. You will get another line, but this may not be quite straight like the line from the eq, but you should be able to fit a line through the points.

Now all you have to do is figure out what to replace the 147% and .47 by in the eq to get a line that matches your measured data. Adding or subtracting from the 147% will raise your line up or down, and increasing or decrease the .47 will make the line steeper or shallower.

I know this sounds complicated if you are not an engineer, and we should make a tool that simplifies this - and we will in time.

[Philip Lochner]

The eq for the baro correction is:

baro_corr = Tot Vacuum value + Rate x baro

where baro is the reading on the baro map sensor. Etc

Thanks for the comprehensive explanation Mr Grippo! It so happens that I am an engineer so should be able to stomach this