Altitude Correction

[Gandalf]

I am new here! Some info on my car - http://www.vems.hu/wiki/index.php?page=MembersPage%2FGandalf

I live in Johannesburg, South Africa, which is 1600m above sea level. I wonder if anybody can help me with a few issues.

Firstly, there is a lot less oxygen in the air at this altitude. When calibrating the WB02 a AFR of 20.9 would therefore not be correct? How can one calculate the correct AFR for calibration at altitude?

Then on tuning, vemsTune has a barometric correction setting. I have set baroCorr to Enable, dBaro to 30 and airden_ignore to 80, but in my logs it shows gBaro as 100all the time? The normal air pressure at this altitude is around 85kPa?

[dnb]

I thought there was less of everything in the atmosphere at high altitudes, so the percentage of oxygen is pretty much constant in the atmosphere.  So the 20.9 constant will be fine because there's proportionally less nitrogen and everything else too.  I could well be wrong because I live 5m above sea level... 

gBaro is the multiplier the engine uses to compensate for pressure changes in the fuelling.  If you have baro_correction turned on, then (I think) the pressure the ECU first sees when the engine isn't running is used as 100% pressure point, not say the 85% point.  So the chances are what you have is correct behaviour.  Since I have an NA car I leave this turned off, so I'm not 100% sure on all the workings of it, so be careful with what I say here!!

Have you calibrated the pressure sensor?  (basic settings->MAP sensor range and map sensor offset)  If not, then this is the first job (assuming they weren't supplied with the unit) test the VEMS against several known pressures and plot a graph in excel.  From this, the two calibration constants can be derived.  If you get stuck, I can help you through the maths.

airden_ignore is an old thing, and personally I wouldn't use it - the replacement is the MAT fuel enrichment table, and it's much more flexible, so use this if it's available and keep the airden_ignore as the last ditch safety feature against hot lean-out.

[Gandalf]

Thanks dnb! I realise most people in Europe would not have this issue

My problem with calibration was that I see 17 AFR in free air and not 20.95! I did some surfing and found this http://www.bgsoflex.com/pwb/0.95/PWBV0.95_QandA.pdf with a calculator at www.bgsoflex.com/partialp/partialp.zip. The is a nice paper in the zip file explaining it all. The calculator shows me that at my altitude and temperatures (it is 32C out there at the moment) there would only be 17.22% oxygen in the air! I thought my WBO2 couldn't be that far out!

On the gBaro, I thought the ECU would read the pressure before starting and then use that in its calculations. I understand that it will remain constant but thougt gBaro should be something like 0.85? I have a VEMS PnP that came with MAP installed - my MAP with car off corresponds closely to my Oregon Scientific Weather Station.

My car has an atmospheric pressure sensor for altitude (F96 in Audi terms) coming in on pin 9 of the Motronic 55 pin. Is there some way that I can use that to constantly calculate the base barometric pressure? And if so, how?

Edit to fix for QNH barometric pressure

[dnb]

If the composition of air is as the paper says, then use 17.2 as the config constant and all will be well - it only needs one point in the curve to calibrate so it'll be fine.   (I'll read the paper when I'm not at work.)

My understanding is that if you turn OFF baro correction gBaro would go to 0.85.  Have you checked the pressure sensor against a range of known vacuums and boost pressures?  This is unfortunately not a single point calibration excercise like the wideband.

My Impreza has a correction for baro pressure too on the standard ECU.  I don't think VEMS supports this yet, but it would be easy enough to come up with a case for supporting it.  You only need the one sensor, and you switch the MAP signal to atmospheric pressure using a solenoid and a misc output every so-often and use this as the baro correction constant... 

It would be nice to have this feature because you can then guarantee boost is a set amount above atmospheric pressure instead of running to an absolute manifold pressure.  And it avoids "work arounds" that can catch out the unwary...

If you want, I can try to nag the right people to get this implemented. 

[Gandalf]

dnb, I think it would be great if VEMS can introduce this functionality. Most of the PnP VEMS go into Audis which nearly all would have a F96 altitude sensors as input. As you say it might be difficult to calibrate though.