ITBs and barometric correction.

[bcal]

I've just finished a trip between Sydney Australia (elevation basically sea level) to our nation's capital Canberra (elevation 2000ft) and experienced an interesting problem.

At about 1500ft the engine began to miss under load and felt very sluggish. I pulled over and hooked up the laptop to see if I could find the problem and saw that the WB O2 was constantly showing max rich.
Thinking I had a problem with the 02 sensor I switched off O2 correction and the problem seemed to get better.

Then as I neared 2000ft it got worse again and it was at this point that it dawned on me what the problem was...Initial baro reading was 93.5kpa and there was no baro correction set on the MS.
Setting baro to initial solved this problem though on the way home I had to stop a couple of times and restart the engine.

So this has got me thinking. Is there anything special about ITB setups that requires a second pressure sensor to measure constant baro?

I mean, ITB setups do operate under much narrower pressure ranges and the VE table is far more radically curved that a typical single TB.
So does this make them more sensitive to barometric pressure changes?

BTW I've just read that barometric pressure changes about 1inchHg (3.5kpa) for every 1000ft.

[Philip Lochner]

In my view, Baro correction has very little to do with the fact that you are running ITBs or single TB.

Without continuous baro, MS uses the baro reading only upon initial power up.

Even though you may have Map/baro as your load index (I prefer to use map only - for timing purposes) it does not change the fact that baro changes at the exhaust affects your engine's overall breathing.

I also live in an area where we have major altitude changes in very short distances and have found continuous baro correction to be a non-negotiable part of an MS build and tune. Provision should really be made for it on the next version of the MS main board.

I prefer to set the "two point table" to 100 and 0 such that that table has no effect on baro changes and then I use only the 6 point baro correction table to correct for baro. What I do is to memorise a specific operating point (say 200rpm, 50kpa) and knowing it has to be, say 15, I adjust the next point on my baro table until I get that figure with my WBO (while EGO is off). You can modify the ini file such that the AFR reading is displayed on the baro correction page.

Once the baro table is tuned it is quite pleasing to be running up and down mountains and seeing required AFRs being maintained!

[bcal]

Thanx Philip.

I ordered 2 new map sensors (one extra for my spare ecu) from a local electronics wholesaler yesterday and they arrived today. At $30 each I thought that wasn't too bad.

I'll hook them up for continuous baro measurement as described in the manual and see how that goes.

We don't really have too many high mountains here in Oz and most climbs are fairly gradual.

In three years of driving on MS this was the first time I experienced this particular problem though I must admit the car has felt a bit flat sometimes and this is most probably the reason.