BARO correction does not seem to work

[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

I've chewed on this a bit... and ended up with more questions if you'll bear with me.

Is AFR directly proportional to pw? (I'm trying to establish if I can use AFRs or if I should be using pw's rather)

How does baro correction come into play when looking at the formula for fuelling: pulse width = Req_Fuel × VE(rpm,kpa) × MAP × E + Injector_open_time ? Does it come through "E"?

Also, I presume that baro_corr is used to calculate the required pw.
Using the formula yields 100% for 100Kpa. This means that an engine tuned and operating at sea level will not have any correction applied. Operating this same engine at 84Kpa barometric pressure, the formula yields 107.52% for baro_corr. I take it that pw calculated from the VE table should be reduced by 7.52% since at higher altitudes less fuel is to be supplied?

[Bernard Fife]

I take it that pw calculated from the VE table should be reduced by 7.52% since at higher altitudes less fuel is to be supplied?

Philip,

No. The MAP sensor output compensates for the lowered intake pressure (unless you are running alpha-N...).

The baro correction compensates for the lowered exhaust back-pressure, so the fuel correction is generally positive (> 100%) as the elevation increases.

This is somewhat application specific, though, which is why it has been made user-settable.

Lance.

[grippo]

Is AFR directly proportional to pw? (I'm trying to establish if I can use AFRs or if I should be using pw's rather)

How does baro correction come into play when looking at the formula for fuelling: pulse width = Req_Fuel × VE(rpm,kpa) × MAP × E + Injector_open_time ? Does it come through "E"?

AFR is air/fuel ratio so yes if the measured AFR is 10% rich you need to reduce pw by 10%, roughly. I use the word "roughly" because your eq, which is correct, is non-linear because we add in the inj open time, while the bar_corr is put into the E term. However, its not going to be a big difference. But if you are looking for every bit of accuracy, you can use the 6 point table, put in the first order corrections based on assuming AFR correction is linear with pw, then repeat the measuremnets and this time your AFRs should be much closer to the target, but if there are any small deltas, correct the 6 points again. However, I think you will find that any remaining deltas are in the noise of the measurements.

[Philip Lochner]

Thanks very much chaps, I now understand much better and am keen to make another trip to the coast to get this right.

[BrentP]

I take it that pw calculated from the VE table should be reduced by 7.52% since at higher altitudes less fuel is to be supplied?

Philip,

No. The MAP sensor output compensates for the lowered intake pressure (unless you are running alpha-N...).

The baro correction compensates for the lowered exhaust back-pressure, so the fuel correction is generally positive (> 100%) as the elevation increases.

This is somewhat application specific, though, which is why it has been made user-settable.

Lance.

So for those of us running alpha-N...

When racing in Seattle or Portland at sea level and going to Spokane at altitude, I go from about 102kpa to 93kpa

The equation given adds extra fuel, and I really need to lean it a bit. You can feel it as well as see it with the datalogs. I have just been adjusting the VE table with a multiplier, but I would like to fix it to work correctly.

If I go with values of total vacuum (%) =0, and rate (%) = 100, then I get a decrease that is the same as the barometer decrease.

Does this sound logical? It makes sense to me for AN, but am I missing anything?

[grippo]

As long as you have a baro sensor you can make the correction do anything you want because it is user changeable. The 2.6 versions allow 6 baro correction points for 6 values of baro pressure. Normally the map multiplier in the fueling eq would exactly comepsate for the 10% drop in air pressure at altitude. With alphaN you can do the same compensation in the baro correction table. This table is an additive +/- % added to the normal baro correction equation.

[BrentP]

The versatility is there, no doubt! Thanks for not sticking with "bum-umptions" like so many systems do.

I was just wondering if my thought process is correct (or close) on how to use it, since A-N is not discussed much.

[grippo]

Your thoughts are correct. The present default eq is

barocor = 147 - (47/100) x baro

This gives a correction of 100% at baro=100 kPa and 105% at baro = 90 kPa.
If you used Map, then at baro = 90 and WOT you would also multiply by map/100 = 90/100. So the total correction would be 90% x 105% = 95% - so there would be an overall reduction at high altitude. However there is nothing magic about the default values. So in your case as you said you could use 0 and 100 instead of 147 and -47. So the code would evaluate this as:

barocor = 0 + (100/100) x baro,

giving you a correction of 100 % (none) at baro = 100 kPa and 90% at bar = 90 kPa. Since you are using alphaN, there would be no Map correction, so 90% would be your total correction.