BaroCor Calculation

[sedd]

I am using a second barometric sensor. How is the BaroCor calculation completed? Is this value calculated in the program and then multiplied times the value from the barometric correction table (and curve) found under the Basic Load Settings drop down? Put another way, is the table used to tweak what is already calculated or is the table the final correction value used in the PW calc.

PW = DT + (ReqFuel * MAP * VE[RPM,MAP] * AirDen * BaroCor * corrections)

I have a barometer reading of 96.2 with a baro cor of 101.8. Then with a barometer reading of 98.6 I show a baro cor of 100.7. this result seems opposite of what I would expect. Shouldn't a lower baro reading mean less dense air and that means less pulse width = less fuel? see values around 507 seconds.

[slow_hemi6]

Move that sensor out of the inlet tract. It is for measuring barometric pressure not your air filter restriction.

[Matt Cramer]

What firmware are you using?

[sedd]

just loaded firmware 1.2.3. on 7-21-13.

[sedd]

slow_hemi6: thanks for the response about my other ongoing post. I would still like to know what the calculation is and what values of stoic are used. I will move the sensor if this is going to help the crank sensor issue. At the moment I don't understand how it would do that. I will try to disconnect baro, and both oil and gas pressure sensors and make some more runs. That should help to sort out if this is an issue or not. but that is another post.

I have cowl induction so my pressure at the air filter will vary. How much variation I have not really looked at closely due to these issues with the crank sensor. Actually I feel this is likely nit picking and would not make much differance. Good for debate in any case.

I understand that the baro sensor is to provide a value for the PV=nRT equation. Mega squirt needs to know what the value of P is so it samples at the start of each run using the map sensor (just once), or in my case I believe it constantly updates the value of P. I thought it would be better to adjust the value of P to reflect conditions at the top of the throttle plates. What happens when my air filter gets dirty or changes in its pressure drop. So maybe we really don't want to measure "barometric" pressure but actully want to measure engine inlet pressure.

In an ideal situation, we would hope that the value of P would not change much with air flow thru the engine. In my case and with original design of the engine filters etc, there appears to be a % of pressure drop. So when my cowl induction opens up I may get back that % or even gain a % or two.

[slow_hemi6]

In the position you have it you keep telling the ecu that the atmospheric pressure acting on your engine and air density have changed when that is clearly not the case. As I said earlier your car didn't just jump over 200m in the air.
Have a read of this thread it gets pretty good towards the end. http://www.msextra.com/forums/viewtopic ... n+strategy Do a search on Barometric correction and you'll find more threads on it. There is one that goes for 4 pages if your up to it.

[sedd]

Is the barometric table and corrections based on 101.325 kPa? Or what value of baro gives a 100% correction?

[jsmcortina]

Is the barometric table and corrections based on 101.325 kPa? Or what value of baro gives a 100% correction?

Whatever value you enter into the table.

James

[sedd]

James, thanks for the response. could you comment on how the pw is calculated per my original posted question? MS2 manual says there is an internal calculation and that the table is used to make small corrections. Did MS3 calculation stay the same?

I am trying to better understand the pw calculation for speed density.

the pw equation shows that the req fuel is multiplied by MAP. Assuming I have multiply by MAP turned on. Would that already adjust for the density of the air inside the manifold? So does the Baro correction need to only adjust for the changes in the air flow efficiency past the throttle plate. I am thinking that for a measured map inside the manifold, the engine would not know or care what the actual barometric pressure is. For a given MAP and RPM the engine will flow the same air at sea level or in the mountains. If true I would think that baro would not be a large correction?

[sedd]

I would like a confirmation about how the barometric correction number is used. I am using the 1.2.3 firmware and have the "at total vacuum" and "rate (%)" value in the calibrate map baro table both set to zero. I use the speed density selection.

If I enter a value of 125% in the table for barometric correction, does it multiply the calculated value for the pulse width by 1.25? Similar if the entry is 75% then the calculated value is multiplied by 0.75?

Can anyone share what they have found for values to enter, based on real use testing and with the newer entry table?

Are users finding that they need to enter lower numbers (less than 100%) as they climb in altitude and in order to reduce a rich condition?

[Matt Cramer]

The barometric correction is a multiplier. (Edited to fix this) Set it at total vacuum to 100% and the slope to 0.

[sedd]

Thanks for the post. I had set these to zero based on the notes in the tuner studio found under the ?, next to the baro table input cells. Is an update needed on that in Tuner Studio?

Is there a quick explanation of what the values in the baro table still do? Can I assume that setting both of these at 100 allows the newer baro table to function as you have confirmed?

What was happening when I had those both set to zero? I have sync issues I am working on, and though I don't know how it would happen, I just want to check to make certain there isn't any way that could have or be causing the issues with sync loss.

[sedd]

After some internet research I found info that noted that for speed density fuel injection, the barometric pressure was automatically compensated for in the VE map vs rpm tables. For a given kPa and rpm, the engine doesn't care what the barometric pressure is and should get the same fuel VE number.

In quite a few other posts it was stated that for almost all engines, the back pressure reduction improves the VE. Unfortunately there was no % correction data to back that up. It appears each engine and exhaust system is unique so it is likely that only field testing will show what the baro correction should be.

For the fun of it I calculated the % baro correction change due to that lower back pressure. See attached spreadsheet. I am curious if real world testing shows anything close to this? Some debate and response would be interesting.

The theory is that with reduced back pressure the compressed volume space at the end of the exhaust stroke has less left over gasses. The change in volume uses PV=nRT equation. That change in volume of left over gas is replaced with an equal amount more fresh air on the intake stroke = better VE. I suspect this is overly simplistic and there are many more variables and interactions to consider. For instance for a given kPa what is the effect of the flow rate change in trying to put more air in the cylinder.

[jsmcortina]

The old internal barocorrection already compensated for exhaust backpressure. However, in the real world, most user found that was in fact the opposite of what they needed for barocor.

Hence, I re-wrote the barocor so the correction curve is fully exposed and adjustable.

James

[sedd]

James, are the settings as Matt has noted, to set total vacuum to 100% and the slope to 0 still needed? Do these still get used?

[sedd]

rechecking this. I think I was originally recommended to run both at 100%. I tried to run with both at 100% and that caused huge pulse widths. The engine barely ran. I switched back to both settings at 0% (also recommended in the help box for those settings) and the engine runs again and pretty well. Best I can tell the pulse width corrections now follows the new baro correction table.

I have also just upgraded to the new ms3 firmware 1.2.4.

I see some edits above, should I still set the % at total vacuum at 100% ? I can easily try this but would rather get it resolved as to the proper setting, since it still runs with it set at zero %. I am thinking I don't want to retune and then find out I had it set wrong and that would have messed up the tune somehow.

[kaeman]

I have cowl induction so my pressure at the air filter will vary. How much variation I have not really looked at closely due to these issues with the crank sensor. Actually I feel this is likely nit picking and would not make much differance. Good for debate in any case.

As I have had some experience with tuning verses under hood pressure.... I found that a lot of change will be made when the pressure under the hood changes, I had my stack injection under the hood with no air cleaner and had the thing tuned so it ran great, then I decided to run an air cleaner and had to cut a hole in the hood big enough for my fabricated air cleaner to fit through. I drove the vehicle into town before finishing the air cleaner assy, the engine ran so lean because the under hood pressure was escaping out the hole in the hood, I had to retune the vehicle.... it was running about 12.4:1 rich rather than the 14.3:1 that it had been running. After getting air cleaner mounted I then had to retune it because of the flow change from the air cleaner element. Finally I got the cowl welded to the hood to cover the air cleaner assy and the tuning procedure had to be done again.... I was surprised how much the hood opening affected the airflow into the engine.

BTW I am running hilborn injected(my own efi conversion) small block chevy 383. The megasquirt made the thing easy to drive... it was originally set up constant flow..

[sedd]

Very interesting. I have been side tracked on this item due to crank sensor issues and due to work items.

I have moved the sensor from after the air cleaner element and tie wrapped it close to where the rear distributor is located on the big block chevy. I have noticed that the baro pressure changed quite a bit during the tuner studio logs. I still have issues with spikes but can see a trend if I view the log compressed. The values changed from 100.2 to 100.7, and that really surprised me. I see dips or low side spikes down to 99.5. I would have thought this reading would be very steady and vary only a very small amount. (Only one log so this is not enough yet to tell for certain) I will try to see if the mph seems to follow the baro in more logs as I have time. The pressure did seem to rise to follow the VSS1 (mph). Could there be a build up in pressure with speed and/or with radiator electric fan? I am trying to figure out how to tell when the fan comes on?

If so, what does this mean to the engine? Where should we place the sensor to get valid readings? From what I can tell the baro seems to only affect the back pressure on the exhaust and then with cylinder filling. The correction for this is minor with a speed density set up? If this is true, then maybe the baro should be sensed at the area of the exhaust tips?

How about a way to average out the baro readings? I have not figured out how to do that? I see MAP averaging but didn't find one for BARO.

[sedd]

I am ready to do some retuning based on new injector settings, so I am back to the baro settings and wondering if anyone has the final answer on this? could anybody who has more data on their real world corrections do a quick comparison to the curves I have attached, and post a response?

I came up with a correction spreadsheet that calculated both the VE change due to better filling with fresh air, and also the flow change across the valves, both of which should lean out the engine.

the values I calculated seem to match one of the posts from real world testing.

I suspect that there are more factors in corrections to consider, IE exhaust flow pressure drop should increase. Possibly these are not significant?

[sedd]

See Delphi tech article 2009-01-0587, access via internet search of: 2009-01-0587

This article is the basis for the attached spreadsheet. Lacking any other info, I have decided to use this spreadsheet for corrections due to baro.

In using the spreadsheet, you should consider a couple of choices, backpressure and the exponent to use. the amount of change due to these is shown by a few of the curves on the spreadsheet.

I used what the author of the article recommended for the exponent, = 0.6

I also chose a value of backpressure that was likely for most of my normal driving, 1.0 psig backpressure.

Caution:

Recent data from Sept 2014 may show the reverse of what is shown in this spreadsheet. I will submit another version with the reverse shape for potential use and testing. If it is decided that this one is not correct, I will delete it.