Baro and MAT ( IAT ) correction.

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Baro and MAT ( IAT ) correction.

Postby Philip Lochner » Mon Mar 02, 2009 4:13 am

Another squirter "tjabo" (http://www.msextra.com/memberlist.php?m ... ile&u=9525) recently asked me specifically about baro correction via PM but I want to reply on the forum in case I talk rubbish (which I know I am quite capable of) hoping that others will correct me.

Both MAT and BARO have a substantial effect on the AFRs resulting from a tuned VE table. EDIT, likewise both MAT and BARO can influence the VE table while being tuned. This is my biggest challenge while tuning the VE table.

Regarding MAT correction (which adjusts PW as a function of inlet air temp)

Do yourself a favour and do the following experiment:
Set values for all correction %'s in the MAT correction table = 0. Then , with stim, keep all inputs constant except IAT and see what happens with inj PW.

My point is that the built-in adjustment for IAT is too aggressive and adds/removes too much fuel. Others have reported likewise. What I now do is that I first adjust the MAT correction table using stim so that PW is the same for all values of MAT. This effectively cancels out the built-in MAT adjustmens that MS make. Then I modify the MAT correction table as needed on the basis of different MAT temps being experienced. Some tuners leave it up to AMC or EGO to correct for the AFR deviations from the AFR table. Personally I prefer to tune the table correctly so that even in open loop, the AFRs will be close to target.

EDIT: I have since realised that one should actually even tune the MAT correction table before the final tune of the WUE table. (Explanation futher down).

Regarding BARO correction (which adjusts PW as a function of altitude)

Highly experienced tuners on this forum hold the view that real time baro correction is unneccesary (when driving with altitude ( baro ) changes). Obviously if you only use the car at only one altitude then this is true. I have not yet been able to understand how that can be, as my AFR gauge clearly tells me that the engine is going richer or leaner for as little as 150m (500') change. This is why I fit a real time baro sensor as std to the MS's I build (not that I do many). Even using "initial map reading" is IMHO not good enough as one can easily experience much baro change on one start - depending on where you live and drive.

Similar to the MAT correction, I set the two point baro correction as follows: "At total vacuum = 100", "rate = 0". This has the effect of cancelling out the effects of the two point baro correction and ALL baro correction is then done with the Barometric Correction table. This gives an interesting insight as to how much fuel needs to be added / removed as a function of barometric changes.

Between MAT and BARO its not easy to get a VE table tuned that was not influenced by either MAT nor BARO during the tune as it means tuning the entire VE table at the same MAT and the same altitude. One needs a very long flat quiet road to be able to do this on the road. Then once the VE table is tuned one should then tune the entire MAT table at constant baro (altitude) and the tune the entire Baro table (ie all altitudes) at a constant MAT (although if the MAT table is now tuned already, MAT changes should not affect the baro table much).

I have found it better to first get the MAT table sorted at constant baro and then when I drive to different altitudes I ONLY adjust the baro table to keep the AFRs where they should be.

EDIT: In an ideal world, the MS tuner (OK, me) would therefore like to first have the MAT correction and Baro tables 100% correct and then tune the VE table. But, like I teach my kids, there's what you want and there's what you can have :D

EDIT 2: Please also read this if you are interested in Barometric Timing Correction for normally aspirated high compression engines: viewtopic.php?f=101&t=46373

Other opinions?
Last edited by Philip Lochner on Thu Aug 16, 2012 5:12 am, edited 6 times in total.
Kind regards
Philip
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Re: Baro and MAT correction.

Postby jsmcortina » Mon Mar 02, 2009 5:24 am

You are somewhat ignoring the ideal gas law though aren't you?

PV= nRT

I agree that evidence says that the theoretical correction is excessive, but cancelling it out seems to be a step too far the other way.

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Re: Baro and MAT correction.

Postby Philip Lochner » Mon Mar 02, 2009 5:47 am

jsmcortina wrote:You are somewhat ignoring the ideal gas law though aren't you?

PV= nRT


Not at all James! I fully subscribe to and acknowledge PV= nRT. Setting the MAt correction table such that all adjustment is cancelled out is just my STARTING POINT for tuning the MAT table, simply to eliminate the over/under adjustment that the internal MAT table produces. As I experience different Inlet Air Temps, this table is then adjusted as required to ensure AFRs are correctly maintained.

To help me know where my AFRs are, I use a gauge called AFRerror = afr - afrtgt1. This eliminates the need to know what exact AFR is called for at a particular operating point. All that needs to happen is that this gauge needs to hover around 0 at all MAP and all RPM, all MAT and all Baro. I also display this AFRerr gauge on my WUE, MAT correction, Baro correction etc pages.
Kind regards
Philip
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Re: Baro and MAT correction.

Postby evo828 » Mon Mar 02, 2009 6:19 am

Philip, is the "overcorrection" making the AFR leaner at higher MAT temps?
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Re: Baro and MAT correction.

Postby Philip Lochner » Mon Mar 02, 2009 6:33 am

evo828 wrote:Philip, is the "overcorrection" making the AFR leaner at higher MAT temps?


IIRC, higher MAT will result in leaner mixtures.

This comes from memory of the early days of MS2 with B&G 2.35 before MAT correction tables were introduced. Starting the engine after a good heat soak would result in a very lean idle.
Kind regards
Philip
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Re: Baro and MAT correction.

Postby Tjabo » Mon Mar 02, 2009 6:46 am

Thanks for a great read Philip!

I'm going to have to ponder it for a while, and I would say it's opened my eyes about a couple of things. . . I had been assuming that the corrections involved were assumed to be accurate. Hrmmmm. . . Seems like every time I turn around I find a new reason to pay ever closer attention to my VE table, and now the corrections to it also. Wow.

Thad
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Re: Baro and MAT correction.

Postby christophe » Mon Mar 02, 2009 10:54 am

I too have been having a few problems tuning the VE with differnet MAT's and air pressures, to try and get round the problem would it not be easier to use a MAF sensor as I assume this measures the actual Mass off air entering the engine and so will give a more precise measurement than having to correct the measured value for different air temps and pressures?

It seems like a no brainer to me but there dont seem to be many people out the using MAF's instead of the MAP so maybe there are other reasons?
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Re: Baro and MAT correction.

Postby evo828 » Tue Mar 03, 2009 1:19 am

Philip - can you please post the MAT correction which "zeroes" the built in ideal gas law correction? I do not have the Stim to do it your way. Thanks.
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Re: Baro and MAT correction.

Postby Philip Lochner » Tue Mar 03, 2009 3:03 am

evo828 wrote:Philip - can you please post the MAT correction which "zeroes" the built in ideal gas law correction?


Table attached. These are the figure obtained for the msq that I was using. I have used 20degC as the reference. I know that my Jag, without the cold air feeds it now has, breathed air of close to 80degC when hot! Now it rarely sees anything above 40degC.

Please note that one can not assume that the same figures wil also zero your table. It is quite possible that a different injection scheme will yield different correction values. I recon not having an accurate opening time for your injectors may also affect the actual result of the MAT correction table as measured by your WBO.

Best is to make sure.
Kind regards
Philip
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Re: Baro and MAT correction.

Postby jsmcortina » Tue Mar 03, 2009 4:29 am

How about a code feature to scale down the correction?
MS1 used a hardcoded air density lookup table, so that was hard, but MS2 does this:

Code: Select all
int aircor_eq(int mat)
{
    // returns air density correction from equation in % (100 is no correction)
    //   mat in deg F x10 or deg Cx10
    /* Following equations used:

       For Farenheit, air density (pounds/cubic feet) = .0391568 x map (kPa x 10)
       -------------------------
       ((mat(degFx10)/10) + 459.7)
       If we take 70 degF as standard temperature (no density correction) and ignore
       map, which is already accounted for elsewhere,then
       air density correction(%) = (70 + 459.7) x 100
       ------------------
       ((mat/10) + 459.7)
       For Celsius, with a 20 deg C reference and mat in deg C x 10
       air density correction(%) = (20 + 273.2) x 100
       ------------------
       ((mat/10) + 273.2)
     */
    if(flash4.Temp_Units == 0)  {    // deg F
        return((int)(529700 / (mat + 4597)));
    }
    else  {                          // deg C
        return((int)(293200 / (mat + 2732)));
    }
}


Could be changed to include something like. (This variable doesn't exist but would be EASY to add.)
Code: Select all
* airden_scaling / 100

Where a value of 100 equals normal correction and zero is no correction. We could then try to "turn down" the airden correction.
From the above equation, the actual values for Philip's table (at those temps) should be:
-7.3
-3.5
0.0
+3.3
+9.3
+14.6

James
EDIT: changed baro to airden.
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Re: Baro and MAT correction.

Postby Philip Lochner » Tue Mar 03, 2009 6:27 am

jsmcortina wrote:Could be changed to include something like.

* barocor_scaling / 100

Where a value of 100 equals normal correction and zero is no correction. We could then try to "turn down" the baro correction.


James, I'm a bit confused now. Are those references to "baro" correct (in which case I am totally lost...) or should they refer to MAT?
Kind regards
Philip
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Re: Baro and MAT correction.

Postby jsmcortina » Tue Mar 03, 2009 6:34 am

Philip Lochner wrote:Are those references to "baro" correct (in which case I am totally lost...) or should they refer to MAT?

Ah, oops. Should say airden. Post edited.

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Re: Baro and MAT correction.

Postby Tjabo » Tue Mar 03, 2009 6:41 am

This is pretty exciting stuff. The only reason I've really been interested in O2 correction was to deal with AFR variation that is probably related to this issue. Not having to deal with any of that (or a PITA MAF sensor as mentioned) would be fantastic! At the very least, the work that has to be done by the O2 correction could be minimized.

By the way, does the new code in the latest Beta that uses the AFR lookup have anything to do with this issue? I am having trouble wrapping my brain around what that is actually intended to accomplish. . . :?

Thanks for the enlightening discussion gents!

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Re: Baro and MAT correction.

Postby Philip Lochner » Tue Mar 03, 2009 6:57 am

jsmcortina wrote:Could be changed to include something like. (This variable doesn't exist but would be EASY to add.)
Code: Select all
* airden_scaling / 100

Where a value of 100 equals normal correction and zero is no correction. We could then try to "turn down" the airden correction.
From the above equation, the actual values for Philip's table (at those temps) should be:
-7.3
-3.5
0.0
+3.3
+9.3
+14.6


It looks like it could work James, and being an easy addition one could ask "why not?", although I'm wondering if such an addition is really necessary?

As long as the tuner knows (perhaps a few more red/bold lines in the manuals in the appropriate places??) to not ignore this MAT correction table (and the baro correction table) and knows that the issue becomes more relevant the further away MAT moves from 20degC (ie engines that draw air from inside the engine bay), he can tune the table in a non-linear manner in much the same way one would tune the non-linear baro correction table.

Perhaps the code could be released with the default values as "zero'ed out" values (no PW change as function of MAT) or perhaps halfway zero'ed?
Kind regards
Philip
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Re: Baro and MAT correction.

Postby jsmcortina » Tue Mar 03, 2009 7:04 am

My thinking was that with a single variable to tune we might discover that there is one magic number that is nearly right for everyone and we can then make that the default. The user would still have the option of tweaking it in a non-linear manner.

It certainly seems that 100% correction is too much, but we also know that 0% is too little.

What numbers have you ended up with ?

James
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Re: Baro and MAT correction.

Postby Philip Lochner » Tue Mar 03, 2009 7:10 am

Tjabo wrote:By the way, does the new code in the latest Beta that uses the AFR lookup have anything to do with this issue? I am having trouble wrapping my brain around what that is actually intended to accomplish. . . :?


This MAT correction has nothing to do with the AFR lookup ( AFAIK).

Consider the following Thad: Your engine is fully tuned - everything 100%.

For some reason you decide that at 2000rpm / 60kpa you want to change the AFR from the current 15 to 15.5 for improved economy.

Without this AFR lookup you would have to go drive the car at 2000rpm / 60 KPA and physcially tune the VE map at this point to affect the change.

With this lookup, you merely go out in your pajamas, switch ignition on (not even starting the engine) , plug your UI in, make the change in the AFR table, and go back to bed. Done. Tomorrow morning when you go drive the car you should find that you are now running at 15.5 at 2000/60. A very handy feature in other words. :yeah!:

.... in theory of course. You WILL see a change in the AFR by adjusting the AFR table but its accuracy will depend on the accuracy of the entire tune including opening time, opening time as function of volt correction, MAT, baro etc etc etc.
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Philip
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Re: Baro and MAT correction.

Postby Philip Lochner » Tue Mar 03, 2009 7:24 am

jsmcortina wrote:My thinking was that with a single variable to tune we might discover that there is one magic number that is nearly right for everyone and we can then make that the default. The user would still have the option of tweaking it in a non-linear manner.

It certainly seems that 100% correction is too much, but we also know that 0% is too little.

What numbers have you ended up with ?

James


It seems to me that the built-in correction sends the AFR more off-target the further away MAT moves from 20degC (either way) as you can see from my figures below (which does not serve as proof of any kind). This calls for non-linear correction which a single parameter (which will help no doubt) will not give.

I suppose the actual position where the MAT sensor is mounted will also affect this - and now we are back at the "what's the correct place?" debate :D (FWIW, mine is mounted in the air filter housing but I want to move it to inside the inlet manifold plenum as I recon the air is heated up quite a bit on its way from the air filter to the inlet manifold)

My current numbers are:
-4
-2
0
2
5
6

BUT: With our climate I never see MAT below 15degC (ie the -2 and -4 dopnt mean much) and with my cold air inlet I now never see temps much above 50. I could remove the cold air inlet and report what I need from 50 to 70degC.
Kind regards
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Re: Baro and MAT correction.

Postby evo828 » Tue Mar 03, 2009 7:56 am

Thanks for the info and tables - will play with it.
Just a comment on MAF sensor - even on a MAF sensor equipped car there are temp related calibration tables (have been playing with Mitsubishi Lancer Evo 8 ECU for some time) - so it is also not the perfect and final solution to fight against MAT corrections.
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Re: Baro and MAT correction.

Postby racingmini_mtl » Tue Mar 03, 2009 8:22 am

There is something that bothers me a lot with this discussion. The fact that there is some error is obvious from people's experience but the cause and the way to go about correcting it doesn't make sense to me. The physical properties of a gas won't change because you change some values in a tuning software. The actual cause has to be that the measured temperature is not the actual temperature of the air going into the engine. To me that's like saying: I have a box measuring A x B x C but my volume is not ABC so I'll change the equation to compute the volume instead of saying my measurements are off.

So, if a correction is needed it should be on the temperature itself and not by changing an equation from a physics law. The advantage of this is also that you will get a temperature that will be closer to what the engine is actually seeing (and you won't be breaking any physics laws).

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Re: Baro and MAT correction.

Postby arlo#1 » Tue Mar 03, 2009 9:17 am

Philip Lochner wrote:
jsmcortina wrote:




I suppose the actual position where the MAT sensor is mounted will also affect this - and now we are back at the "what's the correct place?" debate :D (FWIW, mine is mounted in the air filter housing but I want to move it to inside the inlet manifold plenum as I recon the air is heated up quite a bit on its way from the air filter to the inlet manifold)

My current numbers are:
-4
-2
0
2
5
6

BUT: With our climate I never see MAT below 15degC (ie the -2 and -4 dopnt mean much) and with my cold air inlet I now never see temps much above 50. I could remove the cold air inlet and report what I need from 50 to 70degC.

Philip I am in question of where I have my mat sensor as well (mine is in the side of a throtle bodie but air does not flow over it) I figure up in the air stream will give me quicker readings. But I am thinking in the vacume side of the intake will be a problem because depresureizing air drops in tempurature so at light throttle it would be colder in the intake then wide open (not looking for an aurgument just very interested in what you think) I have had tuning issues with my MAT correction close to zero but thanks for pointing out it corrects on its own. I was wondering if it would be possible to have the mat correction dissabled till the coolent temp was hi enough so they arent fighting each other.
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