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Old 08-13-2003, 09:32 PM   #1
turboICE
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Default Nasty analysis-so why is MAP used as Fuel Load? no dial-ups

OK, the premise - I am messing around with Xede and its flexibility for different loads among other things. I get to thinking why does everyone adjust the MAF signal (how much air the ECU thinks is coming in) based on MAP? I guess for one that is the only load other than TPS many have available. But anyway I want to prove to myself what would be the best load to alter the MAF signal with.

These all come from a thirty minute commute where I get to open it up once in a while. All stock ECU readings in Delta Dash. I reduce the log to the area I care about (and reduce the points to where Excel will graph them) by doing the following:

- I delete all events where the TPS is <= 1.0 volts to focus on open loop.
- I delete all events where the MAF is < 2.0 volts because I notice visually that AFR generally is not in that unloggable realm of below 11.24 unless the MAF is > 2.5 so I figure I have all important events at 2.0 and higher.

I want to lean out the stock maps so I start with IDCs to see what their relationship is with AFR. Well of course fuel flow is only one part of the equation, so correlation is not great but that is due to a wide variance of AFRs at low IDCs.

I figure that the AFR is primarily a function of MAF and IDC and perform the regression. Turns out pretty good though I was hoping for more correlation - still a lot of variance around low IDCs and now around low MAF voltage. Not too surprising but a pretty good regression line actually.

So alright lets throw AFR against MAP, MAF and IDC. Ugh correlation still not great - but the regression looks a lot stronger. OK I am sastisfied that AFR can be reasonably predicted against MAP, MAF and IDC except in their lower operating ranges because the three just move around differently a lot at those levels. Well I don't want to try to build a MAP loaded and a MAF loaded fuel map so lets think of a way to choose between the two.

I decide to regress MAP to IDC and MAF to IDC - why? - because I can only control air flow via the TP and that is going to drive the fuel map to predetermined levels, so I have to control the fuel flow through the injectors just like all of us are.

Long story short - in the operating range that matters to tuning MAF is a much better predicter of the IDCs that the stock ECU will drive than MAP is - and to a greater or lesser extent my AFR.

So if I want to ultimately alter the MAF input anyway why not make the fuel map MAF loaded instead of MAP loaded? Especially since with Xede I can.

That is my question to you all why not MAF?

Keep in mind I come from a high compression small block V-8 background - I still don't buy into this MAP drives everything in tuning.

Note below that the regessions have very good fit but the correlations are a bit flaky when it comes to low operating ranges. But the regression and correlation between MAF and IDC are solid on.
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Last edited by turboICE; 10-18-2003 at 12:00 AM.
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Old 08-13-2003, 09:51 PM   #2
Concillian
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I dunno about the XEDE, but the UTEC uses BOTH RPM and MAP as the load references.

Try looking at a particular RPM range, and that IDC vs. MAP plot will have a MUCH better correlation.

The MAF plot is mostly independent of RPM because it's actually measuring airflow. It damn well better have a strong correlation to AFR, because the ECU uses MAF voltage (airflow) to determine fuel flow.

Using MAF voltage doesn't really tell you the engine load, only how much airflow you have. The engine could be fully loaded at low RPM or much less loaded at high RPM and result in the same IDC and/or MAF voltage.
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Old 08-13-2003, 10:37 PM   #3
turboICE
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I consider RPM to be a given load in any map and negates accross comparison. My discussion is premised on RPM being the first coordinate in any map.

However great idea for further refinement since I need to predict the effect of any given map value on IDC and by inference AFR.

In 400 rpm bands which is the break points of my maps correlation MAF/RPM to IDC and MAP/RPM to IDC both have Multiple R of around 97% and Adjusted R of 93%, great correlation. So I now have band formulas and changes within the band and a better prediction of the outcome of an adjustment - but in no case do they stastically differ between each other in correlation to IDCs within 4 significant digits.

So this additional refinement to the analysis, tells me MAP/RPM is closer to MAF/RPM than I previously thought, good for me to observe.

However with regards to independence to RPM (which if I recall cannot be proven or disproven within this analysis - only suggested) MAF has a significantly higher correlation with RPM than MAP does. Like 50% compared to 23% depending on the band.

Now if my reasoning is sound - in a 2d fuel map RPM actually is a pretty good measure of approximate levels of airflow - but not a good measure of MAP. So given the close relation that MAP and MAF have to IDCs and given that MAF levels are broadly implied by RPM but MAP is not - in total a MAP/RPM fuel adjustment map should be more refined than MAF/RPM - since on a piggy back none of us has MAP/MAF/RPM 3d mapping available to us.

So that is a reason I can now see, feel and relate to in using MAP instead of MAF to drive my MAF adjustements.
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Old 08-14-2003, 05:31 AM   #4
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Quote:
Originally posted by jehcpa
Now if my reasoning is sound - in a 2d fuel map RPM actually is a pretty good measure of approximate levels of airflow - but not a good measure of MAP.
Airflow is dependent on RPM and MAP. Both are equally important to determining airflow, neither should be a good correlation (and they aren't):

displacement * RPM * AbsoluteP * VE / 2 = airflow

VE is what is missing, and indirectly you are tuning for it, just from a couple of orders away.

This is why the TEC products (others probably do too, I just don't know) have an RPM/VE map as well. You pretty mcuh get a full and accurate 3d map of airflow that way.
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Old 08-14-2003, 07:06 AM   #5
turboICE
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Absolutely - it is preferable that my load points for the map are not correlated to each other, but together are highly correlated to airflow and by inference fuel flow.

Thanks for helping me think through this, still trying to make the conversion to forced induction in my thinking.
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Old 08-14-2003, 07:29 AM   #6
turboICE
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Concillian - you rock thanks for being so helpful -

MAP and RPM regressed to MAF Voltage - doesn't even have to be broken down in rpm bands:

Doesn't get much better than that! So I see that VE fills the rest in, but the major drivers are present. Also as you said tuning the AFR compensates for the VE factor not being used in my adjustments.

Doesn't ambient temp play a role in there somewhere though?

Last edited by turboICE; 10-18-2003 at 12:00 AM.
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Old 08-14-2003, 07:34 AM   #7
john banks
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displacement * RPM * AbsoluteP * VE / 2 = airflow

Charge temperature is what matters.

I was planning on using absolute temperature in Kelvin divided by 293 to correct to ambient of 20C for my MAP to MAF conversion. Then use a MAF calibration table in reverse to simulate a MAF voltage. Must get around to doing it.
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