Thursday, February 14, 2008

Temperature Models are important

Both E38 and E40 ECU's have a Temperature Bias table with both Airflow and Speed axis, making us think that it's using both to determine the proper Bias value. But to just make sure that's truly the case, I made a little experiment:

How closely can I can predicting the temperature calculating it from components (Airflow, Speed, Filter, IAT, ECT) to the scanned values of Manifold Temperature?

I wrote few Matlab scripts to take the component values, and use the model I thought it was using. I must admit I went from pure observation and gut feelings on this, but it worked out great.

Test #1:
I made a simple model without any Speed dependencies, the Bias table was purely based on Airflow. I used it to model some E38 data, and it was just rubbish, completely different look and feel to the scanned vs the calculated values.

Test #2:
I expanded the previous temperature model to include Speed in predicting the scanned Manifold Temperature values. And the results were very positive: not only the graphs of predicted vs scanned values were very close to each other, but the set of parameters that yielded the best fit were very similar to the values in the tune which was used to generate this log!

Test #3:
A month or so passes, and I get some E40 data. Blindly, I grab the Temp modeling scripts I've been working with (the ones that worked so well for E38 data), and use them to predict some temps on the E40 values. The result was confusion and big disappointment: the predicted values were somewhat following the scanned ones, but the differences were sometimes quite large (~10 Kelvin). (click on pictures for bigger, more readable versions)

Then I remembered that I'm using the newer model on the data generated by an older ECU. Quick edit of my scripts, and the new predicted values were dead on overlayed the scanned.

This is important for few reasons:
1. We now know the temperature model for E40 (uses Airflow only for the Bias table)
2. We now know the temperature model for E38 (uses Airflow and Speed for the Bias table)
3. We now know that the Bias tables in the two ECU's even though they show the same in HPT (I haven't looked in EFI yet) are actually used very differently. This is a glorious display of phrases like 'looks might be deceiving' and 'verify your assumptions'
4. Newer models aren't necessarily backwards compatible.

Now that we know this, we can do some cool stuff, but more on that later...


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