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# Why do GM use both MAF and VE?

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Why do stock GM cars seem to run a combination of both MAF and VE?

Is there any truth to the MAF not being able to react fast enough in some situtations? (lower rpms and fast throttle transients)

It seems a lot of tuners will only tune the MAF and seem to disable the VE side of things. Is this good practise?

Cheers

Benno

Hi, does anyone know why this is? Or is this covered in detail within any of the HPAcademy courses?

Cheers

this is an explanation on the topic that I was just reading, you can find the entire thread here: https://www.ls1.com/forums/f9/tuning-guide-i-found-based-hptuners-46733/

"If the VCM decides that a throttle transient has occurred (unsteady MAP), the airflow incorporates a "correction" from the SD calculations. This value is the SD calculated air mass multiplied by the previous ratio of measured MAF to airmass to calculated SD air mass (this normalizes it, since you are worried about the transient deviation from MAF to airmass only). The way I understand it is this, imagine you are cruising (MAP is steady), you have a decent vacuum in the manifold and you change the throttle position quickly. Air rushes into the manifold to service the change in air demands from the engine itself but also to try and fill the vacuum. The result is that the MAF reads higher than it should at that point in time (spikes). This is more pronounced at low RPM where the engine airflow is smaller and the relative proportion of extra airflow due to filling vacuum is higher, also the MAF is known to be more inaccurate at lower RPM and more non uniform airflow. IMHO, the SD correction is to account for filling and emptying of the manifold during throttle transients and also to smooth the MAF's spikiness at lower RPM’s.

*The bottom line is that if the engine is at a steady load state or operating at high RPM then the airflow is 100% based on the MAF once you get thru all the filters and calculations. And the SD calcs only get used for transients and smoothing lower speed operation.*At no stage does the MAF get ignored completely in these calcs (the dominating terms of the main filter calc are always MAF based)."

Here is an earlier thread from this forum in which Raymond ( @Arghx7 ) explains the concept quite well, though more from the standpoint of the driver's "torque request" via the throttle pedal: