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ECMLink RPM-based Fuel Adjustments

When you change the RPM-based fuel sliders, the ECU doesn't always use the adjustments you entered. RPM-based adjustments are only made during certain modes of open loop operation in the ECU, primarily the WOT open loop mode. And even at WOT, the full entered value may not always be used. The RPM-based fuel sliders can be thought of as adjustments to the A/F mixture only when you're running beyond the highest airflow curve in the ECU. This curve corresponds to roughly 2.1 g/rev (AirflowPerRev) or about 16-17psi. When you're running wide open throttle with less airflow than that, DSMLink will interpolate according to the fuel curves defined from the factory.

V3 NOTE: V3 changes this even further to take into account the extended load indices provided by V3 functionality. If you view LoadFactor from your logs, you'll see the value used to cap and extrapolate these fuel slider values. In V3, you get no adjustment at all below a LoadFactor of 0.8 (roughly atmospheric pressure) and you get full adjustment above 1.6 (roughly 11psi). Between these two points, the ECU will interpolate your RPM-based fuel settings for you.


Let's assume you're not running enough airflow to land on the highest airflow curve in the ECU (2.1 g/rev) at 6000 RPM. Let's assume instead that you're running on the middle airflow curve (there are 16 or so curves). As outlined in the Tuning Guide, the 2.1 g/rev fuel line results in roughly 55% enrichment over stoich. The middle line results in roughly 35% enrichment. At 55%, the ECU will attempt to run the car at 9.5:1 A/F ratio (14.7 / 1.55 = 9.5). At 35%, the ECU will attempt to run the car at 10.9:1 A/F ratio (14.7 / 1.35 = 10.9).

So the ECU is already pulling a good bit of fuel out for you if you focus primarily on the highest airflow curve for your tuning. What we wanted to do was to keep things simple, so we interpolate for you under the covers. If you were to attempt to dial in 11:1 A/F ratio using the highest airflow curve (by plugging in -14% into the 6000 RPM slider), the ECU will actually use -9% by interpolating between the highest enrichment defined by the ECU and the enrichment that you're actually on. So as long as you're on the highest enrichment curve, you'll get the full adjustment entered via DSMLink. But as your airflow drops off, the ECU starts to lean things out for you already. In those cases, DSMLink attempts to smooth out your entered values so as not to run you even *leaner*.

Now if you run the math, you'll notice that -9% on the mid airflow curve actually results in 12:1 A/F ratio instead of the 11:1 that would have resulted from the original -14% adjustment on the highest airflow curve. The reasoning behind that is that the factory defined a leaner mixture at that airflow curve for a reason. They did all sorts of tests and measurements and such to decide that at those low airflow values, the engine could tolerate a leaner mixture. They defined almost 11:1 at that airflow versus 9.5:1 at the highest airflow. So instead of our trying to maintain the original 11:1 ratio the user entered by plugging -14% into the fuel slider, we allow the mixture to run a little leaner according to the ratio established by the factory.

howfueltableajustmentsareapplied.txt · Last modified: 2012/01/04 13:21 by twdorris