There are three main parts of the fuel adjustment dialog: global fuel, dead time, and the typical RPM-based adjustments that you might expect to see. Global fuel and dead time are adjustments that get applied all the time. These adjustments are provided to compensate for aftermarket injectors. RPM-based adjustments are only made during certain modes of open loop operation in the ECU, primarily the WOT open loop mode. The fuel adjustments dialog is shown below.

Aftermarket injectors are sized "larger" than the factory injectors so as to provide more fuel in the same amount of time. This scaling of fuel flow is easy to calculate and deal with. Simply divide the old flow rate by the new rate and subtract 1 to estimate the "global" adjustment you need in order to compensate for the new injectors. As an example, consider the case of 720 cc/min injectors.

Adjustment = 450 / 720 - 1.0 = 0.625 - 1.0 = -0.375 or -37.5%

So in order to scale the ECU's fuel curve down to compensate for 720 injectors, we would need to reduce the ECU's fuel calculations by 37.5%. But there's a trade off with larger injectors. You don't just get more fuel flow; you also get slower response.

The ECU takes into account the response characteristics of the factory injectors when calculating how long to "fire" them. It first calculates an appropriate time from airflow, knock, and all sorts of other data. It then adds an amount of time based on battery voltage to account for how long it will take the injector to physically open up after being told to do so. This injector response time is referred to as dead time, dwell time, saturation time, and probably a whole slew of other names. It's a characteristic of injector design and size.

The Dead time area of the Fuel Adjustment dialog is where you can make adjustments to this dead time figure. The value entered into this area gets added to the value calculated by the ECU. While there is a way to calculate what this value should be, a trial and error approach tends to work best. Use the long-term fuel trim (LTFT) value calculated by the ECU as your guide. The following outlines a good procedure for getting the dead time right for your setup.

• Car must be fully warmed up then must be operated at cruise for about 10 minutes followed by idle for 5 minutes to get stable LTFT values.
• Compare the LTFT at idle and cruise. You want both to be right about zero.
• If the two are about the same, dead time setting is good.
• If they are equal but non-zero, adjust the global setting.
• If idle LTFT is more positive than cruise LTFT, increase the dead time, less, decrease.

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