Why injectors need a small-pulse correction
For most of their range, an injector delivers fuel in direct proportion to how long it's held open. Double the effective on-time and you get about double the fuel. Right at the bottom, though, the pintle is still settling as it opens and closes, and that neat relationship breaks down. The injector flows a bit more or less than the simple math predicts.
That's the zone the Short Pulse Width Adder fixes. It matters most on big injectors at idle and light cruise, where pulse widths are short enough to land in the non-linear region. It's the reason a large injector can idle smoothly with good data, and hunt for lambda without it.
Where the data comes from
This is characterization data measured by the injector manufacturer on a flow bench. It isn't something you can realistically work out on the car. Makers such as Injector Dynamics publish small-pulse data for their injectors, and most modern injectors follow a similar curve shape. If you can't get published data for your injector, that's a strong reason to choose one that has it.
Setting up the table
When enabled, each injector stage gets its own table. Set it up like this:
- Set the axis to Effective Pulse Width. That channel is the calculated pulse for that stage with the dead time removed, which is how manufacturers present the data.
- Start the axis at 0.000 ms and add breakpoints to match the manufacturer's data points.
- Enter each cell as an adder in milliseconds. Positive values lengthen the final pulse and negative values shorten it, within a limit of plus or minus 5 ms.
- Make the last cell 0 ms. Past the end of the data the injector is linear, and a leftover adder would keep skewing every longer pulse.
The adder sits on top of dead time. If your dead time data is wrong, no small-pulse table will fix it. Enter both from the same manufacturer data set. See injector dead time for more.
How to tell it's working
With correct data, lambda at idle and light cruise should behave like the rest of the map. The fuel table shouldn't need a strange dip or bump just in the lowest load cells. If you find yourself carving odd shapes into the base fuel map down there, check the small-pulse and dead-time data before touching the fuel table. The injector flow settings are the other piece of the same picture.
