Learning to tune....

[XfireZ51]

Since this is a tuning thread, here's a good description about PW. Part of the basics.

"I take it you're referring to the complete concept behind pulse width and fueling, so taking a shot at this: the pulse width is the key to how much fuel will be injected, since the ECM can't influence things like fuel pressure and it don't have a way to directly measure fuel flow through the injector.

So what we do is this: knowing that an injector flows X amount of fuel at Y pressure (let's assume static pressure for now), resulting in a mass flow in pounds per hour, we can assume that holding the injectors open for say a tenth of an hour will result in a tenth of the fuel. The same thing happens when you hold it open for fractions of a second.
Here's your pulse width, or the easier way to refer to it - the short window of time where the injector is open.
Remember that this happens every (other!?) revolution of the engine, and hence that an engine rotating at 6000 rev per min (= 100 rev per sec) will only leave 1/100 seconds (10ms) per revolution for this window at max! This is also where the Duty Cycle comes in, expressing the ratio of time that the injector is open (PW) to the maximum possible opening time due to the engine speed. 50% means your injectors are only open during half of this window.

Furthermore, the injector offsets which we've been looking at, compensate for the time needed for the injector to open and close, because it is a mechanical device. It will take some time for the electromagnetic actuator to energize and lift the pintle of the seat (injector opening process). If you go back to kevm's post, he stated that e.g. an extra 400usec (400 microseconds, 0.4 millisec or 0.0004 seconds) are needed for this. When you have a PW (pulse width, time of injector opened) of 1.2 msec (1200usec), this is quite a big influence (33% difference). At 6 msec PW, 400usec are not quite as critical (6.7%).

By the way the opening injector pintle is also acting against the prevailing fuel pressure, that's why the injector offsets need changing by quite some amount when you raise fuel pressure."

-from Third Gen