Originally Posted by gross polluter

Correct, we use larger injectors to lower duty cycle. When we flow more air, we want more fuel, no? Stock injectors go static with minimal mods. In other words, when we hit 100% duty cycle we lose the ability to supply the engine with more fuel because the injectors are flowing at their maximum.

There's two ways to overcome this:

-Increase fuel pressure thereby increasing injector flow. People do this with an FMU and raise the pressure at ratios higher than 1:1

-Upgrade our injectors so our duty cycles are lowered thereby increasing fuel flow.

So, at the point where the 550s are at 100% duty cycle, our 720s will be at roughly 76.67%. We have another 23.33% duty cycle to go before the injectors go static which means we have the ability to supply more fuel than the 550s.

I think where you're getting really confused is how the ecu calculates fuel. The ecu calculates load and looks up a fuel value on the map at that specific RPM to come up with pulsewidth, not duty cycle. Duty cycle is a function of RPM and pulsewidth. It's a number we use to determine the sufficiency of a particular injector for a given engine.

To correct you:
1) 550 injector on stock ECU:
ECU calculates 5ms, sends 5ms to 550 injector = .046 cc fuel

2) 720 injector on stock ECU:
ECU calculates 5ms, sends 5ms to 720 injector = .06 cc fuel

3) 720 injector with rtek 1.7 or 1.8:
ECU calculates 3.8ms, sends 3.8ms to 720 injector = .046 cc fuel

There's no difference between 1 and 3, but we WANT it that way, otherwise we would be running rich all of the time.

What it all comes down to is that the ecu programmed for larger injectors will always supply more fuel, when needed, than stock injectors.