max injection time available

[Boost]

"A few tuning issues are evident in this particular display. The VE table needs some adjustment because commanded air/fuel ratio is 12.96:1, but the LM1 wide band is reporting 13.3:1. Also evident is a potential fuel supply problem. Pulse width is shown as 18.250 milliseconds at 5,700 rpm. With only 20 milliseconds of time available at 6,000 rpm, the injectors are too small for the amount of power the engine is producing"

Read the above from the main site (Corvette Fever article), and I realized I am not sure how one would know or what limits the injection pulse to 20 milliseconds... may be dumb but I must know to learn. Thanks!

To clarify, is 20 ms @ 6k rpm the given spec for that specific injector, or is that around the general rule for most injectors? He needs bigger squirters to flow more during those same 20 ms, right? Would the driver overheat or the injector hang open with any more?

[joecar]

The time period for 1 complete cycle (intake, compression, power, exhaust) is the time it takes for 2 complete crank revolutions...

rev/sec = RPM/60 so 1 rev takes 60/RPM s.

Time for 1 cycle (2 revs) = 2/(RPM/60) = 120/RPM

So 6000 RPM gives time period 120/6000 = 0.02 s = 20 ms

So at 6000 RPM, if the injectors have at most 20 ms to deliver the required fuelmass;
20 ms at 6000 RPM is 100% duty cycle;
15 ms at 6000 RPM is 75% DC.

[dfe1]

The time period for 1 complete cycle (intake, compression, power, exhaust) is the time it takes for 2 complete crank revolutions...

rev/sec = RPM/60 so 1 rev takes 60/RPM s.

Time for 1 cycle (2 revs) = 2/(RPM/60) = 120/RPM

So 6000 RPM gives time period 120/6000 = 0.02 s = 20 ms

So at 6000 RPM, if the injectors have at most 20 ms to deliver the required fuelmass;
20 ms at 6000 RPM is 100% duty cycle;
15 ms at 6000 RPM is 75% DC.

That's absolutely correct. The point to be aware of is that 20 milliseconds is the maximum amount of time AVAILABLE. Some amount of time is used during the actual opening and closing cycles, so if you're commanding 20 milliseconds at or above 6,000 rpm, you're running short of fuel. The injector doesn't open and close instantly so the amount of time that the injector is open and discharging a meaningful amount of fuel is less than commanded pulse width. To keep things safe, if I see commanded pulse width times of more than 16 or 17 ms at 6,000 rpm, I install larger injectors.

[joecar]

Good point, the injectors take time to ramp open and to ramp closed.

[Boost]

Thank you so much, that makes perfect sense. Cleared up another small thing in the way of developing my understanding of all this.