What method do you use to determine when you're maxing out injectors?

I'm looking at some logs here... these late-model, Gen III, standard-gasoline-only trucks come with 25lb/hr injectors, right? I'm scanning them flowing at 22.X The numbers have my attention.

How much more 'breathing room' do I have?

 

Is that 22 lbs/hr or 22ms pulsewidth?

Most guys calculate injector duty cycle and shoot for less than 80% (ideally). I run up to 90-95% duty cycle on WOT runs at the top end. BigTex was running 125% duty cycle before he upgraded to 42lb injectors.

Here is the formula to calculate injector duty cycle:
%duty cycle = {GM.IBPW1}*{SAE.RPM}/1200 (EFILive PIDs)
or %DC = base pulsewidth * RPM / 1200

 

How do you run more than 100% duty cycle? That just doesn't make any sense to me

 

Oh-no... That's 22ms.

The pretty chart display in HPT "tops out" at the number 25, so I assumed it was the (alleged) injector rating of 25lb/hr.

Thanks for the formula's - FWIW, I got a 79.9% DC.

 

Well, what it really means is that the PCM is trying to command the injector to flow more fuel than it is capable of. Basically the injector is open all the time when this happens.

 

Think of the duty cycle as a work load schedule. Your boss can double schedule you to do 30 hours of tomorrow, but there is only 24 hours in a day. You can't physically do 30 hours of work tomorrow, so 6 hours of work isn't getting done. You've just been asked to do a 125% work load - a 125% duty cycle.

The PCM knows what the injector can flow, and it knows how much fuel is needed for a given combustion event. The problem occurs at high rpms where the time between each cylinder ignition happens very quickly. You get to a point where the PCM needs to open the injectors longer than is possible. Say there is 20 ms between the combustion cycle but the PCM knows it needs to turn on the injector for 22 ms - thats a 110% duty cycle. They can't flow 110%, but are being asked to do it by the PCM. At this point the injectors are open constantly but it still isn't enough fuel to meet demand. You start going lean now.

 

That's theoretical duty cycle NOT actual duty cycle. That reminds me of the TV commercials that say you can refinance your house for 125% of its' equity. Now that's a good trick. When your PCM tries to drive your injectors at 125% your on the verge of letting the smoke out of the injector coils and once the smoke gets out of them it's very hard to get it back in.

 

Injectors start to go static at/about 80% duty cycle.

 

My injectors lasted about one month at 125%, then I lost two. RIP

 

See, you let the smoke out. If you can get the smoke back in you should be OK