Originally Posted by Brains

Actually, 20ms @ 6000 RPM isn't physically maxed out for the injector, it's 50% ... think about it -- it's the max open time for that HALF of the engine revolution. The PCM obviously knows it can hang the injector open longer, because it DOES -- and the motor doesn't starve for fuel where it otherwise "theoretically" should. In order for the injector to go mechanically static (ie. open 100% of the time) your duty cycle would need to be 200%, or have a 40ms pulse width at 6000 RPM. As duty cycle increases, you spray more and more fuel on the back of a closed intake valve. Is it optimal? Heck no, but it works.

Originally Posted by Colonel

Chris, I agree with everything you said there. Don't misunderstand me, I'm NOT advocating running things on the ragged edge of safety. I just want people to have an accurate and realistic understanding of what these fuel systems can do and the danger (or relative lack of danger) involved in running the injectors beyond what some people think they are capable of. In a nutshell, it bothers me that there is a somewhat common belief that stock injectors, even the 28.8 lbers, will safely support operation in the 420-430 range and no more. That is just false and that's my point. I've become a bit frustrated that oftentimes people think their injectors are totally maxed out due to some calculation or PW monitoring when in fact, their injectors will support more.

Now running a power adder, sure, overkill on the fuel system is JUST RIGHT! I've always been an advocate of that. But running NA, you're not going to be running a nice 13:1 fuel/air at 450 RWHP and then all of a sudden an injector stops flowing and you melt a piston. It just doesn't happen...well, anything is possible, but you get my point.

Regarding fuel pressure. Sometimes people *think* their injectors are maxed out because their duty cycle is maxed. Well, if the fuel pressure is 45 psi, you have to increase the duty cycle to maintain the fuel/air ratio. If you bring the psi back to 55+, where it should be, you can bring the duty cycle back down. Were the injectors maxed? Maybe so. Was it the fault of the injectors? No, the fuel pressure was low. How many people running serious power have a pillar mounted FP gage? Not many...but everyone running a PA should have one and it's a really good idea for anyone.

Since we're on the subject, who has had an NA LS1 engine failure due to an injector failing due to being run at a high duty cycle (injectors can fail for other reasons)? I'm sure it's happened before but in my years, I've NEVER heard of it. Not even with all of the 470-520 RWHP large CI engines running around with stock injectors. Why? Because when people notice their O2 readings are lean, they upgrade their injectors and pump. The ones who've not reached the point of leaning the mixture back excessively (even if their injectors are doing all they can) seem to run like this indefinitely and without problem.

Originally Posted by stevieturbo

I havent done the calcs, but I do not believe you are right.

When using 1 injector per cylinder, my ECU wont even allow me to enter numbers on the map higher than 20ms at 6000rpm, and dropping lower as the rpms rise, and naturally time to inject fuel decreases.

I am not currently firing my injectors sequentially. Some ecu's may falsely report higher duty cycles for some strange reason. Ive seen a few do it, but it is false.

I would say the only way to know for sure, is to stick a scope onto the injector output. Id be confident that at 20ms at 6000rpm, it would be flatlined open all the time.

Originally Posted by stevieturbo

The issue can be that on some injectors, above 80% or thereabouts whilst it will still flow fuel, actual fuel flow may not be as linear per duty as it had been at say 2% to 80%

So all cylinders may not be getting equal amounts of fuel and you would not be aware of it.

That said, Ive run injectors on plenty of cars to 95% and left them at that and never experienced any problems.

Originally Posted by joecar

The problem with running more that 70% DC is that the injectors now start spraying against open intake valves... it appears that stock GM calibrations time the injectors to spray against closed intake valves for various reasons (promote evaporation of sprayed fuel, promote cooling of the intake valves).