1000+ HP Supercharged FE

[ByronRACE]

At 100% duty cycle, the theory is that the injector coil will overheat to the point where the impedence changes and causes a catastrophic failure. I've heard this theory many times, but have ever seen an actual demonstrated case. The root cause is always something else once I investigate.

I've wired one of these ford alternative fuel injectors to a 12V, 10A capable bench supply and left it on indefinitely (this is not what a peak&hold driver does, this is flat-out abuse). The current flow and impedance never changes...and the coil never gets hot enough to melt anything. In fact, it gets to 140deg F after about an HOUR, and is quite boring to watch. Nothing interesting happens. No current changes, no impedance changes, no problem.

In my application I'd be lucky to see 70% duty cycle 10 seconds at a time...and I'll be using a true peak&hold system that only delivers less than 1/2A of current, so melt-down is impossible.
I've also done this same test with GM injectors after being told that they would fail. They don't. What fails is PCMs that don't perform true peak&hold, or running a peak&hold type low impedance injector on a PCM that is designed for high impedance injectors. I've seen both cases, and in both cases bad things happen...to the drivers in the PCM. I've never seen a "fried" injector due to current-induced thermal failure.

My pressure change from 40psi to 20psi changes the flow rate through the injector from 160lb/hr to 110lb/hr. That's still 1/3rd of a gallon of fuel per minute at 100% duty. There's no way an injector flowing a gallon of atmospheric temperature fuel every 3 minutes at 100% duty is going to get anywhere near critical temperature in under a minutes time. And, as I said...I would be lucky to see better than 70% duty and the duration will be more like 10 seconds. It's not an issue.

If you saw a failure doing this, I'd be curious to know which pcm and which injector you were using. The injector drive strategy is probably not correct. Peak current must be applied for far too long to build current like that. In a proper peak&hold strategy, on an SEFI injection system, the longest possible "hold time" is around 20mS on a 6000rpm engine. And, the "hold time" represents better than 99% of the entire injector pulse duration. During hold, the driver should supply about 1/2A of current. If the injector is say 5ohm, and you're flowing 1/2A, the coil is consuming 2.5watts of power. There's simply not enough power there to build enough heat to destroy the windings before the heat disspates through the surface area of the injector; whether or not fuel is flowing.

It might also help to know that I've done this before using Siemens Dekka 83's in at least 6 customer cars. It's a well known way to increase fuel pump flow if you are running a large injector on a car that doesn't need the full flow capacity of the injector. Simply drop the delivery pressure and the pump can deliver more volume.

In addition, the same applies to the "oh no your injectors are at 100% duty that will destroy them" myth. 100% duty is fine; it doesn't hurt an injector. It does mean your injectors are too small and you'll probably need to do something about it...but as for the injectors; they're fine at 100% duty. No damage done. I've tuned silver-state-classic cars that ran 170+mph for 80+ miles foot-to-floor with 100% duty railed injectors. Fuel was augmented with a fuel pressure control system with wideband feedback to maintain 11.5:1. Worked fine. No dead injectors. It's amazing how much misinformation is out there.