Injector duty cycle

I have a question. I'm using Direct scan and I'm showing Injector Duty Cycle at 111% at the end of the quarter (5000 RPM)

How can you get any more open than open? Open all the time would be 100%.
The DirectScan number is a calculation, not a "reading" from the ECM. If DirectScan is showing a number that is higher than 100%, then the ECM is commanding a pulse width that is longer than the time available between intake strokes. In other words, the injectors are wide open.

The pulse width being displayed in DirectScan comes directly from the ECM. The % duty cycle is a number that DirectScan calculates.

BTW, if you're getting duty cycles over 100%, you need bigger injectors! :)
Fundamental truth about fuel injectors - they get unstable and non-linear above about 85% duty cycle. There are two schools of thought on injectors:

1. Get smaller (like 42 lbers or less) injectors and go ahead and run them up to 100% duty cycle. Live with the inconsistent fueling.
2. Get bigger injectors (like 55 lbers or higher) and keep duty cycles down (less than 80%). Have more consistent fueling at high rpm.

Most people do Option 1, and they live OK. I was an Option 1 guy for quite a while (red stripes). It did seem like my car was more eratic at high rpm, though. Sometimes it would knock, and sometimes it wouldn't. O2's were reasonably consistent. Recently, I got 57 lbers. Although I've only been to the track once with them, they seem a lot more stable at high rpm's. I've already run as high of boost/timing with them than I ever ran with my red stripes, and duty cycles never exceeded 80%. Didn't ping once.

After doing both, I'm an Option 2 man now. Yeah, it's more money, but I will bet that my head gaskets will last longer with the more consistent fueling.

Food for thought...
You might need a bigger fuel pump. That is the problem I'm having. I can't run open exhuast without leaning out and my injectors are at 100%DC with C-16. Check to make sure you have enough fuel pressure at WOT. If you don't, you probably need a bigger pump.
I would go with the 50's and back them down to get your EGT's where your car runs best. I have my 50's at 76% with alky and the fueling seems to be perfect. Pushing any injector too hard will make it run hot and spray like crap, so in this case a little bigger is better. Not to mention it will save you the money and trouble of having to upgrade so soon when you change your combo....and it will probably chage numerous times over the time you own that car!
Fuel pressure is okay. New Walbro 340 w/wire kit.

I have enough fuel pressure for the amount of boost I am running.

The duty cycle doesn't start to get high until like around 4600 rpm. I go through the traps at 5000. So it's not until the very very very end of the quarter that my duty cycles go higher than 100. It's almost as if I'm running out of injector just as I cross the line. So, I'm border line needing them.
The absolute maximum time the injector can fire is from the time the intake valve opens until the next time that same valve opens 2 crank revolutions later That time, in milliseconds, from intake valve opening to intake valve opening (2 crank revolutions) is = 60,000 x 2/rpm.
At 6000 rpm you can only have a maximum of 20 ms. The higher the revs, the less time there is for the injector to fire.

The % pw is what the computer is commanding divided by the maximum possible above. So if at 6000 rpm the ecm is commanding 15 ms, you've got 15/20 = 75% and you're okay, but if it is commanding 25 ms (start pushing over 5000 rpm) then 25/20 = 125% and you really need bigger injectors.

Yes, you are on the edge and pushing it. IMHO, if you are satisfied with your times and do not wish to go faster, and will rarely be going to the track... you could live with what you have. But... if you want to be safe, knowing that you are going to push it, step up to larger injectors. (It's cheaper that head gaskets and bearings ; )

IMO you need to know how your chip is setup and how much fuel your combo needs. I have a original 50# lubrant chip running 18# with 50's running 130% injector duty cycle via DS. Looking at the chip the PE tables were way off. I had to run my base FP at 32psi just to get 02 mV to 780-800. Obviously the chip had issues. Talked to Eric at Turbotweak and he helped me out with the PE tables to bring my injector duty cycle to 65-70%. My base FP is now at 42-45psi. At 20lbs of boost the duty cycle is ~75% with room to improve. HTH

Mike Banas
How large can you go with the injectors before the low pulse width/duty cycle becomes a problem at idle?

I heard that the mongo injectors become very inconsistent at low duty cycles and idle quality suffers (not just holding RPM but rather becomes erratic and rough).

I had a similar problem in the Opel. I use the FAST system with #160# injectors and duty cycle was >100% at 7,500 RPM. The program was reviewed by Joe Lubrant who immediately suggested a fuel delivery problem. Changed filters, same problem. Sent my 2025 Weldon pump back to Weldon for overhaul. Found the brush head assembly loose. Repaired pump. In the meantime I installed another 2025. DC is now <100%. Another problem may be that your voltage is dropping at high RPM and your pump is not spinning fast enough. If you can check Bat Voltage at 5000 RPM to confirm voltage is still at 14.2V. I am switching to a 16V system for 2004 to insure big volts throughout the pass. No alternator. Good luck.
His voltage was around 13 volts on direct scan.

Not that bad and you shouldn't need over 12.2 or so to keep your pump running supplying fuel one would hope.

If you lose your alternator your battery should be able to keep your motor running properly a bit.

With a 340 hot wired supplying the 38's his voltage would seem okay.

A bit too much timing in the chip for 3rd gear too without the alky.

I think he can get to 21-23 or so psi. with the alky. help.

But I would still get the 50's or the 57's anyway. :)

Just my .02 plenty of things to look at on Direct Scan.
Originally posted by terryk
How large can you go with the injectors before the low pulse width/duty cycle becomes a problem at idle?

I heard that the mongo injectors become very inconsistent at low duty cycles and idle quality suffers (not just holding RPM but rather becomes erratic and rough).


Standard high impedance injectors use a saturated injector driver that has a response time of around 2.0ms It used to be that the largest high impedance injectors were the MSD 50 pounders, but now there are the 57 lb injectors.
The use of low impedance injectors with peak and hold drivers make the larger injectors manageable at low pulse widths as well as more controllable at high pw. The response time of the peak and holds are around 1.5ms

Just wanted to post saying thanks to you guys on this thread. It has helped to better my understanding a lot.

Thanks again,