Good Day,

I want to learn more about fuel injectors regarding size and duty cycle. I have tried doing a search in this sub-forum and went back several years and can't find anything.
So please help answer questions. I'm SURE there are MANY other people here that don't know and would like to know as well.

First off, while it seems like it would be simple, please explain about size of injector. The stock engine has 4.3gps injectors. Does that mean that in a wide open stream/spray, the injector would dispense 4.3 grams per second?
And compared to the 5.5. So the 5.5 is bigger. Now, unlike a carburator where it is just dumped into the manifold, please explain that if the engine signals for fuel, and the injector responds, how is not having more fuel better?

Yes, I understand it could choke it out. I'm old school engine builder. A Holly 850 double pumper, manual secondaries would bog out a small block 289. A built 454 would use every ounce.

I'm also trying to understand the duty cycle of the injector. My next stop is YouTube. But for the benefit of the audience here, please explain.

I understand that when the duty cycle reaches above 85%, a bigger injector should be installed. On the opposite end, what does it mean when the duty cycle is too low?

TIA,

 

https://university.fuelmotousa.com/a...rley-injector/

 

the Ecu doesn’t just send an “on” signal to the injector , and the ejector dumps all it can.. it’s all calculated taking in how much fuel the injector can provide per ms, is it in open loop or closed loop, how much air is calculate to be in the cylinder (VE) and how much fuel to add to make the mixture burn as efficiently as possible which is ~ 14.7 to one ratio( stylometric) . There are many other factors the ECM uses to determine how much fuel to inject. Putting in larger injectors do not make things better… and in
fact can make the engine run worse at slow rpm’s. And your right , 80-90 % duty cycle is when you should consider moving a size up.

 

In the Fuel Throttle Podcast: Episode 6: Harley Fuel Injectors Explained
we go into great detail on fuel injectors; sizing, monitoring duty cycle, design, & tuning.
https://www.fuelmotousa.com/p-34004-...lepodcast.html

 

Many answers you find on the internet will be flatout wrong and/or very confusing to many. Not all injectors are the same even when they are rated the same! Most will rate injectors in GpS, Lb/Hr or CC's. The problem is that only gives you a rating for an injector when in a static condition which is a case that they should never be used in. Some are rated at 3 bar while others are rated at 4 bar, some have dual spray patterns, some have a single spray pattern, and then the angle and taper of the injector pattern all has to be considered for the given application. Then you need to know what was being used when the measurements were being taken too.

Gps = grams per second
Lb/Hr = pounds per hour
CC = Cubic centimeter

For best fuel control you want to use the smallest flow injector that will get the job done as it will give you the best drive ability and fuel economy. That said using a injector with a single spray pattern on a 4 valve head is just dumb but people are selling just that to the public all the time. A perfect example is back in 2006 when HD factory used an injector that had the spray cone angle of 8 degree's in the stock bikes. Once they hit the public people started complaining of having drive ability issues, unstable idles and Cold start issues. HD changed the injector for one that was the exact same in terms of flow but the spray cone angle was change to 25 degree's along with a new calibration to support the injector change and all the issues disappeared. So it is NOT just a flow rating that people need to be aware of!
Injectors DO NOT flow the same across the entire operating range! If you were to look at the flow at very low pulse width and again at very High pulse width things change, and this is just why the industry as a whole uses a 80% rule on injectors. If you do real measurement an injector has a very linear output provided you stay within the 80% DC range. Go outside of this range and things become unstable and unpredictable. A simple example would be you increase the pulse width 5 % and the injector flow will increase 5 % within the 80% range outside this range............... who knows

 

Thank you to Steve, @stroisi12 and @fuelmoto .

Thank you to the ones who know. I appreciate the simple facts, albeit very technical and I am sure other new people are learning as well.
Helping others to understand that a 4 hole is better, the angle make a difference, and bigger is not always better.

 

I have already received a few phone calls on this today, so let me make this perfectly clear, It is NOT OK to run injectors above 85% Duty Cycle! There is a reason the entire Automotive Industry and Injector Manufactures adopted and use the 80% rule. Can you do it, sure, but that in no way, makes it the right thing to do. It ranks right up there with putting a single spray pattern injector into a 4 valve head, its just dumb! Think about this, you spend how many thousands of dollars on your build just to cheap out on a set of proper injectors for the build, does that really make any sense to do? Think about an Evo or TC engine, 1 intake valve so you only want/need a single spray pattern injector, M8 uses 2 intake valves and this is just why you use a dual spray injector with the proper settings. As I previously describe the Injector size, cone angle, Alpha Angle, Beta Angle and single or dual spray all need to be right to get the engine to perform at its best and isn't that what you've built the engine to do?