OK so... Lets say (hypothetically) that the stock injectors flow 30cc at 6000rpm and 15cc at 3000rpm. Now if i were to replace the stock injectors with after market ones that flowed 50cc at 6000rpm and i was making zero boost at 3000rpm, could these new injectors flow as little as the stock(15cc); or would I be dumping unneeded fuel into the engine.

Just to clarify, because I'm pretty sure the above made no sense, it would be from bone stock to FI.

 

Are you talking about just replacing the stock for bigger ones and nothing else? No way to control them, no pump, no fuel rails... Just stock car with bigger injectors?? And then once done, you want to know if they'll be capable to flow as little as the stocks did?

 

For instance if we were to have an injector that flowed 30cc max and another which did 50cc max, the larger could still flow small amounts of fuel as accurately as the 30cc. (no engine involved, just spraying fuel into a bucket)

I really am sorry for the way im wording this, Im trying my best and thanks for your help.

 

Interesting question. I am not sure I follow your logic but I think you're not thinking about this whole thing the right way. Injectors are rated based on flow at certain fuel pressure. Constant fuel pressure is key to correct fueling unless the ECU has some kind of a notion of variable fuel pressure regulation. As far as I know, it's basically constant. So, the ECU controls the injector by issuing pulses which tell the injector how long to spray for. The longer the pulse, the more fuel is added. The flow itself is constant. The bigger the injector, the more fuel is added during a given pulse width since the injector flows more given the same amount of time it is spraying.

So, during fueling operation, the variables in play are size of the injector, pulse width and injector latency. There is some delay between when the ECU tells the injector to open and when the spray actually happens. So, knowing the size and latency, assuming fuel pressure, the ECU can calculate the IPW to hit the desired AFR. This brings me to my point. If you're going to change the injectors, you can't just swap the injectors and call it a day. If you're going to go bigger, you need a tune to properly control them. Bigger is always better is not the right approach in this case. Find a well matched size to what you will be doing with some extra room to spare. So, if you think 500cc is just about enough, go with 550cc ones to have some room for safety. Generally, bigger injectors have bigger latency and will be harder to tune for so only go big if you really have to. So, with bigger injectors, assuming the fuel pressure is the same, the IPW will be shorter to spray the same amount of fuel as the stockers. Latency will play some role here. This is where quality injectors will shine as they'll have small latency and will respond quickly.

Judging the size based on amount of boost is not the right way to gauge what you'll need. You need to look at what your flow will be and go from there. 20psi on a TD04 flows much less than 10psi on GT35. How much flow you'll get will dictate your fueling and determine what size injectors you'll need.

I hope I was helpful

 

that was very helpful. The question wasn't because of any planned modifications, it was just to help clear some things up. I appreciate it!

 

The most common problem is at idle since oem picks the largest available injector to minimize duty cycle at peak flow and one which can deal with the low flow at idle. Injectors at idle don't full open before they are shut off....just a tiny drop of fuel................5-6 HP at idle doesn't use much fuel.

For example say a 370cc is open 11millisecs out of 16.6 ~~60% at peak flow and open only 1.1 millisecs at 650 rpm idle that is a 1:10 dynamic range.

Larger injectors usually will be way to rich at idle, the easy solution is to double the idle rpm as the minimum idle.

Rail pressure can be reduced at idle and cruise [older Q/J used 34 psi at idle/cruise and 43.4 psi at WOT 34/43.4= 0.7834 take square root =0.885 or 11% lower flow per open time vs 43.4.

So at max 370cc =370cc but at idle and cruise it acts like 370 x 0.885= 327cc.

Sorry for using old Q 370cc but that is always in my mind.

The newer returnless systems work at 51 psi and use pulse width modulation to change fuel pump speed and pressure to make idle work.

Unfortunately newer OBD2 systems limit corrections and deviations to +-20% or less from O2 sensors before MIL lights are tripped.

 

As a general rule of thumb, most injectors cannot go below 1-2ms pulse width, and you should not run them at >80% duty cycle.

If you are a high-HP customer, you may run into issues dealing with the dynamic range between 1-2ms pulse width at idle while trying not to exceed 80% duty cycle at WOT.

This is the primary reason for going with bigger/badder fuel pumps, fuel rails, and higher pressures.

Specifically, fuel pressure that increases as boost increases allows injectors to deliver more fuel without going above a certain duty cycle.

As Q45 mentioned, low RPM range and idle etc can give you trouble with injectors running too rich. This will be a fact of life if you have huge injectors because of the 1-2ms minimum pulse (trying to go below that could cause problems, like zero fuel injection, weird idle). And I agree, raising the idle would definitely help with this problem, but this is also addressed with the fuel pressure that increases as boost increases, because if you tune properly for idle fuel pressure, then you tune properly for fuel pressure under boost, you should be golden.

Hope this helps, as I have not really understood the question.

Also, this is why bigger injectors can be so expensive -- because as you go bigger, you start to introduce bigger parts into the injectors. At high RPM and high fuel delivery (duty cycle) the parts in the injector may have trouble keeping up with the RPM cycle of the motor, which could mean fuel leaks out when you don't want it to. So in essence bigger injectors need to be designed with this fact in mind...

 

Wired, that's some good info. Thanks for taking the time to throw it out there.

This one is a bit puzzling to me:

I've seen people throw this number around in the past, or close to it. I know driving the injectors close to their full capacity too long will wear them down faster. This being said, I am not certain if this number is a bit too conservative. I've seen high 80s or low 90s for IDCs at close to redline RPMs on a stock WRX before which makes me wonder. In addition to that, if you consider what IDCs really measure, the number is really relative to the engine speed and IPW. It's not exclusively telling you that 80% IDCs are an indicator that you're close to maxing out your injectors themselves. You could be in the 90s and still be OK. It really depends on what the IPW in relation to what the max pulse could be. I think that will tell you how hard you're driving the injectors. I know the injectors in the WRX wouldn't go static until about 130% IDCs but at this point you'd have other issues to worry about.

I'm curious to hear why you think 80% is a good number. 80% translates to roughly 14.75ms IPW at 6500RPMs. Is this what is considered near-limit pulse width?

 

I use this as a rule of thumb after reading about this in a book:

How to Tune and Modify Engine Management Systems by Hartman



Basically 80% is not a hard upper limit. It's just a guideline. It also depends on how you drive the car. If you are pushing it near redline at WOT ALL THE TIME at duty cycle > 80% your injectors "could begine to fibrillate or overheat" .

If they fibrillate I take that as meaning they will produce an inconsistent fuel stream, meaning you could suddenly run out of fuel or the fuel delivery could oscillate out of control.

If they overheat, they could totally fail.

For N/A applications, horsepower does not jump up by many many times like it does in F/I applications. Of course for N/A your horsepower will go up in a pretty easygoing fashion, so you can usually keep a fixed fuel pressure and just vary the injector duty cycle. If your injectors are sized properly for N/A, they shouldn't have problems.

But for F/I, your injectors have to be able to handle both REALLY LOW fuel delivery to give you a nice idle, and REALLY HIGH fuel delivery for the high HP. There is a lot of fuel difference between idle and 500whp. So the only way to really make that power level SAFE is to pick big enough injectors, and then make sure that the fuel pressure increases with boost pressure (or by some other relationship) so that even if injector duty cycle stays the same, MUCH more fuel will be flowing at higher boost pressures.

 

I've also read that injectors that receive power for 90% of the time are essentially "always open", which is another reason for sticking to roughly 80% to leave some ceiling room.


Now just imagine you have a high HP application and you want to rev your engine to 8000rpm (Note that bigbri had this setup). At 8000rpm the 100% duty cycle is basically 15ms - so you can see how easy it is to run out of fuel when even in N/A applications you can see 16ms pulse width.

Clearly, it's not just your engine internals that you should be concerned about if you want to rev to high RPMs... you need to think about injectors and spark and **** like that.

On top of EVERYTHING, you need to make sure that, while you want higher fuel pressure as boost increases, you also cannot go above a certain fuel pressure (usually somewhere in the 70+ psi range) because the fuel will push the injectors open and fuel will leak out at that pressure, putting a lot of strain in the injectors...



Edit: found a cool website for calculating this stuff
http://www.stealth316.com/2-calc-idc.htm

 

That's the site I use for calculating the IDCs. That's so funny.

Yeah, you bring up good points. If you go beyond 100% IDCs, I don't think you'll run out of fuel but you may as well look at it this way. You'll start overlapping the injector cycle with the exhaust valve opening which may shoot some cool flames out the tail pipes but will not be good for power. Driving the injectors that hard consistantly will induce premature failure and that will be catastrophic in FI applications. Brief lean condition followed by severe detonation or pre-ignition can melt pistons and blow the rings. I guess 80% is a nice safe number to use for good measure. Ofcourse, if you're not spending a lot of time in such high IDCs, you can get away with brief, high numbers. So, I guess the universal answer to all questions known prevails:

"It depends"

 

I calculated it myself first, then found that site to verify