Fuel Fuel Injectors - Explanation

stang89bidges · May 11, 2015

Calling all Air/Fuel mixture guru's. Can someone tell me how these work?

1. I have 19# injectors. If I were to upgrade to 42# injectors would they shoot 42#'s all the time when they are injecting? Or are they variable via the computer or something else?

2. An example I am thinking about. For explanation purposes only, lets say I have a stock factory engine(1989 everything). I replace the 19# injectors with 42# will this put too much fuel into the system and cause problems?

3. Now lets say I have stock bottom, aftermarket aluminum heads, aftermarket lower and upper intake manifold, 70MM TB, 7psi supercharger. What happens with both 19# and 42# injectors? So with 19# would that be a bottleneck and I am simply shoving tons of air and little fuel so HP isn't what it could be or is something regulating the mixture even though the stock computer doesn't know about the supercharger(not tuned)? Then if I switch to the 42# will it simply be pushing 42#'s and still not regulated but since its shoving more fuel in with air I get more HP?

I want to understand this so I can do correct research on different combinations of aftermarket upgrades.

Thanks in advance.

stang89bidges · May 11, 2015

Oh yeah, and I normally hear about 19#, 24#, and 42# injectors like they are the norm. But when I look up injectors today I see numbers like 30#, 36#, and 47#........ Soooooooo, are the 47#ers pretty much 42#ers?

modulistic · May 11, 2015

here, an analogy:

think of it like a firehose and a garden hose. lets say I have programmed you to water the grass one second every minute. I didnt tell you which hose you are operating... your only instructions are "open the nozzle at the end of this hose for one second every minute"

so you are running around operating the garden hose delivering a nice gentle amount of water as per my instructions. Surpise, I swap you out with the fire hose.

now, do I have any right to get pissed at you when you blast the lawn with 50 gallons of water and tear up all the grass?

what will happen is im going to be like "WHOA DUDE, too much water... only open it for .00000001 seconds every minute" you will notice the water coming out and tearing everything up, but you have no idea I swapped out the garden hose.

the problem is I need to change your instructions.

modulistic · May 11, 2015

you could swap it out for 1,000,000,000 lb/hr injectors, you just need to change how long they are open for.

modulistic · May 11, 2015

oh, and Im running 19lb hr injectors with 8 or so lbs of boost. lots of guys do it. its not good for the injectors though, not a long term solution.

liljoe07 · May 11, 2015

The computer may not know there is physically a supercharger on the car. But it will know there is more air entering the engine. So it increases the injector pulse width to compensate for the extra amount of air. You can't just change injectors without letting the computer know or as lots of guys do, install a mass air sensor designed to work the larger injectors. If you install 42lb injectors and keep the same mass air sensor you were using with the 19lb injectors, and don't update the injector tables, the computer will still be opening the injectors based off of the 19lb settings. And it will run very rich. The computer will try to reduce the injector open time. But there is only so much it can do, as there is an 11% or 12% fuel trim limit. Once the limit is reached, the computer can no longer control fuel. Rich codes will be logged at that point.

stang89bidges · May 11, 2015

modulistic said:
here, an analogy:

think of it like a firehose and a garden hose. lets say I have programmed you to water the grass one second every minute. I didnt tell you which hose you are operating... your only instructions are "open the nozzle at the end of this hose for one second every minute"

so you are running around operating the garden hose delivering a nice gentle amount of water as per my instructions. Surpise, I swap you out with the fire hose.

now, do I have any right to get ****ed at you when you blast the lawn with 50 gallons of water and tear up all the grass?

what will happen is im going to be like "WHOA DUDE, too much water... only open it for .00000001 seconds every minute" you will notice the water coming out and tearing everything up, but you have no idea I swapped out the garden hose.

the problem is I need to change your instructions.

and

liljoe07 said:
The computer may not know there is physically a supercharger on the car. But it will know there is more air entering the engine. So it increases the injector pulse width to compensate for the extra amount of air. You can't just change injectors without letting the computer know or as lots of guys do, install a mass air sensor designed to work the larger injectors. If you install 42lb injectors and keep the same mass air sensor you were using with the 19lb injectors, and don't update the injector tables, the computer will still be opening the injectors based off of the 19lb settings. And it will run very rich. The computer will try to reduce the injector open time. But there is only so much it can do, as there is an 11% or 12% fuel trim limit. Once the limit is reached, the computer can no longer control fuel. Rich codes will be logged at that point.

So what you guys are telling me is

1. You have to upgrade your MAF or the car won't run/utilize the new injectors correctly.
2. You may also need a tune along with the MAF, but don't have to if you size your components right.
3. The computer controls how long the injectors spray fuel, but not how much fuel.

Correct?

A5literMan · May 11, 2015

Correct

liljoe07 · May 11, 2015

stang89bidges said:
and

So what you guys are telling me is

1. You have to upgrade your MAF or the car won't run/utilize the new injectors correctly.
2. You may also need a tune along with the MAF, but don't have to if you size your components right.
3. The computer controls how long the injectors spray fuel, but not how much fuel.

Correct?

1- Not necessarily. If you leave the stock mass air in place, then you must adjust the injectors parameters in the tune. But, it depends on the situation.

2- Correct, kinda goes with what I was saying in #1. It depends on the situation. With the people that use a so called "calibrated" mass air sensor, an on the stock tune. There is a point that as the larger you go with the injectors, the mass air sensor that tends to be used with them gets to far away from the stock mass air curve. Calibrated mass air sensors are generally scaled off of the stock curve, and the difference between injector sizes. Without a tune, the computer still thinks the engine is using stock injectors, so open and close times are based off of that. So the calibrated mass air sensors will lower the voltage being sent to the ECM to make it think the airflow is less, so the ecm will shorten the open time, in an attempt to get the larger injectors to spray an amount of fuel similar to what the stock injectors will spray for a given airflow.

3-The ECM is programmed already with what the stock injectors should flow for a given open time. Its basically calculations. Since it knows the injector size already, it knows how much fuel should be delivered during the time its open. But its not always accurate, and its another reason why you have 02 sensors. They are the feedback sensors to the ECM for fuel control. There are a lot of things being looked at by the ECM as far as 02 behavior goes. And depending on the behavior, the ECM will add or subtract fuel. Thats really kinda of simplified, and if you had something like the Moates Quarterhorse, you could see all the things that goes into the calculations for fuel.

jozsefsz · May 11, 2015

Adding to liljoe's comment (which is all good & correct)

2. The calibrated MAF, in an attempt to fool the computer into correct fueling, actually messes with your load calculation which can have detrimental effects on performance (things like timing).

3. A tune will more comprehensively adjust for larger injectors than a calibrated MAF. There are aspects of the tune (such as the cranking pulsewidth and the voltage tables) that the MAF cannot address. This can cause a rich condition at idle and hard starts when warm.

I personally wouldn't recommend a calibrated MAF without a tune. With a tune, it's just fine, provided you've been given a nice transfer sheet.

stang89bidges · May 11, 2015

Great explanations. I'm understanding a bigger picture now.

So I'm now under the assumption when changing fuel injectors, in order to do it correctly, you should always get a tune. And I'm assuming it would be best to get an adjustable MAF to go along with new injectors so the tuner can get it as close as possible. Then of course if you change again later your adjustable MAF can be retuned to different settings.

2 questions

1. What about picking the right injector sizes for your upgrades, is there someone or a company you can call and tell them your plans and get help with correct injector size? I'm talking specifics here, not just saying well 19# is good up to 300hp. Or is that seriously all there is to it and the tuner will take care of the rest?

2. Are the tuner kits that have pre-built tunes good enough to run without a dyno? In retrospect to upgrading injectors.

jozsefsz · May 11, 2015

The tune becomes more important with the larger injectors you select, as the "fudge factor" that is applied by the calibrated MAF grows bigger. For example, if you pick 24lb injectors and a calibrated MAF, you probably won't notice the driveability problems I mentioned. At 42lbs, you definitely will notice so a tune will be very important.

An adjustable MAF is not at all important, just one that's sized for the maximum airflow you'll expect to generate. Even if you make other modifications in the future, the MAF transfer sheet (voltage vs. airflow) is all that's entered into the tune and should not need updating again, unless you change the MAF again.

Basically yes, that's all there is, pick an injector that gives you a factor of safety over your horsepower goals, running at a max of about 80% duty cycle. Some power-adders (like a turbo / supercharger) you'll want a bigger factor of safety as those engines sacrifice a little efficiency (i.e. burn a little more fuel for each horsepower generated).

For simple bolt-ons and upgrading injectors, a mail-order or canned tune (like Bama) can work just fine. Personally I like the QuarterHorse with Binary Editor for DIY, but there's a substantial learning curve and expense associated, so I'd go down that road only if it sounds like fun (which it did to me).

stang89bidges · May 11, 2015

I see your an IT Architect. No wonder I like your explanations lol. I'm a Systems/Network engineer. I'm just used to reading whitepapers and instructions all the time and I enjoy learning every little detail of everything I do.

Anyway, thanks for answering so quickly, I'm still learning and this really helps confirm my ideas on the subject.

mikestang63 · May 11, 2015

1. Injectors don't make any hp, they support hp
2. On a stock motor, 19 lb injectors are what you want
3. Use any of the online injector size calculators to determine what size you need. Know that power adders lower tow duty cycle for injectors. Ex- a n/a motor can run at 80-100% of duty cycle if needed. Nitrouss or blown 50-75 % max, meaning the same injector will support less hp in a blown vs. n/a motor
4. I prefer Ford injectors
5. when you start to deviate from the facory size injectors, you need either/also a tune/aftermarket MAF.. An aftermarket MAF 'tricks" the EEC by adjusting the voltage so that it thinks the voltage is less to compensate for the larger injectors.
6. Adding to large an injector than needed without a tune will result in all kinds of bad things- running rich, running like crap, bad mpg, washing down cylinder walls.

As a GENERAL rule

For most n/a motors with the typical HCI top end kit making around 300rwhp, most guys go with 24 or 30# injectors. I prefer 30's so you are not maxing out the injectors

For the typical basic supercharger setup, 42# injectors and a tune.

jozsefsz · May 11, 2015

As a fellow IT guy, I think you might really enjoy doing the tuning yourself. You can download binary editor for free to see what you can mess with, and there's a lot. (But only a total of about half a dozen parameters to change if you change injectors & MAF).

Here's a good fuel injector calculator I've used: Fuel Injector Calculator | Fuel Calculators | Resources | DeatschWerks

You'll find it spits out roughly mikestang's numbers (it adjusts for volumetric efficiency for forced induction outside of the duty-cycle fudging mentioned).

Fuel Fuel Injectors - Explanation

stang89bidges

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At least it is lumpy...

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Fuel Fuel Injectors - Explanation

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