What determines how much fuel is delivered by an injector?
Fuel Injector Help
- Thread starter 4AU4LIFE
- Start date
I believe the answer is "pressure".
its a function of injector size, pressure, and the duration that the injector is open. The injector size is fixed. I think the pressure is fixed by a regulator also. The duration that the injector is open for each intake cycle is variable and is controlled by the fuel compter in response to all sorts of input, especially throttle position.
I think thats how it works heheh correct me if I'm wrong.
I think thats how it works heheh correct me if I'm wrong.
Alright then let me ask more. If you have two injectors one stock and one a/m that is a higher flow, would they both spray the same amount of fuel if all other factors are equal? Or would the higher flow one spray more?
I'm no expert, but I'd expect the higher flow injector to spray more because of its orifice being larger in diameter. That is why it is higher flow. . Its larger.
So it would be detrimental to replace OEM injectors with higher flow injectors if all others factors are equal?
Absolutly! The reason to go to high-flow injectors is to be able to supply more fuel than the stock injectors are capable of flowing to keep the air/fuel ratio the same when your forcing alot more air in the engine. That means forced induction.
You don't want to dump more fuel in without dumping more air in as well. Only forced induction will do that. Headers, intake, ect on a normally asparated engine will NOT require you to go to oversize injectors. To do so will just waste fuel, and you won't see a performance gain. It's possible that the fuel computer would try to keep the air/fuel ratio proper and keep the injectors from flooding the engine, but if it can't, results would not be good. Too rich a mixture leads to low power, carbon build-up, and poor lubrication due to diluted oil.
Hope this helps, and this is just my opinion, but I have had aftermarket forced induction cars in the past (turbo), so I have run into these issues.
You don't want to dump more fuel in without dumping more air in as well. Only forced induction will do that. Headers, intake, ect on a normally asparated engine will NOT require you to go to oversize injectors. To do so will just waste fuel, and you won't see a performance gain. It's possible that the fuel computer would try to keep the air/fuel ratio proper and keep the injectors from flooding the engine, but if it can't, results would not be good. Too rich a mixture leads to low power, carbon build-up, and poor lubrication due to diluted oil.
Hope this helps, and this is just my opinion, but I have had aftermarket forced induction cars in the past (turbo), so I have run into these issues.
That does help a lot. I know have the answers I need to make an informed choice. 
The answer is pulse width. But the info given above is correct. Bigger injector would be too much feul. Mo - Elite
Elite said:
In lamens terms huh??!!
Thanks Mo.
The flow through the injector is controlled by an electric valve built into the injector which is either full ON or full OFF. For each cycle the amount of time that it is on can be referred to as the "pulse width". When the engine is idling, the pulse width is very narrow, meaning that the injector is off most of the time and just pulses on for a short duration every intake cycle. As you step on the gas, the pulse width widens and the injector stays open longer for each cycle. That way more gas flows through it into the engine. At WOT, the injector is open just about all the time. For sake of argument, it never closes, and is flowing its maximum amount of fuel.
And that is why if you need more fuel into the engine than what it was designed for, you can't do it by modifying the fuel computer because at WOT the injector is already open continuously, and that’s that.
So, if you need more fuel in there because your going forced induction, you have two ways of doing it, or a combination of the two.
1. Raise the fuel pressure. Increasing the pressure flows more fuel through the injector. There is a limit of how high you can go because at some point the injector valve will not be able to open because of the pressure load behind it.
2. Get a bigger injector (high flow). Now, for any given pulse duration, the larger injector is flowing more fuel simply because its orifice is larger in diameter. (For example, a 1 inch pipe flows more water volume than a 1/4 inch pipe).
Hope this helps, but the bottom line is that larger injectors should not be considered to boost power unless your doing something else that requires more fuel to maintain the correct air/fuel ratio
And that is why if you need more fuel into the engine than what it was designed for, you can't do it by modifying the fuel computer because at WOT the injector is already open continuously, and that’s that.
So, if you need more fuel in there because your going forced induction, you have two ways of doing it, or a combination of the two.
1. Raise the fuel pressure. Increasing the pressure flows more fuel through the injector. There is a limit of how high you can go because at some point the injector valve will not be able to open because of the pressure load behind it.
2. Get a bigger injector (high flow). Now, for any given pulse duration, the larger injector is flowing more fuel simply because its orifice is larger in diameter. (For example, a 1 inch pipe flows more water volume than a 1/4 inch pipe).
Hope this helps, but the bottom line is that larger injectors should not be considered to boost power unless your doing something else that requires more fuel to maintain the correct air/fuel ratio
I'm not wanting to boost power by installing larger injectors. I'm wondering if going from FI to NA and leaving in the higher flow injectors is a bad choice?
Now that is a good question. I don't know if the fuel computer will compensate and maintain the correct a/f ratio, but I bet there are experts out there who know the answer. Common guys. Lets hear from you.
No the feul computer cannot compensate enough for that kind of change. You would need to at least go back to a smaller feul injector. If you have other feul supporting mods they should be fine. Mo - Elite.
In the case described it sounds like you are asking about a 440cc injector and going back to a 330cc injector. The 440cc injector will deliver 30% more fuel for a given pulse width (measured in milliseconds)
The real issue is not if the 440cc injector is capable of less fuel, since in an FI system, when not on boost they will work properly, rather it is that if you remove the fuel metering system that "normalized" the 440cc injector there is no way to adjust it to function with the normal ECU.
"Normalization" - this function is used to compensate for a change in injector size. If an engine is updated to a 440cc injector from a 330cc injector, and at idle the pulse width is 40 ms for the 330cc, it would need to be 30ms for the 440cc to supply the same fuel amount. (My numbers are for example, not 100% accurate, but you get the idea).
Many add-on type systems, like a Split Second for example, have this type of normalization as part of their functions. This is opposed to systems like the AEM which starts from a blank sheet and allow complete recalibration/total adjustment of the system.
So if you remove an add-on unit like a SS Box, you must return to the original injector size, since the stock ECU fuel maps are based on the original injector size.
HTH,
LarryB
The real issue is not if the 440cc injector is capable of less fuel, since in an FI system, when not on boost they will work properly, rather it is that if you remove the fuel metering system that "normalized" the 440cc injector there is no way to adjust it to function with the normal ECU.
"Normalization" - this function is used to compensate for a change in injector size. If an engine is updated to a 440cc injector from a 330cc injector, and at idle the pulse width is 40 ms for the 330cc, it would need to be 30ms for the 440cc to supply the same fuel amount. (My numbers are for example, not 100% accurate, but you get the idea).
Many add-on type systems, like a Split Second for example, have this type of normalization as part of their functions. This is opposed to systems like the AEM which starts from a blank sheet and allow complete recalibration/total adjustment of the system.
So if you remove an add-on unit like a SS Box, you must return to the original injector size, since the stock ECU fuel maps are based on the original injector size.
HTH,
LarryB
I'm curious: Why are you taking FI off your car?
Thanks Larry.
mikec said:
Long story.
