Injectors, Duty Cycle, And Hp Question. Jr No Chart Necessary, I Get That Part.

90lxwhite · Dec 13, 2014

Pardon the extreme ignorance in the subject but I have a few questions. I'm not trying to figure out what injector size for a certain horsepower level ( @jrichker
I've seen the chart ) I'm just trying to get a better understanding how they work. So x pound injectors "max out" or reach 90% duty cycle at x amount of horsepower, this I understand. My question is are they "maxed out" all the time or just when the motor reaches max horsepower? Say a car makes 400 hp @ 6k rpm and say x pound injectors will support 300 hp, are the injectors "maxed out" all the time or just when the engine surpasses the rpm at which the the 300 hp happens? I hope I worded the question well enough, the wife and kiddo are out of town and I've had a few. Bought em w the crappy gift card that came as part of my Xmas bonus...
Thanks
Mikey the troll
Merry Christmas and I hope Santa brings y'all all of your mustang needs.
Ps: long live the pushrod 302

90lxwhite · Dec 14, 2014

No clue huh?

A5literMan · Dec 14, 2014

As your engine builds the need for fuel consumption the injectors will reach their limit. Just like your present set up won't let you get above 5600 rpm. The car will "break up" and you run the risk of going lean also. Remember that the fuel itself has a certain cooling effect. Develop a "hot spot" and it will blow a hole into your piston etc.

RangerJoe · Dec 14, 2014

What he said. You could run a 500 hp capable motor with 19lb injectors as long as you never pushed the motor to the rpms where the hp exceeded the injectors ability.

Joe

90lxwhite · Dec 14, 2014

A5literMan said:
As your engine builds the need for fuel consumption the injectors will reach their limit. Just like your present set up won't let you get above 5600 rpm. The car will "break up" and you run the risk of going lean also. Remember that the fuel itself has a certain cooling effect. Develop a "hot spot" and it will blow a hole into your piston etc.

RangerJoe said:
What he said. You could run a 500 hp capable motor with 19lb injectors as long as you never pushed the motor to the rpms where the hp exceeded the injectors ability.

Joe

Thanks guys. I got it now

Noobz347 · Dec 14, 2014

90lxwhite said:
Thanks guys. I got it now

Do you? I ask because when I read your post I thought you were asking a different question.

The answer to what I thought you were asking is 80%.

That is approximately the duty cycle you want injectors to be as max power. Too much in either direction or up and down in fuel pressure, things begin to happen to spray patterns.

90lxwhite · Dec 14, 2014

Noobz347 said:
Do you? I ask because when I read your post I thought you were asking a different question.

The answer to what I thought you were asking is 80%.

That is approximately the duty cycle you want injectors to be as max power. Too much in either direction or up and down in fuel pressure, things begin to happen to spray patterns.

No I wasn't asking what's a safe % duty cycle or which injectors to use, I was asking when does the 80% take place. For example, are the injectors "maxed out" on the way to 7-11 or when wot at the drag strip. The other guys answered well enough I think.

addermk2 · Dec 15, 2014

I might have a good data log for this. I'll post it tonight.

Noobz347 · Dec 15, 2014

90lxwhite said:
No I wasn't asking what's a safe % duty cycle or which injectors to use, I was asking when does the 80% take place. For example, are the injectors "maxed out" on the way to 7-11 or when wot at the drag strip. The other guys answered well enough I think.

80% at WOT.

90lxwhite · Dec 15, 2014

Noobz347 said:
80% at WOT.

Got ya

addermk2 · Dec 15, 2014

The 80% at WOT is really just a "rule of thumb" kind of thing. I've run injectors right up to 100% before. Should I have been running larger injectors? Sure. But I wasn't about to drop $750 for 4 1000cc injectors, or $650 for 750's.

So I dialed back the boost and stuck to 310hp.

jlangholzj · Dec 15, 2014

Noobz347 said:
80% at WOT.

I saw this carousing the front page so I thought I'd poke my head in.

The furthest you can push a modern injector is about 92-93% duty at any RPM range. Beyond that what happens is that you start to lose resolution in your fuel map. Typically this means you'll be going lean but that's not always the case. The reason for this is because the injector can't respond sufficiently fast, causing the loss in resolution. Doesn't matter if your "WOT" or at 1500 rpm or 12,000 rpm, 90% duty is 90% duty. This comes from the fact that at a certain rail pressure, you can only flow so much through the injector. Need more fuel, the IDC goes up but you can only get so much out of that injector.

Noobz347 · Dec 15, 2014

jlangholzj said:
I saw this carousing the front page so I thought I'd poke my head in.

The furthest you can push a modern injector is about 92-93% duty at any RPM range. Beyond that what happens is that you start to lose resolution in your fuel map. Typically this means you'll be going lean but that's not always the case. The reason for this is because the injector can't respond sufficiently fast, causing the loss in resolution. Doesn't matter if your "WOT" or at 1500 rpm or 12,000 rpm, 90% duty is 90% duty. This comes from the fact that at a certain rail pressure, you can only flow so much through the injector. Need more fuel, the IDC goes up but you can only get so much out of that injector.

Not sure what this means.

80% at WOT with stock fuel pressures ensures that there's enough headroom at the top so that the EEC can adjust pulse width to compensate for weather, combo changes etc.

WOT, 1500 rpm, or 12,000 rpm does in fact make a difference. If the injectors are sofficient to provide an adequate length pulse at 5000 RPM (let's say that the motor gets adequate fuel at this rpm at max pulse) then at 10,000 rpm (for instance) the same injector even at 100% duty cycle will not remain open long enough to provide the fuel volume necessary.

The 80% at WOT throttle allows for a little more or a little less fuel (longer/shorter pulse) within the EECs ability to add/remove fuel without having to change fuel pressure resulting in a less efficient spray pattern.

Of course, we all know that some fudging of the spray pattern can be done without much headache but this can reduce fuel volume as well.

jlangholzj · Dec 15, 2014

Noobz347 said:
Not sure what this means.

80% at WOT with stock fuel pressures ensures that there's enough headroom at the top so that the EEC can adjust pulse width to compensate for weather, combo changes etc.

WOT, 1500 rpm, or 12,000 rpm does in fact make a difference. If the injectors are sofficient to provide an adequate length pulse at 5000 RPM (let's say that the motor gets adequate fuel at this rpm at max pulse) then at 10,000 rpm (for instance) the same injector even at 100% duty cycle will not remain open long enough to provide the fuel volume necessary.

The 80% at WOT throttle allows for a little more or a little less fuel (longer/shorter pulse) within the EECs ability to add/remove fuel without having to change fuel pressure resulting in a less efficient spray pattern.

Of course, we all know that some fudging of the spray pattern can be done without much headache but this can reduce fuel volume as well.

Okay I guess that's my fault then. Let me try and spell this out a bit harder.

At the end of all the calculations and compensations if you're past 90-ish duty you're hosed. It doesn't in fact matter what RPM that you're at, if you required enough fuel to hit above the critical duty then either you need to vastly increase your fuel pressure or use a larger injector. Generally yes you are correct in that as RPM increases IDC will increase. This however is not exclusively the case. That's because whether or not you're at WOT doesn't matter, its resulting airflow. Smash your foot to the floor and you're "WOT" at 2.5k rpm and at 6k rpm when you shift. Which is why the point I was trying to make is that the concern is not the RPM at which max IDC occurs but rather the fact that you're hitting max IDC.

So okay, the 80% number gives you enough headroom so that as air temps change and pressures change you've got some extra injector to keep the motor happy. I guess i should have said the above paragraph first. My point was that your delivered IDC to the motor will function fine up to low 90% without any adverse effects to what the ECU thinks it's doing. Once you get into those upper injector ranges though you don't get any "inefficient spray patterns" but as i previously stated, a loss of resolution in the fuel table, which will often (but not always) cause a lean condition.

Noobz347 · Dec 15, 2014

jlangholzj said:
Okay I guess that's my fault then. Let me try and spell this out a bit harder.

At the end of all the calculations and compensations if you're past 90-ish duty you're hosed. It doesn't in fact matter what RPM that you're at, if you required enough fuel to hit above the critical duty then either you need to vastly increase your fuel pressure or use a larger injector. Generally yes you are correct in that as RPM increases IDC will increase. This however is not exclusively the case. That's because whether or not you're at WOT doesn't matter, its resulting airflow. Smash your foot to the floor and you're "WOT" at 2.5k rpm and at 6k rpm when you shift. Which is why the point I was trying to make is that the concern is not the RPM at which max IDC occurs but rather the fact that you're hitting max IDC.

So okay, the 80% number gives you enough headroom so that as air temps change and pressures change you've got some extra injector to keep the motor happy. I guess i should have said the above paragraph first. My point was that your delivered IDC to the motor will function fine up to low 90% without any adverse effects to what the ECU thinks it's doing. Once you get into those upper injector ranges though you don't get any "inefficient spray patterns" but as i previously stated, a loss of resolution in the fuel table, which will often (but not always) cause a lean condition.

Inefficient spray patterns come from changing fuel pressure too far one way or the other (one of the reasons why FMUs are less than desirable). Not from duty cycle.... What you are saying though, is correct. When you start getting into the upper pulse range on an injector (indication that the injector is too small) then it gets dicey.

In case we've not beaten this thing to death.... stock fuel pressures and 80% duty cycle at WOT. lol

I think we took the long way around.

bentley429isBAC · Dec 17, 2014

Oh 80% is max? Guess 120% isn't working out...

Noobz347 · Dec 17, 2014

bentley429isBAC said:
Oh 80% is max? Guess 120% isn't working out...

Magic injectors (like those that can remain open 100% of the time plus be open an additional 20% of 'always open') are not the subject of this discussion.

bentley429isBAC · Dec 17, 2014

Noobz347 said:
Magic injectors (like those that can remain open 100% of the time plus be open an additional 20% of 'always open') are not the subject of this discussion.

Might in reality only be open 100% but the ecu can be calling for more and show higher then 100%. But yeah! I know what your saying.

Noobz347 · Dec 17, 2014

bentley429isBAC said:
Might in reality only be open 100% but the ecu can be calling for more and show higher then 100%. But yeah! I know what your saying.

Now I know what YOU are saying and you bring up a good point!

On more than one occasion, I've come across setups where the ECU has not been updated (by chip, piggyback, etc.) and the EEC value differs from the size of the actual injectors that are installed. Pretty common to find 24# injectors this way but I've seen some 36s and 42s as well. To compensate, what they did was to run percentages like you mentioned. This would not be my first choice unless whatever tuning method I was using, did not support larger injectors. It usually ends up being rich at idle and lean(er) as RPM goes up because the EEC doesn't know what the minimum pulse is for the larger injector.

90lxwhite · Dec 17, 2014

So when's it maxed out again? Redline??? Not what percent is safe, not what injector is recommend for what hp level. When does the motor reach whatever the safe limit is? Does it come on in increments like say at 1,200 it's cycling 20% and at 3k it's at 50%, so on and so forth?

Injectors, Duty Cycle, And Hp Question. Jr No Chart Necessary, I Get That Part.

90lxwhite

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90lxwhite

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A5literMan

At least it is lumpy...

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90lxwhite

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