ls7 stock injector flywheel hp

[miahbuchholtz]

im tring to figure out how much stock ls7 injectors hp will they make. im building a ls427 stroker with ls3 heads and it should make 630hp at flywheel

[Mike Norris]

At about 540-550 RWHP I see about 95% duty cycle which is about 625 crank give or take.

Mike

[miahbuchholtz]

thanks then thoses should close

[Gandalf]

Mike, what would you consider a good/healthy margin for duty cycle? ie: when should you consider stepping up in injector size?

Gregg

[Mike Norris]

Mike, what would you consider a good/healthy margin for duty cycle? ie: when should you consider stepping up in injector size?

Gregg

Some swear by 80% no matter what. I take each car as an individual basis and look at what the driver is doing with the car.

If someone is never really pushing their car and it is a cruiser most of the time with an occasional WOT run, then I can live with 95-100% DC at max RPM. This is because most folks will not run their cars to the edge, even at WOT. In this case if I see 80-85% DC in the 5500 area it is usually okay for these folks.

If someone is going to run the car to 6500 or so everytime during a run, then I keep in the 85-90% DC max if at all possible.

With that said, not too long ago (6-7 years ago) before we had a lot of injector choices, I would see 110-120% DC on a forced induction setup and not have issues. Not saying that this was right, justs stating what we used to see from time to time.

Hope this helps.

Mike Norris

[andrewb70]

Mike,

How do you even interpret DC over 100%? Doesn't 100% duty cycle mean that the injectors are static, as in wide open all the time. How can they be more than "on" all the time?

Andrew

[Mike Norris]

Mike,

How do you even interpret DC over 100%? Doesn't 100% duty cycle mean that the injectors are static, as in wide open all the time. How can they be more than "on" all the time?

Andrew

That is precisely why I do not allow myself to get caught up in the Injector Duty Cycle drama. I have seen over 100% (110 to 115 on a couple cars) on HP Tuners scans several times and the fueling still controlable (is that a word?). I know how the theory should work and then I see data to prove otherwise. I can see how this would drive an engineer crazy, but for a simple old Irish guy, real world is what I go with.

Mike

[andrewb70]

That is precisely why I do not allow myself to get caught up in the Injector Duty Cycle drama. I have seen over 100% (110 to 115 on a couple cars) on HP Tuners scans several times and the fueling still controlable (is that a word?). I know how the theory should work and then I see data to prove otherwise. I can see how this would drive an engineer crazy, but for a simple old Irish guy, real world is what I go with.

Mike

I didn't mean to get into a "theory vs. real world" debate. I can see how HP tuners and other software can show a duty cycle over 100% because the duty cycle is a calculation which incorporates a time element. The time element is addressed by the RPM. So it is technically possible to have a DC value of over 100% but any value over 100% is meaningless because by definition the injector is wide open and is no longer pulsing. So if the injector is already wide open there is no way to control it any longer because it is flowing all it can flow all the time. It has become an open garden hose. LOL

Just sayin....

Andrew

[Gandalf]

+1 for Real World - thanks Mike!

[andrewb70]

+1 for Real World - thanks Mike!

Again, this is not a debate between real world and theory, but people have to understand exactly what DC means. The duty cycle is the ratio of time that the injector is open to the amount of time that is available. It is expressed in the following equation where IDC is the duty cycle and IPW is injector pulse width:

IDC = IPW * (RPM / 120)

So for instance, at 7000 RPM the maximum time that is available for an injector to be open is approximately 17.14 milliseconds. If your engine requires more fuel than a particular injector (at its flow rate) is capable of providing in that time frame to maintain a desired A/F ratio, then you need a bigger injector. This is the logic that you must use when sizing an injector for a desired level of power. There is no magic here, it is just algebra.

Like I said before, some software packages may indeed show a DC cycle more than 100%, but as I said before, that isn't possible, by the very definition of duty cycle. There is no magic way that an injector can be open longer then it has available to it, end of story, period. So whatever the software is displaying, it isn't duty cycle, some some contrived value that it computes from god knows what variables.

Incidentally, the Holley EFI system actually uses the IPW directly in the fuel table, and it will highlight cells red if the pulse width exceeds the time available, thus indicating that the injector is fully open and at 100% DC.

Andrew

[EFI69Cam]

Again, this is not a debate between real world and theory, but people have to understand exactly what DC means. The duty cycle is the ratio of time that the injector is open to the amount of time that is available. It is expressed in the following equation where IDC is the duty cycle and IPW is injector pulse width:

IDC = IPW * (RPM / 120)

So for instance, at 7000 RPM the maximum time that is available for an injector to be open is approximately 17.14 milliseconds. If your engine requires more fuel than a particular injector (at its flow rate) is capable of providing in that time frame to maintain a desired A/F ratio, then you need a bigger injector. This is the logic that you must use when sizing an injector for a desired level of power. There is no magic here, it is just algebra.

Like I said before, some software packages may indeed show a DC cycle more than 100%, but as I said before, that isn't possible, by the very definition of duty cycle. There is no magic way that an injector can be open longer then it has available to it, end of story, period. So whatever the software is displaying, it isn't duty cycle, some some contrived value that it computes from god knows what variables.

Incidentally, the Holley EFI system actually uses the IPW directly in the fuel table, and it will highlight cells red if the pulse width exceeds the time available, thus indicating that the injector is fully open and at 100% DC.

Andrew

Agreed about the 100% being just that.

It seems what is needed to have an accurate picture is a conversion from the software defined DC to the actual. Who's to say that the 110% seen in HPTuners is not a perfectly safe 95% actual?

[Gandalf]

Again, this is not a debate between real world and theory

Andrew - chill mate. I have no time for nor did I intend to enter any type of "debate". I'm merely thanking Mike for answering my question and offering up his real-world experience. I personally defer to real-world experience over numbers when it comes to this EFI stuff (and running an I.T. company - trust me when I tell you that I am a numbers guy by nature and I appreciate your detailed approach to the math).

Peace.

[andrewb70]

Andrew - chill mate. I have no time for nor did I intend to enter any type of "debate". I'm merely thanking Mike for answering my question and offering up his real-world experience. I personally defer to real-world experience over numbers when it comes to this EFI stuff (and running an I.T. company - trust me when I tell you that I am a numbers guy by nature and I appreciate your detailed approach to the math).

Peace.

I am chill and I like Mike. Hell, he tuned my car!

The original poster asked how much a certain injector size will support in terms of HP. Mike responded.

Then you asked the question as to when to step up in injector size, to which Mike replied how we has seen DC numbers of over 100% in software. I merely jumped in to clarify, for anyone that should stumble into this thread, what DC really means. So in the future, when someone is looking for a way to determine what size injectors they need, they don't mistakenly think that a small injector is OK because they have seen that Mike Norris said you can run them at 115%.

Bottom line: When choosing injectors you should still use 80% DC in the calculation.

Andrew