What size of injectors?
Thread Starter
On The Tree
Joined: Jan 2005
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What size of injectors? I built a 383ci LT4 with 210cc AFR Competition Heads.. I some 383ci LT4 engine packgae 30lb injectors are used, and people said to me that I should use 30lb injectors. But now I'm totally confused because racetronix said to me that I should use minimum 42# injectors. I guess I will have 550hp at flywheel,so is it not enough "30lb" injectors??? Its a daily car,not a race car,not a drag car..
Sugestions very important for me..
thanks
Sugestions very important for me..
thanks
TECH Apprentice
Joined: Feb 2005
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According to the math, Racetronix is correct -- if you are going to be running 550hp at the fly.
Fuel injector requirements = [Peak HP x BSFC / Duty Cycle / # of Injectors]
[550 X .5 / .8 / 8] = 42.97# injectors -- Which means you could support 550 horsepower at 80% injector duty cycle with ~42# injectors at stock fuel pressure (3bar/43.5psi). It's recommended to keep duty cycle at or below 80-85%.
Unless you boost your fuel pressure to a whopping ~6bar (87psi), those 30# injectors aren't going to support 550 horses. (Not that you'd ever try going that route, but the injectors would no doubt fail at that kind of pressure anyway.) Otherwise, you'll run out of injector with that kind of power = lean running condition at WOT = not good.
IMHO, fueling is not the place to skimp. And, there's nothing wrong with using a larger injector than what you actually need. -Gives you a safety margin...
Fuel injector requirements = [Peak HP x BSFC / Duty Cycle / # of Injectors]
[550 X .5 / .8 / 8] = 42.97# injectors -- Which means you could support 550 horsepower at 80% injector duty cycle with ~42# injectors at stock fuel pressure (3bar/43.5psi). It's recommended to keep duty cycle at or below 80-85%.
Unless you boost your fuel pressure to a whopping ~6bar (87psi), those 30# injectors aren't going to support 550 horses. (Not that you'd ever try going that route, but the injectors would no doubt fail at that kind of pressure anyway.) Otherwise, you'll run out of injector with that kind of power = lean running condition at WOT = not good.
IMHO, fueling is not the place to skimp. And, there's nothing wrong with using a larger injector than what you actually need. -Gives you a safety margin...
TECH Apprentice
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To get 37# injectors to perform like a 42# injector? You'd need to run ~56.05psi of fuel. To get the volume of fuel you need, you're probably going to have to upgrade fuel pumps if you havent already. -Perhaps not a bad idea to upgrade pumps no matter which route you take...
If you are curious, here's how I get that number:
-The standard formula for finding 'injector performance' at increased fuel pressure is:
sqrt(new fuel pressure / old fuel pressure) X old injector size = equivalent NEW injector size.
-If you swap that all around to solve for 'new fuel pressure', you get:
(desired fuel injector size / regular injector size)^2 X old fuel pressure = New required fuel pressure.
(42/37)^2 X 43.5psi = 56.05psi
So, reader's digest version, you'd need to run a fuel pressure of roughly 56.05psi to get 37# injectors to run the same as 42# injectors. Again, to get the volume of fuel you need, you're probably going to have to upgrade fuel pumps...
If you are curious, here's how I get that number:
-The standard formula for finding 'injector performance' at increased fuel pressure is:
sqrt(new fuel pressure / old fuel pressure) X old injector size = equivalent NEW injector size.
-If you swap that all around to solve for 'new fuel pressure', you get:
(desired fuel injector size / regular injector size)^2 X old fuel pressure = New required fuel pressure.
(42/37)^2 X 43.5psi = 56.05psi
So, reader's digest version, you'd need to run a fuel pressure of roughly 56.05psi to get 37# injectors to run the same as 42# injectors. Again, to get the volume of fuel you need, you're probably going to have to upgrade fuel pumps...
Thread Starter
On The Tree
Joined: Jan 2005
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Oh its great thank you Alex..!
BTW: I have racetronix 255lb Walbro pump kit (with harness) ,and aeromotive Adjustable Fuel Pressure Regulator..
So if I go with 37lb injectors with ~60psi pressure that will not be a problem?? is this pressure not hurt the injectors??
BTW: I have racetronix 255lb Walbro pump kit (with harness) ,and aeromotive Adjustable Fuel Pressure Regulator..
So if I go with 37lb injectors with ~60psi pressure that will not be a problem?? is this pressure not hurt the injectors??
TECH Apprentice
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As for the pump -- that's good. 
I just ordered the same for my buildup...
As far as running higher pressure with smaller injectors, you might talk to Racetronix about that. High-impedance injectors tend to get sloppy with higher fuel pressures. If you push it too far, they can lock up on you. I'm not really certain at what pressure this becomes a problem, but Racetronix would know...
I'm guessing that they'll simply recommend stepping up to 42# injectors -- keeping the fuel pressure lower. That's the route I would recommend. The 37/42# injectors are the same price, so why not?
(-That is, unless you already have the 37#'ers, or found some used...)
I just ordered the same for my buildup...As far as running higher pressure with smaller injectors, you might talk to Racetronix about that. High-impedance injectors tend to get sloppy with higher fuel pressures. If you push it too far, they can lock up on you. I'm not really certain at what pressure this becomes a problem, but Racetronix would know...
I'm guessing that they'll simply recommend stepping up to 42# injectors -- keeping the fuel pressure lower. That's the route I would recommend. The 37/42# injectors are the same price, so why not?
(-That is, unless you already have the 37#'ers, or found some used...)
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Thread Starter
On The Tree
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No, I didn't buy them yet ,but I ordered and I can still change my order, but some people say too large of injectors (like 42lb) will make the car very hard to tune and have hurt the drivability..
BTW:I know someone runs the duty cycle at about %90 without a problem. How we can adjust the duty cycle at 80-85% ???
BTW:I know someone runs the duty cycle at about %90 without a problem. How we can adjust the duty cycle at 80-85% ???
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Verdad Gallardo TECH Apprentice
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Well, setting up injectors properly is somewhat multifaceted -- there are a lot of possibilities. Keep in mind that all of these formulas are merely estimates!!! It's impossible to predict exactly how your motor is going to run. Perhaps you wont hit exactly 550hp, and perhaps your brake-specific fuel consumption (BSFC) will be slightly lower than .5 -- it's impossible to know.
Duty cycle isn't something you can adjust directly. It is dependant on how hard the injector has to work to get the proper amount of fuel into the motor (horsepower, fuel pressure, etc). It simply refers to 'how much of the time' the injector has to remain open. Naturally, injectors can't produce a pulsewidth longer than 100% -- that's open full-time.
*Once you start getting closer to 100% DC, the injectors will again start to get sloppy and/or fail. This is the reason for keeping it at 80-85%. You can go higher, but you risk running into fueling problems.
You can use that first formula to determine how much power you can support with a certain injector / duty cycle. To maintain an 80% DC, you could use either 42# injectors @ 43.5psi, or 37# @ 56psi. If you want to push a 37# injector past 80% DC, then you could run a lower fuel pressure. It's as simple as that. At stock fuel pressure with the amount of horsepower you're wanting to run, your 37# injectors would be nearing 93% DC.
That being said, yes, I would assume it's more difficult to tune for larger injectors / offsets. I'm not really computer savvy, but I'm sure it can be done. But just the same, I don't like the idea of running high fuel pressures and/or LARGE high-impedance injectors -- either can get sloppy. Because of this, I'm going to run the Acceleronics box so that I can use low impedance injectors at low fuel pressure...
Duty cycle isn't something you can adjust directly. It is dependant on how hard the injector has to work to get the proper amount of fuel into the motor (horsepower, fuel pressure, etc). It simply refers to 'how much of the time' the injector has to remain open. Naturally, injectors can't produce a pulsewidth longer than 100% -- that's open full-time.
*Once you start getting closer to 100% DC, the injectors will again start to get sloppy and/or fail. This is the reason for keeping it at 80-85%. You can go higher, but you risk running into fueling problems.
You can use that first formula to determine how much power you can support with a certain injector / duty cycle. To maintain an 80% DC, you could use either 42# injectors @ 43.5psi, or 37# @ 56psi. If you want to push a 37# injector past 80% DC, then you could run a lower fuel pressure. It's as simple as that. At stock fuel pressure with the amount of horsepower you're wanting to run, your 37# injectors would be nearing 93% DC.
That being said, yes, I would assume it's more difficult to tune for larger injectors / offsets. I'm not really computer savvy, but I'm sure it can be done. But just the same, I don't like the idea of running high fuel pressures and/or LARGE high-impedance injectors -- either can get sloppy. Because of this, I'm going to run the Acceleronics box so that I can use low impedance injectors at low fuel pressure...
Thread Starter
On The Tree
Joined: Jan 2005
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Now everything is clear! I looked at Racetronix website and saw this:
For 37lb/hr Delphi Injector:
Static Flow Rate: 37.27 lb/hr @ 43.5PSI (300kPa) or 282gm/min
Static Flow Rate: 43.04 lb/hr @ 58.0PSI (300kPa) or 326gm/min
So I understood that we can go with 3 or 4 bar fuel pressure on their injectors without a problem.. As you say we should stay at %80-85 DC for injectors' health..
thanks Alex you are the one
For 37lb/hr Delphi Injector:
Static Flow Rate: 37.27 lb/hr @ 43.5PSI (300kPa) or 282gm/min
Static Flow Rate: 43.04 lb/hr @ 58.0PSI (300kPa) or 326gm/min
So I understood that we can go with 3 or 4 bar fuel pressure on their injectors without a problem.. As you say we should stay at %80-85 DC for injectors' health..
thanks Alex you are the one
TECH Apprentice
Joined: Feb 2005
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D'oh -- didn't see that on their site. If they have that listed, then yeah -- they obviously believe the injector can function correctly at that much fuel pressure. Otherwise, you wouldn't expect them to advertise it. 
Well, I hope that all works out OK. My only other concern would be the factory fuel lines. Let us know how it goes~
Well, I hope that all works out OK. My only other concern would be the factory fuel lines. Let us know how it goes~
