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Joined: Jul 2002
Posts: 123
From: Enid, Oklahoma
Help
How do I figure out what size of fuel injectors and fuel pressure/fuel pump I need. What I have so far, is in my signature. I plan on getting Kook headers and a 58 mm monoblade. I am not for sure on what the horsepower is going to be. Any help would be appreciated.
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Joined: Nov 1998
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From: Hell was full so they sent me to NJ
I tried to give you an answer in your earlier post, but you deleted it.
Are you saying after investing all that money in a high end build-up, you don't have the slightest idea what kind of HP to expect?
How does a person decide on what size of injectors they should run?
By knowing the flywheel HP of the engine, and whether its normally aspirated or running a power adder. The latter allows you to guesstimate the brake specific fuel consumption (BSFC) of the engine. You also have to know the fuel pressure you will be running, and at what fuel pressure the injectors were flow rated.
For a short cut, try multiplying the estimated flywheel HP by 0.07 to get you in the ball park on the minimum injector size you should consider.
For a short cut, try multiplying the estimated flywheel HP by 0.07 to get you in the ball park on the minimum injector size you should consider.
Last edited by Injuneer; Apr 19, 2007 at 05:52 PM.
Thread Starter
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Joined: Jul 2002
Posts: 123
From: Enid, Oklahoma
I am shooting for 600 at the flywheel. My problem was that when I received my heads the chamber cc on my receipt didn't reflect on what we talked about. So I don't know if they are what shows on my receipt or what we discussed. I really hope that they are what we talked about over the phone. The heads look awesome. AI did a wonderful job on the heads. I can't wait to put the car on a dyno someday. Unfortunately I have a long way to go. Transmission needs rebuilt and I would like to get a beefier rearend. Now that all my money went to the motor I will have to wait awhile til I get some more money built up. It may take awhile but I bet it will be worth it in the end.
Last edited by silver95; Apr 19, 2007 at 07:28 PM.
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Joined: Nov 1998
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From: Hell was full so they sent me to NJ
OK.... 600 flywheel x 0.07 = 42 #/HR. That assumes a BSFC of 0.476 #/HR/HP and a max duty cycle of 85%. Also assumes the injectors were flow rated at 43.5psi, and your fuel system is running at 43.5psi. If you need more, consider running the fuel system at 58psi, and the same injectors will flow 48.5 #/HR.
Last edited by Injuneer; Apr 20, 2007 at 05:27 AM.
Thread Starter
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Joined: Jul 2002
Posts: 123
From: Enid, Oklahoma
OK. Now what percentage does a person want to work around when it comes to duty cycle? Is 80% better than 50%? If I decide to run nitrous does it make a difference on the injectors or since it is a wet system that has its own fuel and nitrous nozzles I won't need to worry about my fuel injectors? Sorry for all the questions. Thanks for you help!!
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Joined: Nov 1998
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From: Hell was full so they sent me to NJ
If you add a WET nitrous kit, the sizing of the injectors will not be affected.
Duty cycle is the percent of the available time that the injectors are open. If you are turning 6,000rpm there is only 20 milliseconds available in two revs of the crank. If the PCM was setting the injector pulse width at 20mSec, the injector would be open continuously, and would become unstable since the PCM is trying to open and close it.
You generally size the injectors to limit the duty cycle to no more than 80-85%. Higher is bad, because the injectors are overworked (= overheated), wearing them out, and the spray pattern and flow capacity become unstable. A lower duty cycle is not a problem. You only run into trouble when your injector is so large that is can't be "turned down" to the less-than-1mSec pulse widths required for the engine idle at a reasonable speed.
The number I gave you is a "safe" guideline. A efficient combustion chamber, with a good tune will use less fuel then 0.476 #/HR/HP. But it also does not leave you with a lot of room for additional HP in the future. You might consider an even larger injector. Once you go over 42#/HR, you may be looking at switching to a low impedance injector, and the stock PCM needs an external driver box to run low impedance injectors. 42 #/HR used to be the upper limit for high impedance injectors, but there are now some larger sizes available with high impedance.
Its also possible to increase the fuel pressure to cause the injector to flow more fuel than it is rated to flow. I run injectors that flow 64 #/HR at 39.15psi, but I run them at 58psi fuel pressure, so they flow 78 #/HR. Even running 800 HP with DRY nitrous, the injector duty cycle is under 70%.
Duty cycle is the percent of the available time that the injectors are open. If you are turning 6,000rpm there is only 20 milliseconds available in two revs of the crank. If the PCM was setting the injector pulse width at 20mSec, the injector would be open continuously, and would become unstable since the PCM is trying to open and close it.
You generally size the injectors to limit the duty cycle to no more than 80-85%. Higher is bad, because the injectors are overworked (= overheated), wearing them out, and the spray pattern and flow capacity become unstable. A lower duty cycle is not a problem. You only run into trouble when your injector is so large that is can't be "turned down" to the less-than-1mSec pulse widths required for the engine idle at a reasonable speed.
The number I gave you is a "safe" guideline. A efficient combustion chamber, with a good tune will use less fuel then 0.476 #/HR/HP. But it also does not leave you with a lot of room for additional HP in the future. You might consider an even larger injector. Once you go over 42#/HR, you may be looking at switching to a low impedance injector, and the stock PCM needs an external driver box to run low impedance injectors. 42 #/HR used to be the upper limit for high impedance injectors, but there are now some larger sizes available with high impedance.
Its also possible to increase the fuel pressure to cause the injector to flow more fuel than it is rated to flow. I run injectors that flow 64 #/HR at 39.15psi, but I run them at 58psi fuel pressure, so they flow 78 #/HR. Even running 800 HP with DRY nitrous, the injector duty cycle is under 70%.
Thread Starter
Registered User
Joined: Jul 2002
Posts: 123
From: Enid, Oklahoma
What brand of injectors do you use that seem to be pretty dependable? How does one figure on how many pounds a fuel injector will flow at a certain PSI. Also, how did you figure the duty cycle? thanks again
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Joined: Nov 1998
Posts: 71,094
From: Hell was full so they sent me to NJ
I use Bosch. Bosch (or Denso, under license to Bosch) makes the various FMS and SVO injectors used by Ford. Popular replacements for the stock LT1 injectors. They are all pintle style injectors and work well at high pressure.
Lucas and Siemans make good injectors. The stock ball & seat RP (now Delphi) injectors are very good, but do not like high fuel pressures. I think Delphi now makes some types other then the ball & seat. Check Racetronix for injectors.
An injector is rated to flow at a specific pressure. The stock injectors were flow rated at 3bar (= 43.5 psi). The FMS/SVO injectors seem to be flow rated at 2.7bar (39.15 psi). You can calculate the flow rating of an injector when it is operating at a different pressure by dividing the new pressure by the flow rating pressure, then taking the square root of the result. Example:
Running a stock injector (flow rated at 43.5 psi) at 58 psi:
(58 / 43.5) ^0.5 = (1.3333) ^0.5 = 1.1547
A 24 #/HR injector X 1.1547 flows 27.7 #/HR at 58 psi.
You calculate duty cycle by using a scanner to simultaneously measure rpm and injector pulse width. Then you use the formula:
duty cycle = (pulse width x RPM) / 120,000
If your data log shows you have a 16.64 millisecond pulse width, and the engine is running at 5,700 RPM:
(16.64 X 5,700) / 120,000 = 94,848 / 120,000 = .79 = 79% duty cycle.
Some scan software will do the calculation for you, like TTS DataMaster, but it appears that the DataMaster calculated duty cycle may be off a little because it bases the calculation on "base pulse width", and I think that the base pulse width is the one calculated by the PCM before applying certain corrections. I run my engine on a MoTeC M48Pro computer, and the built-in data logger calculates duty cycle and displays it real time.
Lucas and Siemans make good injectors. The stock ball & seat RP (now Delphi) injectors are very good, but do not like high fuel pressures. I think Delphi now makes some types other then the ball & seat. Check Racetronix for injectors.
An injector is rated to flow at a specific pressure. The stock injectors were flow rated at 3bar (= 43.5 psi). The FMS/SVO injectors seem to be flow rated at 2.7bar (39.15 psi). You can calculate the flow rating of an injector when it is operating at a different pressure by dividing the new pressure by the flow rating pressure, then taking the square root of the result. Example:
Running a stock injector (flow rated at 43.5 psi) at 58 psi:
(58 / 43.5) ^0.5 = (1.3333) ^0.5 = 1.1547
A 24 #/HR injector X 1.1547 flows 27.7 #/HR at 58 psi.
You calculate duty cycle by using a scanner to simultaneously measure rpm and injector pulse width. Then you use the formula:
duty cycle = (pulse width x RPM) / 120,000
If your data log shows you have a 16.64 millisecond pulse width, and the engine is running at 5,700 RPM:
(16.64 X 5,700) / 120,000 = 94,848 / 120,000 = .79 = 79% duty cycle.
Some scan software will do the calculation for you, like TTS DataMaster, but it appears that the DataMaster calculated duty cycle may be off a little because it bases the calculation on "base pulse width", and I think that the base pulse width is the one calculated by the PCM before applying certain corrections. I run my engine on a MoTeC M48Pro computer, and the built-in data logger calculates duty cycle and displays it real time.