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  #1  
Old 11-01-2012, 11:30 PM
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Default fuel injectors

well im gonna be building a 390 strocker for my truck, and i know the factory injectors wont be nearly enough, so whats a good brand/size ect to get?
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Old 11-04-2012, 11:56 AM
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Originally Posted by mr.lopar View Post
well im gonna be building a 390 strocker for my truck, and i know the factory injectors wont be nearly enough, so whats a good brand/size ect to get?
Fuel injector sizing is going to be based on your expected BHP output, fuel pressure, BSFC, and other variables. I would say try maybe 32lb/hr injectors. I like the Holley brand injectors, they're very good. This is for a 43 PSI supply and good for around 425 HP.
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Old 11-06-2012, 11:06 PM
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im sure its outta reach, but im tryin to get at least 500 horses out of it. goin with forged crank and pistons, h-beam rods, eddy heads, hughes airgap intake, big gulp throttle body, hi volume oil and water pumps, 8qt oil pan ect. just tryin to figure out injector size and cam size to get
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Old 11-07-2012, 04:14 PM
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im sure its outta reach, but im tryin to get at least 500 horses out of it. goin with forged crank and pistons, h-beam rods, eddy heads, hughes airgap intake, big gulp throttle body, hi volume oil and water pumps, 8qt oil pan ect. just tryin to figure out injector size and cam size to get
Well again the only way to determine exactly what is the best fuel injector size you need to know how much BHP the engines produces, the BSFC (if you engineer the engine to be very efficient you will lower BSFC. But for practical purposes for a V8 producing 550 HP with a fuel pressure at rail of 43.5 and an 80% duty cycle with a .5 BSFC you should use a 46 lb/hr injector. 500HP from a 390 is very possible, just be sure to try and keep the rod to stroke ratio close to 1.8:1 (longer lasting). The smaller the stroke the less piston rock and the less wear occurs to the bearings, cylinder wall, etc...
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Old 11-09-2012, 12:05 AM
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ok wth is BSFC?
how do i figure out the duty cycle?
not sure bout the stroke ratio, im ordering a 390 stroker kit (4" stroke crank) and .030" over bore
dont remember how much pressure the stock fuel pump puts out at the rail, will it be enough, or do i need to get a high voulme fuel pump to?
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Old 11-12-2012, 01:27 PM
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Originally Posted by mr.lopar View Post
ok wth is BSFC?
how do i figure out the duty cycle?
not sure bout the stroke ratio, im ordering a 390 stroker kit (4" stroke crank) and .030" over bore
dont remember how much pressure the stock fuel pump puts out at the rail, will it be enough, or do i need to get a high voulme fuel pump to?
BSFC is brake specific fuel consumption, it's basically how much fuel the engine consumes under a load. engineers generally simulate everything on a dyno so they will have flow meters hooked up and measure bsfc. but the rule of thumb (which i hate because im an self made engineer, soon to go to embry riddle) rule of thumb is .5 you can reduce bsfc (better MPG's) by increasing the pressure inside the manifold (this is where forced induction comes in) increase pressure by super charge, turbo charge, or by getting closer to the center of earth, in other words sea level is 14.5-14.7 psi, i always design by 14.7 and the lower in sea level you go the higher the pressure but it's very small.

the stroke ratio is the rod length divided by the stroke, if you were to draw out a sketch of a connecting rod and crank and look at the angles of the wrist pin and connecting rod when the angle of the crank is the connecting rod is 90 degrees then you would see that the longer the stroke well the throw then the larger the angle of the wrist pin. if the wrist pin angle is larger then the piston has a higher tendency to rock and cause piston slap etc... this would damage the walls and cause more bearing load mostly on the sides and would cause slight vibrations, this is why F1 engines have a very small rod and stroke to reduce the vibrations because vibrations at high speeds become deadly to the engine. but for all practical purposes engine builders say divide the rod length by the stroke, stay as close to 1.8 as you can and if you divide say the 318's rod and stroke 6.123/3.31 then you get rounded. 1.85:1. which is why the 318 is a pretty durable long lasting engine. to make more torque you can get a large stroke (works because w=fd so the large the distance than the more force) but you can also get a large bore to create more area and p=f/a so more pressure within the cylinder can be made if the area is larger. but the most effective is to enlarge the stroke but if you want a fast running engine that can keep up operating at high speeds you don't want to get a large stroke, or you want to make sure everything is balanced as well as a german engine.

as for the stock fuel pressure, it runs at 43.5 PSI. that is perfectly fine because if you can run a lower fuel pressure but you need larger injectors to compensate, same way as if you have a large BSFC you need either more fuel pressure or larger injectors. the duty cycle is setup by the computer, most vehicles run 85% duty cycle tops for production vehicles, race engines run 100% and they are also a low impedence so they are essentially faster but sadly they burn out faster too. but you aren't going to put enough RPM's to need 100% duty cycle and low resistance injectors. if you remember from HS physics, lower resistance causes more heat. basis of joule heating effect.
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Old 11-12-2012, 11:02 PM
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god i hate math lol

ok so you say i need 46# injectors, im thinking on goin with 42#, and in an email from heifever, he says i only need 24#
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Old 11-13-2012, 11:45 AM
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god i hate math lol

ok so you say i need 46# injectors, im thinking on goin with 42#, and in an email from heifever, he says i only need 24#
Well BSFC is based on A/F ratio, air pressure, density, pressure differential, among other things. But again rule of thumb. But fuel injector size is than based off of the BSFC, estimated BHP, duty cycle, and fuel pressure. I would go with 46# just incase, you really only need If I remember it is a 44 but it is common practice to go over a little just incase because of fuel pressure loss and changes in altitude etc... Only good to have a safety.

The math isn't bad, math is fun. 24# Is to small. 24# is too small for even 380HP. It is difficult to determine without doing the math and actually having a good idea of how much hp you will be able to produce, this is why simulation software is helpful! It isn't always accurate but it gives you a idea which is better then buying fuel injectors only to return them later for bigger ones.

The ECM does all the math to correct for excess fuel, etc...
I did the math for you to get the fuel injector size, as far as HP production I gave it an educated guess based on numbers from similar builds. Just remember it's easier to work with larger injectors because if it is too large you can always reduce the fuel pressure which is much easier than trying to increase it with a smaller injector. Especially since these systems run at 43 PSI.

Last edited by ramvan2500; 11-13-2012 at 11:47 AM.
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Old 12-31-2012, 03:01 PM
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Where r u plaining to get ur Pcm programmed at, because im doing almost the same thing
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Old 01-08-2013, 09:55 PM
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Just to let everyone know deachwerks injectors maybe wrong spelling they spell it weird has a calculator for injector sizeing in cc and lbs u should check them out they have great stuff for your fuel systems there big in the nissan world well import world and have some stuff for domestic cars theres alllways universal parts like 320 lph fuel pump. You guys should check them out can't hurt
Here's a link to http://www.deatschwerks.com/resources/fuel-calculators/

Last edited by sr20boostjunkie; 01-09-2013 at 11:46 AM. Reason: added a link
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