Performance injectors, hurt or help mileage?
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From: baren desert wasteland, s
Performance injectors, hurt or help mileage?
I'm wanting to add more horses to my 99 f150 supercab short bed, I have done several mods already and am running out of things to do but still want more power. I do not want to do anything to my truck that will hurt the mileage because it already isn't too great, usually about 12-13.5 miles to the gallon. I have been looking at performance injectors and there are different flow rates you can get based on your horses, acording to how many horses i have i should be getting probably about 36lb/ hour flow rate. If I do this will it hurt my mileage or help it since I have other mods that probably need the higher flow rate?
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Joined: Nov 2002
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From: Memphis, TN
Performance injectors, hurt or help mileage?
36lb/hr is INSANE! 
The stock ones shouldn't be bigger than 19lb/hr but probably 17lb/hr, and you're considering double that. The engine probably won't even run with those.
The stock ones shouldn't be bigger than 19lb/hr but probably 17lb/hr, and you're considering double that. The engine probably won't even run with those.
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From: Copley, OH
Performance injectors, hurt or help mileage?
Rule of thumb for injector sizing is assume a BSFC of .5 lbs/HP-hr and an 80% duty cycle. This gives a conservative sizing. Usually a well built engine can do a bit better than 0.5, somewhere around 0.47 or so and you can push the duty cycle up to 90%
Based on that 36# injectors would produce enough flow to support :
(36 x 8 x 0.8)/0.5 = 460 HP
If you assume .47 BSFC and 90%
(36 x 8 x 0.9)/.47 = 551 HP
Somehow, I think 36s are just a bit too big. Most well built Mustang 5.0s run nicely on the 24# units.
FWIW bigger injectors will add exactly zero HP to your engine. All they will do is provide enough fuel to support the other mods you've made. Unless you've added a hotter camshaft, aftermarket heads, a bigger intake and T/B and headers and a free-flow exhaust, you're unlikely to need bigger injectors. The stock 19# units are good for 240-290 HP.
Based on that 36# injectors would produce enough flow to support :
(36 x 8 x 0.8)/0.5 = 460 HP
If you assume .47 BSFC and 90%
(36 x 8 x 0.9)/.47 = 551 HP
Somehow, I think 36s are just a bit too big. Most well built Mustang 5.0s run nicely on the 24# units.
FWIW bigger injectors will add exactly zero HP to your engine. All they will do is provide enough fuel to support the other mods you've made. Unless you've added a hotter camshaft, aftermarket heads, a bigger intake and T/B and headers and a free-flow exhaust, you're unlikely to need bigger injectors. The stock 19# units are good for 240-290 HP.
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From: baren desert wasteland, s
Performance injectors, hurt or help mileage?
Thanks for the info. Just so you know I have done several mods to my truck which is why I'm asking about the injectors. I have completly free flowing exhaust, dual 3 inch exhaust with a magnaflow dual in dual out muffler, no cat converters, gibson headers. For the engine I have put an airaid intake, 75mm bbk throttle bodie, Helix throttle body spacer, a tornado (wich I'm not sure if that does anything or not) and a superchip. I figure I have somewhere in the neighborhood of 380-400 horses so by your calculations I should probably be getting about 30# injectors. So my question is will this hurt my gas mileage or will it just help the other mods that I'v done?
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From: baren desert wasteland, s
Performance injectors, hurt or help mileage?
Well seeing as how my truck came with 260 horses I doubt thats all that it has after putting dual 3 inch exhaust with a magnaflow dual in dual out muffler, no cat converters, gibson headers, airaid intake, 75mm throttlebody, mass airflow sensor, throttlebody spacer and a superchip. Being generous in my calculations (the minimum hp gains that it says like 15-25 hp I would calculate 15 instead of the 25) I came up with 425 hp but I don't think thats quite right which is why I said I have probably about 380-400hp. and I'll I was really asking was with all those mods would putting the correct flow rate of injectors in if it would hurt my mileage, I understand more gas is being put into the engine but it would be getting a better fuel air mixture and you wouldn't have to push down as much on the gas pedal to get the same results. Just to clarify what my situation was and what my point of view and why I was asking. I do appreciate the replies.
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Joined: Dec 1999
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From: Copley, OH
Performance injectors, hurt or help mileage?
Missed the '99 part. The stock normally aspirated 4.6L 4 cam makes what, 320 HP? You're not making any more and probably quite a bit less. In no case are you making more than your injectors allow and in no case will you need more than 24s.
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Performance injectors, hurt or help mileage?
STRANGERANGER .. sounds like you have done some research with EFI systems .. I'm in the building stage and was looking to set a base line for the EFI .. i'm building a 392 stroker .. using dart sr heads with 202 intakes 165 exhaust .. comp 35-518-8 cam with around 10.25 compression..I have a 90 mustang eec harness and processor with mass air and i'm going to try to modify the 351 truck efi manifolds to run on this combo .. Where would you start ? .. also where can i get the info to plug into your formula to figure out the injector size to start out with ? How much flexability is there in the EEC system .. ?
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From: Copley, OH
Performance injectors, hurt or help mileage?
24# injectors work well up to around 310-365 HP.
30# are good to around 380-460 max.
36# are good to around 460-550 max.
How much power do you intend to make? I'm guessing that a set of 30s would do the job for you, depending on how fast you're going to spin the engine. If you rev the pee out of it, it might make enough power to need 36s. Wouldn't that just be too bad
30# are good to around 380-460 max.
36# are good to around 460-550 max.
How much power do you intend to make? I'm guessing that a set of 30s would do the job for you, depending on how fast you're going to spin the engine. If you rev the pee out of it, it might make enough power to need 36s. Wouldn't that just be too bad
5th Wheeling
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Performance injectors, hurt or help mileage?
I'm going to cap it around 6K .. a 351/392 is kinda heavy to spin up to high without some serious bottom end work .. and thats about where the cam fizzels out too ... I think my real handicap is going to be the intake system .. like i said I'm going to try to open up the stock 351 EFI manifolds and throttle body and give it a shot before i take the plunge on the after market .. if you have any more info or info sources I'm all ears .. I was also thinking of throwing this project out in the forum and seeing if anyone else has been this route ...
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Originally Posted by ssKable
Well seeing as how my truck came with 260 horses I doubt thats all that it has after putting dual 3 inch exhaust with a magnaflow dual in dual out muffler, no cat converters, gibson headers, airaid intake, 75mm throttlebody, mass airflow sensor, throttlebody spacer and a superchip. Being generous in my calculations (the minimum hp gains that it says like 15-25 hp I would calculate 15 instead of the 25) I came up with 425 hp but I don't think thats quite right which is why I said I have probably about 380-400hp. and I'll I was really asking was with all those mods would putting the correct flow rate of injectors in if it would hurt my mileage, I understand more gas is being put into the engine but it would be getting a better fuel air mixture and you wouldn't have to push down as much on the gas pedal to get the same results. Just to clarify what my situation was and what my point of view and why I was asking. I do appreciate the replies.
Your 260 stock HP probably would have put 210-220 to the wheels on a dyno. Add maybe 30-40 RWHP total for the intake/exhaust mods and another 20 or so for the Superchip and you end up with around 280 horses to the rear wheels. Give 20% for parasitic losses and you are looking at flywheel HP in the mid 330's.
If you really want to run 36 lb injectors you can, but you will need a blower....a big one.