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Discussion Starter #1
i have 38lb injectors that came with my roush m90 s/c kit but i have a steeda cai and im putting on headers, stage 2 blower cams, possibly a pully, and a gt500 fuel pump, are the injectors going to be enough for this setup? Im shooting for 450-500 rwhp
 

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The Mustang Shop
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No, 39lb injectors are good for around 475whp.

There's a formula you can generally go by. It depends on duty cycle and fuel pressure as well.

HP X BSFC (.5 for n/a or .6 for boost)
----------------------------------------- = injector size
# of injectors (8) X max duty cycle (80%) .80

It would look something like this:

HP x .6
------- = ?
8 x .80

HP x .5
------- = ?
8 x .80

500hp x .6 = 300

(# of injectors X 80% injector duty cycle)= 6.4

300 divided by 6.4 = 46lb injectors
 
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Discussion Starter #3
eventually i plan on having 600+hp would buying injectors for the future that are over kill for this setup hurt it at all right now?
 

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eventually i plan on having 600+hp would buying injectors for the future that are over kill for this setup hurt it at all right now?
Buy 60lb injectors.
 

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Discussion Starter #6
just two last questions is that formula for rwhp or flywheel hp? and what injectors do you recommend, are frpp injectors good?
 

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just two last questions is that formula for rwhp or flywheel hp? and what injectors do you recommend, are frpp injectors good?
I have always used the FRPP injectors and had good luck. The formula is for rwhp.
 

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With current setup 39lb. injectors is all you'll need, BUT to get to 600 hp you'll need the 60lb. injectors.
 

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When I was at 490whp, the 39lb injectors were maxed.
 

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Ford Racing says: 39lb 530hp na and 408hp fi at 85% duty cycle. 60lb na 816hp and 628hp fi at 85% duty cycle. Of course these are for the Ford Racing ones. Get the 60lb if you plan on upgrading you set up for more hp. Make sure you get a tune set up for what injectors you have.
 

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Oh and the formula I have is:

(Inj. size you plan to use)x(number of cylinders)x.85( .85 is the duty cycle) divided by .50 for Natrualy Asp. and .65 for Forced Ind.
 

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True, but with his setup he will be lucky to hit 450-460.
Exactly. My previous Whipple H.O. kit came with the 38#s, and were at around 90% at 505/468. OP you should be fine for now, but when you upgrade blowers and internals get different injectors. I did have GT500 pumps too if that makes a difference.
 

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Oh and the formula I have is:

(Inj. size you plan to use)x(number of cylinders)x.85( .85 is the duty cycle) divided by .50 for Natrualy Asp. and .65 for Forced Ind.
I have used that method for flywheel HP.

60lb injectors x 8 x .85 / .50 = 816 HP.

816bhp x .85 = 694whp x .5 = 347 / 6.4 = 54lb injectors. Round up to 60lb.

True, but with his setup he will be lucky to hit 450-460.
You are probably right. I suggested 60's so he has room to grow.

I think we are all saying the samething. 39lb for under 475whp, 60lb if you want to make more than 475whp.
 

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I have used that method for flywheel HP.

60lb injectors x 8 x .85 / .50 = 816 HP.

816bhp x .85 = 694whp x .5 = 347 / 6.4 = 54lb injectors. Round up to 60lb.



You are probably right. I suggested 60's so he has room to grow.

I think we are all saying the samething. 39lb for under 475whp, 60lb if you want to make more than 475whp.
:yes:yes
 

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I am running the injectors that came with my m90 kit and I have lt headers and vmp 500 horse kit(2.57 pulley, frpp cai, tuner). I just finished doing some data logging so I'll check into what duty cycle I was at and get back to you:)
 

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Yeah let me know cause I'm running a simalar set up and never data logged it. I had 38lb. , stock pump, Roush M90 w/ 2.57 pulley, Steeda CAI 90mm mas air housing. Having my gt500, 3.73 gears and 47lb injectors installed as we speak.
 
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