Calculating HP vs Fuel Injector Size
11-21-2006, 04:14 PM
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Calculating HP vs Fuel Injector Size Okay, I've tried the Sticky several times and the links no longer work so I figured it was time to post something useful that doesn't rely on another site. If the other site doesn't come back up, perhaps this should become a sticky?
I've been going over injector sizes to figure out if the injectors I have are appropriate for my application or not.
If someone wants to chime in and/or correct my math please do so.
I'm deriving my formulas from the link below:
http://www.cranecams.com/?show=newsLetters&no=160
(They use 48# injectors in their example.)
This means that if you have a 28lb injector (which I do) you multiply it by 2.
So one 28lb injector can theoretically support 56hp.
Now since injectors spend 10% of their time in the off position, you can actually only flow 28 x .9 = 25.2lbs of fuel out of it, which would support 50.4hp per 28lb injector.
So to be safe we should actually take the 28lb multiply it by .8 yielding a safe estimate of 22.4lbs/hr of fuel or 44.8 hp per 28lb injector.
Since I have 8 injectors 44.8 x 8 = 358hp max safely on 28lb injectors.
From my reading good tuning can yield better than .5 BSFC but .5 is a good safe number to use for NA applications.
My basic equation is
(Injector size) times (Number of injectors) Divided by (BSFC) times (Duty Cycle)
28 x 8 / .5 x .8 = 358.4
By those calculations I do not have large enough injectors since my H/C setup should put me in the 400-450HP range, prior to my wet N2o Kit, thus I should be using 32 if not 36# injectors.
The simplest equation I've derived is for every 1lb of injector, it should safely produce 1.6hp @ 80% duty cycle, or 1.8hp maximum @ 90% duty cycle.
Other sizes:
26LB injectors = 41.6HP per injector safely (8 = 332.8 HP Safely [374HP MAX])
28 = 44.8HP (8 injectors = 358.4 HP Safely [403HP MAX])
32 = 51.2HP (8 injectors = 409.6 HP Safely [460HP MAX])
36 = 57.6HP (8 injectors = 460.8 HP Safely [518 HP MAX])
40 = 64HP (8 injectors = 512 HP Safely [576 HP MAX])
This obviously reinforces why GM decided to switch from 26# injectors in 2001 and go to 28#. You can see that a stock LS1 in 2000 was dangerously close to the safe duty cycle/hp calculation since the WS6 could come with 325HP.
I've been going over injector sizes to figure out if the injectors I have are appropriate for my application or not.
If someone wants to chime in and/or correct my math please do so.
I'm deriving my formulas from the link below:
http://www.cranecams.com/?show=newsLetters&no=160
(They use 48# injectors in their example.)
Keeping in mind the "old rule of thumb" that it takes 0.5 lbs of gasoline per hour to make 1 HP (this is referred to as the Brake Specific Fuel Consumption-BSFC);
So one 28lb injector can theoretically support 56hp.
Also, in operation, a fuel injector must be turned off at least 10% of its operating cycle (duty cycle).
To be on the safe side, we recommend fuel injectors that would provide the necessary fuel flow at 80% of their free flow rating.
Since I have 8 injectors 44.8 x 8 = 358hp max safely on 28lb injectors.
From my reading good tuning can yield better than .5 BSFC but .5 is a good safe number to use for NA applications.
My basic equation is
(Injector size) times (Number of injectors) Divided by (BSFC) times (Duty Cycle)
28 x 8 / .5 x .8 = 358.4
By those calculations I do not have large enough injectors since my H/C setup should put me in the 400-450HP range, prior to my wet N2o Kit, thus I should be using 32 if not 36# injectors.
The simplest equation I've derived is for every 1lb of injector, it should safely produce 1.6hp @ 80% duty cycle, or 1.8hp maximum @ 90% duty cycle.
Other sizes:
26LB injectors = 41.6HP per injector safely (8 = 332.8 HP Safely [374HP MAX])
28 = 44.8HP (8 injectors = 358.4 HP Safely [403HP MAX])
32 = 51.2HP (8 injectors = 409.6 HP Safely [460HP MAX])
36 = 57.6HP (8 injectors = 460.8 HP Safely [518 HP MAX])
40 = 64HP (8 injectors = 512 HP Safely [576 HP MAX])
This obviously reinforces why GM decided to switch from 26# injectors in 2001 and go to 28#. You can see that a stock LS1 in 2000 was dangerously close to the safe duty cycle/hp calculation since the WS6 could come with 325HP.
