i got super-bored in my marketing class earlier this week, so i made out a chart listing the rated flowrates, 85% and 80% duty cycle flowrates, and approximate horsepower figures that are supported by the above rates using the injector sizing equation with corbins BSFC of .525. ideally, i would like some help in filling out the rest of the chart to include the approximate idle, static and boost fuel pressures at each 1 pound increment of boost pressure for each injector size at each duty cycle. sounds complicated, but if it can be done it would be an extremely useful resource for lots of people.

using the formula:

horsepower x BSFC (.525) x 10.5 (converting # to cc)
---------------------------------------------------- =
number of injectors (4)



horsepower x 5.5125
------------------- =
4


horsepower x 1.378125 = injector size required (actual flow)

so, also: injector size / 1.378125 = supported horsepower

the CHART:

injector flowrate
(rated 100% DC) 85% DC 80% DC ... HP 85%/80% IDLE FP (est.)
310-315cc (30#) ...263cc ...248cc... ........190/179 .....?
. . . .330cc (32#) ...280cc ...264cc... ........203/191 .....?
370-378cc (36#) ...314cc ...296cc... ........227/214 .....?
. . . .400cc (38#) ...340cc ...320cc... ........246/232 .....?
. . . .420cc (40#) ...357cc ...336cc... ........259/243 .....?
. . . .440cc (42#) ...374cc ...352cc... ........271/255 .....?

so, does anyone know the formula for determining the necessary fuel pressures to run at each of these flowrates? or at least the idle FPs?
i can make the chart into an actual spreadsheet or table once all the values are available. i didn't have time to reasarch the rest. but whats here will at least give people an idea of what size injectors they will need if they are aiming for a specified horsepower, and want to run the injectors at a specific duty cycle.

i understand that the needed fuel pressures vary for each car, but as i understand it, there should be a mathematical IDEAL fuel pressure at each boost level, and at idle. all of this assumes no SAFC is used and the injectors are at 100% DC. i looked over the formulas again but i cant crunch the numbers.

for example, if you use the formula here:


and you wanted to make the 319hp that you should theoretically be able to make with 440s at 100% DC

then:

approximate injector size = 440cc
injector size also = 440cc (because you are running them at 100%)

440/440=1
1 squared = 1
1 x 43.5 = 43.5psi <-- this would be your recommended idle FP???
add 1 to that for each psi of boost, and your 15psi boost fuel pressure would be 58.5? is this correct?

OR

you could decide you want to make 319hp with an 85% DC, also on 440s:
approximate injector size = 517cc (440/.85)
injector size = 440cc
517 / 440 = 1.175
1.175 squared = 1.3817
1.3817 x 43.5 = 60 psi idle (75psi at 15 pounds of boost)?

if these numbers and my math are correct i will finish the rest of the table.

PS - do you have to subtract values from the final total to account for vacuum at idle (for example, instead of saying 43.5psi idle FP in the first example, say 43.5 - idle vacuum?

Thanks cyan, this really helps me understand it a lot better. This should be put in a sticky for future reference. Good job man!:thumbsup :bowdown

no problem, i know how confusing it all is, especially at first. i'm working on a word table right now that i will post as a jpeg in a few minutes.

its all chinese to me :) good job tho

Great info man :thumbsup..

When I had my old FMU setup with 32lb injectors, I did 270whp on the dyno last year...Your 100% DC for 32lb is 239hp :shrug

Edit: I'm guessing the higher FP and SAFC gave me extra ponys with those 32lb injectors?

or you were running lean. FP doesn't make any difference because if the DC is 100% the injectors are flowing at their max regardless of the fuel pressure. of course the higher pressures would cause the injectors to spray a little above their rating i suppose, but not enough to make a massive difference. of course...the first thing i thought of when i saw these HP numbers were that they are pretty low. also consider, these horsepower numbers are using a BSFC of .525. if you were running a lower BSFC then you could get more power from the same size injectors. but 319 WHP does sound pretty low for 440s at 100% because i think we've seen a few dynographs here with 440s and SFMU that are well over 319whp. maybe corbin can shed some light on this.

Originally posted by cyan
or you were running lean. FP doesn't make any difference because if the DC is 100% the injectors are flowing at their max regardless of the fuel pressure. of course the higher pressures would cause the injectors to spray a little above their rating i suppose, but not enough to make a massive difference. of course...the first thing i thought of when i saw these HP numbers were that they are pretty low. also consider, these horsepower numbers are using a BSFC of .525. if you were running a lower BSFC then you could get more power from the same size injectors.

All that information is completely wrong. Fuel pressure plays an important role in the flow rate of a fuel injector. It is why injector flow rates are calculated at 43.5psi. At a different fuel pressure the flow rate for the injector would be completely different.

Also, you're bringing in duty cylce into all of this as if it's something that you can control on the stock ECU. It's not. It's exactly why all the turbo kits come with RRFPR (greater than 1:1). We supply more fuel by increasing the fuel pressure because we cannot adjust duty cycle.

Lastly, using a BSFC of .525 on a turbo engine isn't correct. You should be using .600 or higher.

I didn't look through the calculations you made but they are most likely incorrect because of the conclusions that you've drawn from them.

I'm not trying to be an asshole but this stuff is just completely off. I know what Corbin has written is correct on his site. I would take a look at it again. You may also want to read the tech section on RC Engineering's site.

corbins site says specifically that our cars use a .525 BSFC when turbocharged. he does say that that is for our cars stock engine, but then he goes on to use that number in calculating the needed injector size for 275 HP, which is inachievable without forced induction, so apparently, at least according to him, .525 is the number we should be using for our cars when they are making 275-400 hp or more.

Find out how much fuel you will need per injector. Assume a BSFC of 0.525 for this engine, based on its design. You have 4 fuel injectors. Your approximate maximum horsepower is 275. Your approximate fuel flow rate per injector is calculated below.


in addition to that, all of the numbers above i got from using the formulas on his site. all of the formulas are in the first post. please look them over and compare them to the ones on corbin's site and you'll see they are the same...at this point, go through the numbers and check my math...thats why i asked someone who knows more about it to double check it for me.

i checked RC's website, and my math is correct according to their tech info as well. here is an example using their formulas here:


using 42lb (440cc) injectors, thats 42 x .8 = 33.6
33.6 / .525 = 64
64 x 4 = 256 hp
i wrote 255, so i was off by 1hp

if i use you suggestion of .6 BSFC, it changes that final number to 224hp. but you are correct about the pressures making changes to the hp outputs that arent exactly as negligible as i thought they would be so i will have to modify the table a little bit.