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Overlooked
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4,677 Posts
Discussion Starter #1 (Edited)
As the title says, this should be the end all to injector sizing questions. I'll try to explain the math behind it all also so you can try it out yourself. This is only a guide and should not be taken as written in stone. Alot of things can change the outcome such as atmosphere (altitude, humidity, temp) and fuel types. But this WILL allow for some of these changes such as lean spikes and the likes.

First, to explain some of the things involved. I took into account A/F ratios, BSFC, VE, injector duty cycles, and a few other things. I used a common 13:1 A/F ratio, a .5 BSFC (I'll explain this in a minute), 95% VE (this too) and 80% duty cycles for common injector sizes.

Most know what A/F ratios are. It's the amount of air per unit of fuel. So 13 cups of air to 1 cup of fuel so to say. If you richen it up a bit to say 12:1 or 11:1 you might say that it will help but you will actually raise the BSFC makeing less power per unit of fuel.

BSFC (Brake Specific Fuel Consumption) is a bit tricky. It's kind of like HP measurements as it doesn't really fit anywhere but to refrence something. This something is a ratio of fuel consumption to HP produced. Not all fuel is turned into mechanical power (driving the piston down) but turned into heat and "wasted" energy. This ratio shows how much fuel is being used to make mechanical power. Side note, most forced induced motors have a slightly lower BSFC .40-.45 meaning more fuel is turned into more mechanical energy. This also realates to VE and A/F ratio. You really only see this number on a dyno.

VE, or Volumetric Efficeincy, is how much fresh air and fuel actually get into the cylinder on each intake stroke. 2 valve carb engines are around 85%, 4 valve carb - 88-90% 2 valve injection - 90% 4 valve injection - 92-95% and 4 valve force induced (us) are from 95-100%, sometimes 105-110%! This can happen from the sheer volume of air being forced (but will create heat) or, more preferably, intake/exhaust manifold design and valve size and timing.

Injector duty cycles are just how hard/long the injectors work per stroke.

The math behind it. You can skip down if you haven't already lol. Simple algerbra really with some key conversion numbers.

The equations:
Air flow - (((x*A/F*BSFC)*60)/A/F)*VE ***(VE needs to be intverted. 95% would be 1.05, 90% would be 1.10 etc.)***
Fuel - ((x*10.5092)*6)*.8 "x" = injector size 6 = number of cylinders .8 = duty cycle

First is this little equation - x*13*(.5/60) where (x) is the desired HP goal (flywheel HP), 13 is the A/F ratio, and .5/60 is the BSFC converted from hours to minutes. This will give you the flow rate in lbs/min of air.

Next, we convert this to lbs/hr by multiplying by 60 which is 3,250lbs/hr.

Now to take our desired injector and convert it from cc/m (cubic centimeters per min) to lbs/hr. It uses this equation: x/10.5092. 10.5092 is one of those key numbers I was talking about to take cc/m straight to lbs/hr.

So the common 550cc injector flows 59.58lbs/hr. Now we need to multiply this by 6 to show the amount of fuel for the whole engine which is 314lbs/hr. Now to imply our duty cycle or 80% - 251lbs/hr

Next we imply the VE and A/F ratio. x/13*1.05. "x" is the 3250lbs/hr. The reason I multiplied the 1.05 instead of .95 for our 95% VE compensates for the fuel requirements in the charts. This equation gives us the individual unit of air so we can see how much fuel we would need. 3250/13*1.05=262.5lbs/hr

Take the two numbers in red. 251 is the amount of fuel at 80% the 550cc injectors can supply. The 262.5 is the amount of fuel needed to make 500HP. Now to show that 550cc's are not recommended for 500HP. This HP figure is FLYWHEEL HP. Manual transmission rob 5-10% power where autos can take up to 25%!!! If you have a manual, the most you could put down, SAFELY, is around 425-450WHP. Ouch.

Here are some charts. First one is MaxHP by injector size along with required/supplied fuel.
-------------------------------------
l 550cc l 475 BHP l 249/251 l
l 650cc l 560 BHP l 294/296 l
l 680cc l 590 BHP l 309/310 l
l 750cc l 650 BHP l 341/342 l
l 780cc l 675 BHP l 354/356 l
l1000cc l 865 BHP l 454/456 l
l1200cc l1040 BHP l 546/548 l
l1600cc l1390 BHP l 729/730 l
-------------------------------------

You can see how close some of them are to 80%. The reason they say not to go past this, IMO, is for a lean spike. 80% pretty much guarantees you're ok up to 17:1 A/F. Please don't do that. Heat, things melt, fly apart, boom, people cussing...it's just not good.

Next is the HP with min and max injectors. Anything under 50% duty I think is a waste soo...
BHP Min Max
----------------------------
l 400 l 550 l 680 l
l 425 l 550 l 750 l
l 450 l 550 l 780 l
l 475 l 550 l 780 l
l 500 l 650 l 780 l
l 525 l 650 l 780 l
l 550 l 650 l 1000 l
l 575 l 680 l 1000 l
l 600 l 750 l 1000 l
l 625 l 750 l 1000 l
l 650 l 750 l 1000 l
l 675 l 780 l 1200 l
l 700 l 1000 l 1200 l
l 725 l 1000 l 1200 l
l 750 l 1000 l 1200 l
l 775 l 1000 l 1200 l
l 800 l 1000 l 1200 l
l 825 l 1000 l 1200 l
l 850 l 1000 l 1200 l
l 875 l 1200 l 1600 l
l 900 l 1200 l 1600 l
l 925 l 1200 l 1600 l
l 950 l 1200 l 1600 l
l 975 l 1200 l 1600 l
l 1000 l 1200 l 1600 l
l 1025 l 1200 l 1600 l
l 1050 l 1600 l 1600 l
l 1075 l 1600 l 1600 l
l 1100 l 1600 l 1600 l
l 1125 l 1600 l 1600 l
l 1150 l 1600 l 1600 l
l 1175 l 1600 l 1600 l
l 1200 l 1600 l 1600 l
l 1225 l 1600 l 1600 l
l 1250 l 1600 l 1600 l
l 1275 l 1600 l 1600 l
l 1300 l 1600 l 1600 l
l 1325 l 1600 l 1600 l
l 1350 l 1600 l 1600 l
l 1375 l 1600 l 1600 l
l 1400 l Yeah Right!!! l
----------------------------

Good Luck, Have Fun, Be Safe.


I'd like to see this stickied. If anyone else feels the same, let the mods know. I feel that this will answer alot of redundant questions about fuel injector size. And please feel free to let me know what you think, good bad or indifferent.

- Matt
 

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Overlooked
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4,677 Posts
Discussion Starter #3
BTW, this is good for any 6 cylinder engine. Also, if you just breezed through take a look at the paragraph above the first chart. I think alot of you will like it lol.

If you want to know about other sizes let me know. Next on the list is a turbo application...
 

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I gots deep pockets cuz
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952 Posts
Good job Bass, now I know i can get in the 400hp range with my 550's. Awaiting your turbo writeup to see if I need to get a new turbo or not
 

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Registered
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861 Posts
Nice! Gotta love more stickies.
 

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Addicted to oil
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Thanks. I've upgraded mine this week already.
 

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475RWHP 449TQ and climing
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That yellow bold high light will save losts of engines and help many to see why I have been saying 500rwhp and 550ccs don't match for reliable power.

I truely believe once we start making over 350-400rwhp that we need a standalone for optimum tuning
 

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475RWHP 449TQ and climing
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5,905 Posts
BTW, this is good for any 6 cylinder engine. Also, if you just breezed through take a look at the paragraph above the first chart. I think alot of you will like it lol.

If you want to know about other sizes let me know. Next on the list is a turbo application...


This will be interesting I'll bet. There's loads of misconceptions here, and it's also going to be based on where your emphasis for power goal rpms are going to be, and what type of tranny, and diff will also be needed to really be accurate in achieving these goals I think.

When Blackdevilsupra, and I were the 1st to put a GT40 on a automotive engine instead of truck engine we were told that lagg would kill our power goals, and that a truck trubo was over kill. Now I am looking to go from a gt4067r to a hybrid 74mm turbo.:rockon: :stickpoke
 

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Overlooked
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Discussion Starter #9
I looked at the GT4086 and the GT4088R. There's not a whole lot of difference between that and the GT35R in terms of lag, maybe 500 RPM tops.
 

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Registered
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why f**k isnt this a sticky???????????? HELLLLLOOOOOOOO!!!! Great info here..
 

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4750LBS of pull power pit
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323 Posts
Great info. Next weeks post. What size injector do I need for 400hp? Sticky this shit to save a lot of grief.
 

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again.

Zummtizzle..pretty please??? Sticky???
 

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Addict
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Lol serious hardcore bump I've had to search for this thread 2/3 times in two years :) haha Someone please sticky this thing
Kudos to Bassjunkien for writing this thing Up. still loving it :)
 

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Fantastic thread thank you Bassjunkie.... The only thing I want to ask is regarding this sentence "the common 550cc injector flows 59.58lbs/hr. Now we need to multiply this by 6 to show the amount of fuel for the whole engine which is 314lbs/hr. Now to imply our duty cycle or 80% - 251lbs/hr"
If you multiply 59.58lbs/hr x6 it equals 357lbs/hr not the 314 you came up with or am I missing something?? Does this change the results for the other size injectors as well???
Like I said, thanks so much for doing this, it is by far the clearest and best laid out guide I have seen in regards to injector sizing.
Did you end up doing one for turbo's too??
Cheers, Pete
 

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7M Fanatic
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3,042 Posts
Someone please sticky this thing
A couple of notes:
Injector design has come a long way since this was written, and larger injectors than needed are easier to tune.

All this info can be used to size a fuel pump, and will reveal things like pump deficiency at max flow, and REQUIRED pressure.
Just look at the old standby pump, the Walbro 255, which starts to fall off the scale at 45psi, and will hardly support 550 injectors.
Remember, all injectors are sized at 45 psi.

And one last thing, with bassjunkiens analogy of air/fuel ratio (13 cups of air to 1 cup of fuel) is incorrect.
It's 13 POUNDS of air to one pound of fuel.
Whole lot of difference between a pound of air, and a cup of air.
A pound of air is nearly 12.4 cubic feet (29265cc.)
A pound of gas is 377cc.
So 13:1 A/F, by weight is 77.6:1 by volume.
 

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Unlimited Time Attack A70
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3,571 Posts
And one last thing, with bassjunkiens analogy of air/fuel ratio (13 cups of air to 1 cup of fuel) is incorrect.
It's 13 POUNDS of air to one pound of fuel.
Whole lot of difference between a pound of air, and a cup of air.
A pound of air is nearly 12.4 cubic feet (29265cc.)
A pound of gas is 377cc.
So 13:1 A/F, by weight is 77.6:1 by volume.
Lambda would have been a better choice all around to use.
 

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7M Fanatic
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3,042 Posts
I make my best power at 12.5:1 (NA under full load).
I eat pistons at anything leaner 13.7:1.
But at 1/4 throttle, cruising down the highway, it runs just fine at less than Lamda, like 16:1.
All tunable on a stock ECU using a Super-AFC, and even easier with a stand alone.
 
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