The "What size Injectors/Fuel Pump Do I need?" Reference thread

[marleyskater420]

I have been meaning to do this for quite some time now, and currently I am tired of staring at blueprints and finally felt like writing this up.

First off, we need to establish a few main terms that I will be referencing.

1) B.S.F.C.= Brake Specific Fuel Consumption. Definition= "Measure of how efficient the engine is combusting fuel at a given RPM".
Basically, how much fuel does your motor require at a certain RPM to make that power. Forget worrying about the RPM you will spin, the greater concern is the actual BSFC number.
A) Keep in mind the BSFC of the same fuel type CAN vary from motor to motor. Since one of the operative words in the definition is efficient the BSFC for a OHV N/A V6 with crappy head flow will be higher than a DOHC N/A V6 with 4 valves per cylinder, and lots of headflow.
B) Why does the crappy flowing motor have a higher BSFC? Because the crappy-flowing motor requires more fuel to make the same power due to the inefficiency of the motor.... basically because your motor sucks at having well-flowing cylinder heads, it takes more fuel to make the same power as a car with well-flowing heads...similar to how guys with stage 2 motors and crazy flowing heads can make power on a turbo at 20psi that it may take a guy with ported iron heads 28-29psi to make. Get the general idea? (before some douche posts "well there are other things that factor in too!" may I remind you that this is a basic 101-level thread and I am trying to simplify this, so stfu and go pick a e-fight elsewhere)

C) For reference:
N/A gas motors have a BSFC of .5-.55 (now some motors may be less, some more)
Turbo gas motors have a BSFC of .6-.65
N/A methanol motors have a BSFC of about 1.1

D) The BSFC of E85 is somewhat contested/varied over what you may find on the internet. I have seen some DOHC 4 cylinders able to use a BSFC .79 for E85, and others I have seen almost hit .90-again, it depends on the engine combination, but to solve that for us, I will be using a number of .85 that will pretty much cover any Buick combination you have-so guys with entirely stock setups can use .85 and be completely fine with the number they arrive at. Obviously those who have high-flowing heads may choose to use something more around .83 or .81, but I have learned that the LAST thing you want to do, is skimp on your fuel system when making high horsepower..

For the sake of argument, and to keep things on the safer side, we will use a BSFC of .85 for E85. Kinda easy to remember, no?

2) How the hell do I calculate what this "BSFC" is? Or better yet, now that I know what number I am going to use for my BSFC on my E85 motor, how can I put that to good use?

A) If you want to calculate your own BSFC for your car, and you are able to log and accurately determine how much fuel your engine is consuming, you can use the following equation:

BSFC= LB/Hr in fuel flow (for the entire motor) /(divided by) Observed Horsepower

B) Example: BSFC = 160lb/hr fuel flow consumption / 400hp motor = 160/400=.4 BSFC
- That would mean that this motor makes 400hp at the crank, and only consumes 160lb/hr in fuel..that would mean on a 6 cylinder motor, the engine would only be using 26.67lb /hr fuel per-cylinder for all 6 cylinders, and creating 400hp. For reference, .4 BSFC is a very efficient motor.

C) If we know that BSFC = LB/hr in fuel flow / obs. HP, then we also know that ->
-> LB/hr fuel flow= BSFC x Obs Hp

a) This means that if we know what our target Horsepower level is (how much power you want to make) and what our BSFC we want to use is (we already decide on .85) then we can multiply those by each other and determine how much fuel flow the engine as a whole needs to make that power!

b) EX: If our BSFC is .85, and we want to see how much fuel we need to make 700hp ...
.85 (BSFC) X 700hp (our HP goal) = 595 Lb/Hr in fuel flow!

c) If we need 595 lb/hr fuel for our 6 cylinder engine, we need to divide 595 by 6 -> 595 / 6 = 99.17lb/hr PER INJECTOR IN ORDER TO MAKE 700FWHP USING A BSFC OF .85 FOR E85.

d) If you need to convert Lb/Hr into GPH (Gallons Per Hour) you take the lb/hr number and divide it by 6, and it gives you the GPH total. If you want to go from GPH to Lb/Hr, just multiply the GPH number by 6 and you have the Lb/hr number.

D) What if we want to calculate the fuel needed for our motor using different fuel injector duty cycle percentages?

a) Why does this matter? When you are selecting your injectors, you do not want to pick an injector that would make the HP you are aiming for at 100% duty cycle because of issues with the injector that I won't get into here. Just know that most reputable shops that actually give 2 shits about you and your car will calculate the fuel flow for each injector using a duty cycle of no more than 80% or 85%.

b) To calculate the duty cycle at various percentages, we first need to use our equation to find out how much fuel is needed per injector for our motor.
Formula: Q= Max H.P. X BSFC / # of cylinders (that means= The max HP you want to make, multiplied by the BSFC for that fuel, divided by the number of cylinders in the engine)
Q= Injector flow rate in lbs/hr
Max hp= estimate of engines max HP
BSFC= Brake Specific Fuel Consumption for given fuel. We are using .85 for E85

c) EX: Q = 400 (max HP) X .85 (BSFC) / 6 (# of cylinders) = 56.67 lb/hr PER INJECTOR at 100% flow

d) Now that we have our flow rate required per cylinder, we want to find out what the size injector we need if we want to have the injector work at less than 100% duty cycle.
*The formula for this is : lb/hr per injector / .90 (90%, .8 is 80%, etc etc) = injector size needed to flow that fuel amount with a 90% duty cycle.
*EX: If we use our 56.67, it would look like this to calculate what size injector we need if we wanted it to flow that at 90% duty cycle : 56.67 / .90 = 62.97lb/hr
*If we wanted to see what injector was needed at 80% duty cycle, it would look like this:
56.67 / .8 = 70.83 lb/hr


So why does all this math matter, and how can I use it?

1) If you want to figure out how injector you need for a certain horsepower target, you can now use the formula Q= Horsepower X BSFC / # of cylinders (where Q is the injector size in lbs/hr)

So if you want to know what size injectors you need for 850fwhp on E85, you can use that formula easily=
850 X .85 / 6 = 120.41 lb/hr (this is at 100% duty cycle)

If we want to know what size injectors we need if we only want an 80% duty cycle, we can use the other formula= 120.41 (calculated from the previous formula that tells us how much fuel we need) / .80 = 150.51 lb/hr injectors to flow enough for 850fwhp on E85 an only have a 80% duty cycle.



So, some basic HP ratings for those who just want the easy way out and don't bother to learn. THESE NUMBERS ARE THE FLOW RATES PER INJECTOR AT 100% DUTY CYCLE:
600fwhp = 85 lb/hr
650fwhp = 92 lbs/hr injector at 100% duty cycle
700fwhp = 99 lb /hr
750fwhp =106.25 lb/hr
800fwhp = 113 lb/hr
850fwhp = 120 lb/hr
900fwhp = 127.50 lb/hr
950fwhp = 135 lb/hr
1000fwhp = 141 lb/hr



Now there is another way to calculate all of this and still be on the safe side, and that is to calculate the fuel requirement (BSFC X HP = fuel flow required) and multiply that number by 40% to be on the safe side. Yes, E85 uses roughly 30% more fuel, but in life there are things like tolerances/pressure increases from the fuel line plumbing/safety net, so using 40% is a very safe way to calculate things as well. You would use a BSFC of .65 for a gasoline turbo engine, and calculate the flow with that, and then add 40%.

So- a 600fwhp gas turbo motor X .65 BSFC = 390 lbs/hr
Then- 390 X .40 = 156 lb/hr
Then- 390 lb/hr + 156 lb/hr = 546 lb/hr, which when divided by 6 (number of cylinders) gives you 91lbs/hr, which you will see is even higher than the number I calculated using .85 for E85 BSFC.

You can do it either way. For whatever it may be worth, Kinsler Fuel Injection uses the 40% addition method-calculate the fuel required on a boosted gas motor, and add 40% to that.

Hopefully this comes in handy to someone looking to find out what injector size will work for them.

Alright Go down a few posts to learn how to select what type of fuel pump will work with your setup.

 

[dynoman]

This worked out almost perfect for my setup . 706 hp @ 82% duty cycle with 120lb injectors .
Nice write up !

 

[marleyskater420]

This worked out almost perfect for my setup . 706 hp @ 82% duty cycle with 120lb injectors .
Nice write up !

I'm glad man, really.

Like I said in the beginning, we are doing our best-guess for this stuff, so it may not be 100% accurate. This lack of precision is why I try to err on the side of caution, and use either the 40% method like Kinsler does, or the .85 BSFC for E85.

I know some of what I wrote may be kinda boring, and it may take a few times reading it to really grasp all of it, but trust me-this is the kinda stuff that makes the difference between a properly set up system and something just thrown together.

If I can get to it later today, I'll try to add some info about fuel pump selection.

 

[BEATAV8]

Sir, excellent post. I corroborate, agree with, and support it.

Good Work!

I've attached an excel file which I threw together some time back. The numbers therein are very very close to the numbers you have.

 

[marleyskater420]

Sir, excellent post. I corroborate, agree with, and support it.

I've attached an excel file which I threw together some time back. The numbers therein are very very close to the numbers you have.

Thank you!

The danger I see with your chart, is that it does not look like it calculates the fuel pump flow at 75-80psi, which are the numbers that really matter.

At 45psi a 450lph fuel pump could support 900hp, but at 80psi, that may be more like 700hp due to flow decrease/etc.

I will try to post more on this later, but thank you for the kind words!

 

[BEATAV8]

Thank you!

The danger I see with your chart, is that it does not look like it calculates the fuel pump flow at 75-80psi, which are the numbers that really matter.

At 45psi a 450lph fuel pump could support 900hp, but at 80psi, that may be more like 700hp due to flow decrease/etc.

I will try to post more on this later, but thank you for the kind words!

it's there. There's a fuel pressure column going up to 80psi in most cases.

lol it's kind of a mess as it's been tossed together over time

 

[marleyskater420]

it's there. There's a fuel pressure column going up to 80psi in most cases.

lol it's kind of a mess as it's been tossed together over time

Ah, I see it.

Your HP numbers seem a bit high, especially when I do the 40% method (which I prefer, since it allows more safety room).

Like on the twin V1's, I got 815hp at 80psi and you got 898hp... big difference.

The nice thing on the 40% method is that it can also account for any issues with the fuel flow in the lines/filters and a corresponding increase in pressure the fuel pump sees (which would make it flow less).

I'm sure your 898 is pretty close, but with stuff I would rather be very safe than riding the line, yanno?

 

[BEATAV8]

Ah, I see it.

Your HP numbers seem a bit high, especially when I do the 40% method (which I prefer, since it allows more safety room).

Like on the twin V1's, I got 815hp at 80psi and you got 898hp... big difference.

The nice thing on the 40% method is that it can also account for any issues with the fuel flow in the lines/filters and a corresponding increase in pressure the fuel pump sees (which would make it flow less).

I'm sure your 898 is pretty close, but with stuff I would rather be very safe than riding the line, yanno?

Absolutely, my cheat sheet there only has a cushion of 5% (arbitrary). In practice, I run my car much more conservatively.

my per injector numbers look really close to what you have listed above.

For the fuel pump stuff I have (in most cases) 2 different voltages listed.

 

[marleyskater420]

Alright this post is about how to select what type of fuel pump will work with your setup.

First we need to understand a few things:

1) When calculating if a fuel pump will flow enough for you, we need to know that for every pound of boost we have, we need to add 1psi to the fuel pressure as well.
A) So if we have a fuel pressure base of 45psi, and we plan to run 25psi, we need to look at what the fuel pump puts out at 70psi..and for safety reasons, I like to look about 5psi higher than that, so for a base of 45psi fuel pressure with 25psi of boost, I would be looking at what the fuel pump flows at 75psi.

2) As fuel pressure increases, a fuel pump's flow decreases. This is due to tolerances within the pump, and various other factors, but remember that if the fuel pump you are looking at flows enough for your fuel needs at 45psi, it will flow LESS than that at the actual pressure the fuel pump sees when you at full boost.
A) Most people overlook the restrictions that can occur in a fuel system and neglect to add in extra fuel flow to account for these increases in pressure... IE: any kinks, or "hard" bends (sharp 90degree fittings) and especially poorly-flowing fuel filters, can all add extra pressure to the system and thus make the fuel pump see a higher pressure than what you are seeing at the rail. This is why I stress so much on getting a well-flowing fuel filter, like the ones that Kinsler Fuel Injection sells. They test their filters for pressure drops up to 1000 lb/hr, whereas most companies only test to 600 lb/hr. This is important because their fuel filters have almost no effect on the fuel pressure, and thus are not a restriction in the fuel system, so we have a more accurate idea of the fuel pressure the fuel pump sees if we can remove any potential pressure increases from the fuel flowing through the lines.

3) Always look at what voltage the fuel pump flow numbers are rated at. Fuel pumps will flow more with more voltage applied to them. If you do not have proper wiring, and the fuel pump flow numbers are advertised at 16v or even 14v, your pump may only be seeing 13-13.2v depending on how you have it wired up, which means that the fuel pump you are looking at will actually flow LESS on your application than what you are staring at on the advertised flow numbers.
A) Well, how do we calculate the flow numbers at different voltages?
*To calculate the fuel flow for a pump at different voltages, use this equation:
(voltage you want to know the flow at / voltage the pump flow numbers are at) X fuel flow of the pump at the voltage that is given = approx. flow at the voltage you want to know
EX: If a fuel pump is advertised to flow 200GPH at 14v for a given PSI, and you want to know what the pump would flow at 16v, your calculations would look like this:
(16v [voltage we want to know fuel flow at] / 14v[advertised flow voltage] ) X 200gph[flow at 14v] = 228.57gph

4) So how do we calculate how much fuel pump is needed for our build? If we go back to my first post and look at the Injector Size calculation, which is Q = (Max HP X BSFC) where Q= Total injector flow rate required, we can figure how much fuel flow we need for a certain HP level.
A) If your fuel pump is rated in GPH and you want to know lb/hr, just multiply the GPH by 6 (GPH X 6= LB/HR) and if you have LB/HR and want to know GPH, divide the LB/HR by 6 (LB/HR / 6 =GPH) to find the GPH needed.

5) Now we can start playing with numbers for fuel requirements. Our sample for this will be an E85 car (obviously) looking to make 750fwhp with a fuel pump at 14v.
A) First we use our fuel flow equation Q= HP X BSFC, which we know that our target HP is 750 and the BSFC is .85 -> Q= 750 X .85, which gives us -> 637.5 Lb/hr = 750 x .85
B) Now we know that we need 637.5 lb/hr of fuel for this motor, OR if we want GPH, we do LB/hr / 6 = GPH, so 637.5 / 6 = 106.25 GPH. This is the fuel flow we want to look for on the pump after we add in our boost psi on top of our base fuel pressure, and add 5psi on top of that for safety.
C) So if we need 106.25 GPH for our engine, and we know our base fuel pressure is 45psi and we plan to run 25psi, and we add 5psi for some safety room, we arrive at 75psi.
D) Now we go and look for a fuel pump that will flow 106.5 GPH at 75psi, or 637.5 Lb/hr at 75psi.

6) I have attached 2 flow charts for two separate Weldon pumps.
A) If we look at the 2015 flow chart, we see that this fuel pump at 75psi flows right about 100GPH at 14v, which is not enough.
B) If we look at the next pump, the 2025, we see that at 75psi this pump flows just over 140gph at 14v. If our target fuel requirements are about 106.5GPH, we know that the Weldon 2025 pump will flow plenty for our 750hp motor, and will give us room to grow.
C) Here is a link to the Racetronix 450lph : http://www.racetronix.biz/itemdesc.asp?ic=F90000267&eq=&Tp=
If we look at the flow chart at 75psi we can see that the pump flows about 90GPH. Since we need 106.5 GPH, we know that the 450LPH pump will not be enough fuel pump.


Hopefully using these equations can help you guys figure out what to look for, and how to determine what fuel pumps will work for you!

Best of luck, and if you have any questions please feel free to post in here or shoot me a PM!

 

[huntsg]

Nice post. Easy to follow. I've already started my own calculations.

 

[marleyskater420]

Nice post. Easy to follow. I've already started my own calculations.

Great I'm glad to hear!