Topic: Fuel Pump/Injector sizing

[FlaBouy]

Didn't see a Fueling area to put this in so I figured the Forced Induction area would be the spot.

Random thought. How many times do you hear people question if larger injectors or fuel pump will add more horsepower and how does one know how to size injectors/pump prior to running out of injector duty cycle or have lean conditions due to exceeding pump capacity?

This is really quite easy to ballpark if you know what to look at.

First let's look at airflow real quick. To figure theorectical airflow, or filling of the cylinders,  one just needs to run the numbers through on 100% filling of a four stroke engine with 323 ci motor (5.3L). If you assume that ambient air (temp and barometric pressure) has effect, and assume for arguments sake that a standard constant for ambient is 0.00004671, then the formula to use is:
(CI) x (ambient) x (RPM/2) = 100%fill of the cylinder.

So if you want to see what your airflow should be on an efficient 5.3L motor at 1000 rpm, you would just say 323 x .00004671 x (1000/2) = 7.54 lb/min. You can log this in HP Tuners to verify your results. I have noted many stock Av's running about 35 lb/min at WOT in the 5000-5200 range (Stock shift points)

Why is this important? It's not unless you know that if you are seeing 7.54 lb/min @1000 rpm and seeing 1.40 lb/min at idle that your HP Tuners log should be registering in the 25% VE range.... since 1.40/7.54 = .25

OK, so beyond the airflow theory what the hell does that have to do with fueling? Hmmmm.. if you are trying to hit AFR targets, would you not want to know how much air there is to add fuel too?

But I digress...

Say you want to make 650 HP at the crank, knowing that it will take that much on a Z-71 to make 500 RWHP due to the 20%+ drivetrain loss on a 4wd shoebox.

Again you have to make an assumption unless you have HP Tuners to figure out what engineers call Brake Specific Fuel consumption. Without getting into a lot of detail just assume a NA motor has a BSFC rating of .5 and a FI application uses anywhere from .55 to .65 for the constant....This is in laymans terms the fuel requirement to make a constant power.

So using the following formula, we assume (650 hp) x (.6 lb/hp-hr) = 390 lbs/hr of fuel. This is using a BSFC of mid range  (.55 - .65) on a blown application.

So if we know we need 390 lbs/hr of fueling to make 650 HP, then using eight cylinders, we only need to divide 390 by eight cylinders to know our injector size.  390/8 = 48.75 lb/hr injectors. But wait. This is at 100% duty cycle. You want to load an injector at only about 80%, so 48.75/8 =60.93 lb/hr or 60# injectors......

If you are still awake, if we need 390# hr of fuel for 650 hp then we can say 390# hr / (6#/gal) = 65 gallons an hour or 246 liter per hour to make 650 hp. That boys and girls is a Walbro 255 lph pump which history has taught us will support right up to 650 HP.

So to recap:

Wanted horse power x BSFC = Fuel Requirement
Fuel Requirement / 8 = minimum injector size
Minimum Injector size / 8 = Properly sized injector
Fuel requirement / 6 = gal per hour
gal per hour x 3.79 = liter per hour

Simple math...... 8)

Edit: Too bad my spelling isn't as good as my math............ :-[

[KY_BOB]

Very nice write up.  Every word of it makes since if you read it one step at a time.


The only thing that I have to add is that the part of the equation to calculate injector sizing assumes the fuel pressure is constant and at the same pressure as the injector is rated at.  If the fuel pressure is increased the actual flow rate is also increased too, so that can be taken into consideration.   The '02-03's idle at 52psi but it increases (as vacuume decreases) to 58psi.  With FI it can go above that.   2004 and newer trucks hold a constant pressure (52psi, I believe).

With that said, it's limited as to how much the injector flow will increase.  With my stock injectors (24#) and over 100psi of fuel pressure I was still seeing injector duty cycles as high as 120%.  With the 42#'ers that I bought from Flabouy and dropping the pressure back to the mid 60's, I'm now running 85-90% duty cycles max.


I will add that I hadn't done the math as to how many lph of fuel I was going to require with the addition of nitrous but after reading this calculation, I feel that I'll be OK.

[FlaBouy]

All things being equal, injectors and pumps are rated at known fuel pressures.  Say an injector is rated at 330cc at 40 psi, the same injector is rated at 369 cc@ 50 psi. What Bob is talking about is the difference between a "returnless" fuel system and a fuel system that has a return line ran back to the tank. That type of system is regulated at the rail for pressure with an "FMU". Unused fuel is returned to the tank. There are numerous disadvantages to that system, one being heat is returned to the tank also with the fuel. Baffles are now needed in the tank to reduce foaming, and the PCM has to compensate for the increase in fuel pressure, and when running boost, ever pound of boost introduced into the intake manifold, increases fuel pressure in a linear fashion. So in a fuel system that uses a return line, the injector size would "grow" due to pressure increase every time rpm or boost is introduced if the PCM did not compensate for the "growth".

Now on a "returnless" system, you don't have all this nonsense. The fuel pump is a constant pressure, flow on demand setup with no returning fuel from the rail. It returns unused fuel right at the pump in the tank. No heat transfer, no baffles to combat foaming, and no PCM compensation for fuel pressures bouncing all over the place. It is very easy in this type of system to calculate needed flow since pressure remains relatively constant at the rail.

With all that said.... if you know what your fuel pressure is, and you know what your injector size is, and you know what your duty cycle is, would it not be a safe argument to state, "You now know what your horsepower level is on your motor, within reason?"
 
 8)

[justJR]

Wow! Well said! :thumb:

[Main One]

If I recall correctly, injector fuel flow changes with the square root of fuel pressure.  Doubling your pressure therefore instead of doubling your injectors' flow, increases it by only about 40%.