patman

iTrader: (3)

I have had a couple people asking me about this lately and I thought I would type something up in case it will help some people. I know most people just guestimate this stuff, but maybe some would like to know the proper way to do it.

I am going to make some assumptions to try and simplify this as much as possible, but basically, fuel produces horsepower, so if you have a power goal, you can do a bit of math and figure out your fuel system. Note that all assumed variables I am using are on the conservative side, but all are reasonably typical.

Most people think of power as wheel horsepower, so we will start with that.
Say you want to make 400whp.

Step 1: Convert to BHP (brake horsepower at the flywheel): Since drivetrain loss is around 15% on our cars, multiply by 1.15:: 400x1.15 = 460bhp.

Step 2: Fuel flowrate: A boosted rotary has a BSFC (brake specific fuel consumption) of around .8 lb/hp-hr. In other words, you will need .8 lb/hr of fuel flow for each HP you make. Continuing from before: 460bhp x .8 lb/hp-hr = 368 lb/hr.

Step 3: duty cycle: Fuel system components are typically rated for 80% duty cycle. This means that a 160lb/hr injector should only really be expected to flow 128 lb/hr, and so on. Fuel pumps are the same way. Almost all these components will actually flow 100% duty cycle or even more, but why take the risk? Anyway, we will assume 80%, so 368 lb/hr required /.8= 460 lb/hr for components sizing purposes.

Step 4: Injectors: this one is easy, take the lb/hr rating, divide it up however you want, and go shopping. Note that Fuel injectors are rated at 45 PSI, so if you run higher pressure (most imports do) you can cheat on the sizing a little bit. Note also that metric fuel injectors are sized in cc/min, which is very nearly 10 lb/hr. In other words, a 1680cc/minute injector is the same as a 160 lb/hr injector. In this case, my choice for 460 lb/hr would be 2x70 lb and 2x160 lb = 460 lb/hr.

Step 5: Lines: This is one that most people worry way too much about. If you buy the right size pump, you can pretty much push enough fuel to make any power through whatever size line. What you need to know is the pressure drop through the line so that you can size your pump accordingly. This is a pretty involved calculation, if anyone wants to read up on it look in some fluid flow books or websites for pressure drop in a pipe, and you will likely come across the following equation: DP=.0000034*Ff*Leq*W^2/(rho)/d^5. I am not going to get into that here as it is too complex for a general estimate. Instead I calculated the values for you:
for 5/16 fuel line: .112 psi/ft
for 3/8 fuel line: .047 psi/ft
for 1/2" fuel line: .012 psi/ft
Taking it as simple as possible, this is the amount of extra pressure your fuel pump will have to produce for each foot of fuel line length (NOT counting return line, which doesnt matter) Note that these factors are based on the fuel flowrate for 400whp, so if you are shooting for 800, either do the calculation yourself or scale these up some.
One more issue with the the lines- fittings account for most of the pressure drop. It is possible to get data on most fittings to quantify this pretty closely, but for this discussion we will just assume that each fitting is equivalent to a foot of pipe, which is conservative, but not unreasonable.
Continuing with our example: If you are shooting for 400whp, theres no reason to change out fuel lines, so we will assume stock 5/16" lines. I don't remember how long stock fuel lines are so I'm gonna take a WAG (wild *** guess) and say 20'. I'm also going to assume you will have 4 90 degree fittings from the pump output to the fuel rail. Hence your equivalent length of fuel line is 24 feet. If your rail pressure is 45PSI, your pump will have to produce 45+(20x.112)=47.2 PSI.

Step 6: Pump: You need 2 things to spec out a pump- pressure and flow. Since you now have both of these, it should be easy. Pick out a pump and look at the literature from the manufacturer, and you will see some flow curves of flow vs pressure. Most fuel pumps are rated at 45psi as mentioned already, so for example a walbro 255 will flow less than that at any pressure above 45 psi.
One thing you will likely need here is to convert from lb/hr to either liters/hr or gallons/hr.
Using the density of gasoline as 6lb/gallon, and our previously calculated flowrate:
460lb/hr /6lb/gallon= 76.7 gallon/hr
keep going if you need liters: 76.7 gallon/hr x3.8 l/gallon =291 liters/hr.


Just to sum this up, for our 400whp example, with some safety factors thrown in, you are looking for 460lb/hr worth of total injectors, stock fuel lines, and a pump capable of 291 liters/hr or 76.7 gallons/hr at 47.2 PSI

The same assumptions and equations are valid for pretty much any turbo rotary car. For NA, use .7 instead of .8 for BSFC. As mentioned, in many cases you can get away with skimping on one component or the other a little bit by adjusting fuel pressure.

Also before anyone comes on here and screws up my thread with some crap about "i made 401whp on a walbro and some 650 injectors i robbed off my mom's minivan". I just want to clarify one more time: this is the process you should use to buy the right components for a safe fuel system that won't fail. The constants I used are also somewhat on the safe side. If you do the research and learn about what they mean, you can make a more accurate calculation based on your parts and preferences, but for most people, this will get you on the road and keep you there. Can you run higher duty cycles? Yes. But should you? That's up to you.

Hopefully this is easy enough to understand and helps some people out.

Pat

Evil Aviator

Just some additional information:

- Most fuel injectors are rated at 3bar (43.5psi), not 45psi.

- Easy rule of thumb for fuel line sizing from A. Graham Bell:
1/4" OD - 165bhp EFI, 110bhp Carb
5/16" OD - 300bhp EFI, 200bhp Carb
3/8" OD - 500bhp EFI, 335bhp Carb
1/2" OD - 1000bhp EFI, 675bhp Carb

FullFunctionEng

iTrader: (13)

Those rule of thumbs for fuel line sizing are real real real generalized...btw.

For those looking into injection flow rates/DC/etc, Injector Dynamics has a good excel sheet that will give estimates (that are actually good estimates)

http://www.injectordynamics.com/IDHo...Calculator.xls

Carlos Iglesias

iTrader: (6)

Attached is the Aeroquip tech reference chart I like to use when calculating hose/pipe flow performance...