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**Re: Fuel 101**

**How much fuel do I need to meet my power goals?**

Before you buy a pump, fuel lines, fuel pressure regulator, and injectors you should have an idea of your target power. This will help you determine what components you will need to use in your fuel set up.

There is one other thing you need as well, and this is the

**Brake specific fuel consumption (BSFC).**BSFC is a measure of an engine's efficiency. It is the rate of fuel consumption divided by the rate of power production. BSFC is specific for the reciprocating engine. It is

**not**weather dependent.(1) The formula for gasoline powered reciprocating naturally aspirated engines is

**0.5 lb/(hp·h)**. Generally, SFC within a particular class of engine will decrease when the compression ratio is increased.(2) That is to say, less energy from fuel will be lost to heat, making the engine more efficient.

A good general rule of thumb for gasoline engines is:

* For naturally aspirated estimate BSFC to be 0.4 to 0.5

* For nitrous estimate BSFC to be 0.5 to 0.6

* For forced induction estimate BSFC to be 0.6 to 0.7

(3)

Remember, Engine size is not important in this calc, engine compression is. If you are using stock pistons and staying NA you would use 0.5, infact, that is the lowest I would go. 0.52-0.55 will add some margin of error.

So, now that we have the hard part out of the way we can start figuring out our fuel needs.

Lets go with a

**target horse power of 260**. This will be approx 245-250 at the wheels.

**260HP x 0.5lbs fuel/1HP an Hour = 130 lbs fuel/hour**

This is the fuel requirement of the motor at this power. Running at 100% is not good for the components though, and it is best to have room to grow. A fuel pump that can provide more fuel would be desirable.

Well, we have the total fuel needed, how big do the injectors need to be?

Injectors are rated at a specific pressure.

**43.5psi is pretty much the standard**, but be sure to ask the manufacturer prior to purchasing a set. Upping the psi does add more fuel, but it can also stress the pump more. Infact, as pressure rises, pump flow capacity decreases substantially. Furthermore, because injector rating is a measure of the max volume the injector can flow at that psi, it is the same as 100% duty cycle. This is not ideal as the injector will become overdriven and could eventually fail. Not only that but the ECU programs the pulse width (time between injector opening and closing) with cylinder firing order and cam timing. Having the injector pour fuel into a closed intake port is not good at all.So keeping all of this in mind.

Now, lets get some basic numbers going.

Our total fuel is 130 lbs fuel/hour. We are using 4 injectors, so each must flow:

**130 lbs fuel/hour / 4 injectors @ 43.5 psi = 32.5 lbs fuel/hour @ 43.5 psi**

We want this flow rate, but we want it at 80% duty cycle, so we will need a larger injector.

To solve simple multiply by 1.25. This takes our needed size and makes it 80% of the product of the calculation.

**32.5 lbs/hr * 1.25 =**

**40.625**lbs/hour.By now there are two things that stand out.

One is the psi, Honda calls for 50~55 psi.

Two is the unit of measure, Honda uses cc/min, not lbs/hour.

Fuel flow increases almost proportionally to the square root of the ratio of pressure increase.

Ok, rated pressure was 43.5psi. We are gonna run 55psi as per Honda. The ratio of increase is:

**55psi / 43.5psi = 1.2643678160919540229885057471264**

and the square root of the ratio is:

1.1244411127720090718243445638738

and the square root of the ratio is:

1.1244411127720090718243445638738

Take the injector rating at 80% and 100% duty cycle and multiply them by this new number from the pressure increase.

**80% duty cycle + 32.5 * 1.1244411127720090718243445638738 = 36.54 lbs/hr**

100% duty cycle = 40.625lbs/hr * 1.1244411127720090718243445638738 = 45.68 lbs/hr

100% duty cycle = 40.625lbs/hr * 1.1244411127720090718243445638738 = 45.68 lbs/hr

That means that by increasing the psi at the fuel pressure regulator by 11.5 psi we saw an increase fuel flow of about 12%:up:

Solving for cc/min is easy. The conversion factor for gasoline from pounds per hour to cc per min is 10.5

So,

**40.625 lbs/hr multiplied by 10.5 nets 426.56 cc/min**

This is our target size for an injector

**as per the 43.5 psi standard at 100% duty cycle.**

This injector will flow

**341 cc/min at 80% duty cycle at 43.5 psi and will flow near 382.2 cc/min at 55psi.**

Great link to a huge list of injectors with part numbers and specs

http://users.erols.com/srweiss/tableifc.htm

Notes:

*1

*2

*3