Calculating Your Fuel Injector And Fuel Pump Requirements

[straughsberry]

Hi All,

Recently I've decided to take on building my 1000+HP project myself with the help of cous and a couple of friends. I've been looking at the fuel system for requirements and all I can say is, it ain’t going to be cheap. However, I’ve learnt a lot and thought I’d share for the benefit of members who wish to go down this path.

Of course, most of us just go with what others have successfully used but when you step outside that realm, we need to make some calculations when choosing our fuel system. All this information is out there in some form or another but I thought I'd try and make it as simple as possible.

Firstly, lets assume we have adequate sized fuel lines and fuel rail to carry our fuel requirements. The 2 main things we’re trying to ascertain here is fuel injector and fuel pump size.

To start, we are going to need to know the following things:

1. Target power figure (at the crank - BHP)
   
2. Boost pressure to reach target power figure
   
3. Brake Specific Fuel Consumption (BSFC) figure
   

Then we can tackle:

1. Choosing an injector
   
2. Choosing a fuel pump
   

Target power figure

This is easy. Pick a number that we think we want to make, and try and make it. 1000 sounds about right. So, our target peak power is 1000 BHP.

Boost pressure to reach target power figure

This will require knowledge of the turbo you need to use for your target power. Lets use a GT45R and I reckon 28 psi will see us at 1000 BHP. And the fact that the MAP sensor on our car runs out at 28 psi dictates the max on stock ECU.

Brake Specific Fuel Consumption (BSFC) figure

The BSFC is basically the amount of fuel per hour (lb/hr) required by the engine in order to make 1 brake horsepower. Is it calculated like this:

power / fuel consumption

In the USA, the units typically used are brake horsepower (BHP) and pounds of fuel per hour (lb/hr).

This figure changes with different types of engines and different throttle position and even different fuel. A diesel engine being very efficient due to its high compression could have a BSFC of .38. A forced induction 4 stroke may be as high as .65. As we are interested in supplying enough fuel for a peak power figure, for this exercise, we are only interested in the BSFC at WOT. Forced induction engines will typically have a BSFC between .55 and .65 at WOT. We will use .60 to be safe. However to work this out exactly, we would have to use dyno figures at WOT and peak power to calculate the BSFC.

It is also very important to note that an engine actually requires a certain mass of fuel and not volume of fuel per hour per Different fuels have a different mass or gravity and therefor the BSFC will change. and we will need to recalculate our flow requirements for both injectors and pump.

Choosing an injector

To choose the correct injector we need to know our fuel flow requirement. This can be calculated using the following simple equation:

fuel flow per injector = ((BHP x BSFC) / # of injectors) / safe duty cycle

Using our figures of 1000 BHP, BSFC of .6, number of injectors at 6 and duty cycle of .9 we have:

((1000 x .60) / 6) / .9 = 111 lb/hr.

So we are going to need 111 lb/hr injectors to make our 1000 BHP. OR are we? Remember that most injector’s flow is rated at 43.5 psi. So if our fuel rail pressure is higher than this, the rated injector flow will increase as well. The stock fuel pressure in BA-BF is 58 psi or 4 bar. We can work out an injectors new flow rate with yet another simple calculation:

new flow rate = old flow rate x v(new pressure / old pressure)

Using the above formula and our known fuel flow requirement of 111.11 lb/hr, lets see if a Siemens 80 lb/hr injector will do the job for us:

80 x v(58/43.5) = 92.38 lb/hr

92 lb/hr isn’t going to cut it. This tells us we either need to increase the fuel rail pressure, or find an injector that will flow 111.11 lb/hr @ 58 psi. Even if we raise fuel rail pressure to 72.5 psi (5 bar) we still only have 103 lb/hr of fuel from the Siemens 80’s. How much “safe” power can a flow rate of 92 lb/hr support? We can use the below formula to get this figure.

max safe BHP = ((injector size x number of injectors) x (duty cycle)) / BSFC

Plugging in our numbers:

((92.38 x 6) x .9) / .60 = 831.42 BHP

OK so how big of an injector do we need given our 58 psi fuel rail pressure and required flow of 111.11 lb/hr? We can calculate it using:

required injector size = v(rated pressure/new pressure) x required flow

Plugging in our numbers:

v(43.5/58) x 111.11 = 96.22 lb/hr

So it looks like we are going to need a 100 lb/hr fuel injector rated at 45 psi.

Choosing a fuel pump

Because of our forced induction scenario, the fuel pump needs to operate against both the fuel rail and manifold pressure. This is why a fuel pump has the potential to make more power in an N/A situation. Because it doesn’t have to work against the extra pressure brought into the equation by boost. This is typically called pressure across the injector or delta pressure. This is another simple calculation:

delta pressure = fuel rail pressure + manifold pressure (boost)

Now get your calculator out for this one. Plugging in our numbers we have:

58 + 28 = 86

An important piece of data we will require is the pumps flow at a given pressure and voltage. Increasing voltage will also increase flow. This information is available from the pumps fuel flow chart.

As always, we want to factor in some safety margin so we’ll use 90% of the pumps flow when considering it’s suitability. To calculate a pumps power potential, do this:

fuel pump BHP potential = (fuel pump capacity (lb/hr) / .9) / BSFC

Below is the fuel flow chart for the Aeromotive A1000 fuel pump.

At 86 psi, the Aeromotive A1000 will flow approximately 550lb/hr of fuel at 13.5 volts. Lets plug in those numbers:

(550 / .9) / .6 = 1018.52

We have a winner. With plenty of safety margin calculated in, this pump will provide enough fuel for 1018.52 bhp. If it was me, I’d be moving up to the next available size pump so I had room to move in the future. After all, how long will 1000 bhp keep you satisfied

One more important thing to remember is that all of the above figures are not the "real world" so to speak. Sure, they’re based on the peoples real world experience but in the end, every car and every component will vary slightly. Having said that, with the safety margins we've factored in, you’re probably not going to under estimate your requirements.

Cheers,

Steph.

Edited 03/05/09 09:05 AM by straughsberry

[straughsberry]

While we're here, is there anyone who can get decent prices on Aeromotive gear? Drop us a PM if you can

[Tocchi]

very handy cheers

[Mastika]

thanks for the help very useful, even for people who can only dream of achieving 1000bhp

[Madwhip]

Good stuff mate your onto it. I'm not looking for anymore power but that is good reading. Nice work

[Xrfranjo]

What aeromotive gear are you after... I can get from ally sheets, frames, lights, engines to whole aircraft( not working) as I work in a metal recycling plant

[Xrfranjo]

cancell that ........ I was thinking of something else.... I now know what you want..... try rocket at eastern creek

[Impellor]

Fuel system requirements calculator. Not sure of accuracy, but looks OK.

http://www.eastcoastrollingthunder.com/jmiller/finject1.htm

Cheers

[trbo 6]

I've actually just gone through this myself, however I worked out the fuel requirements based on rpm, motor cc, boost, VE, A/F ratio to work out the mass of air the motor takes in at a certain rpm and therefore work out the injector size requirements from that.

Doing it this way is I can change the required A/F to suit E85 and E70 to then work out the injector size and pump requirements for each type of fuel.

[trbo 6]

Also just punching some numbers into my spreadsheet and 1000hp requires around 35psi of boost and close to 92lb/hr per injector.