Tech Tip: Fuel Injector Sizing

[JDM Sam]

Many people ask, what size injector do I need to run XX hp?

The answer is simple if you know how to do a simple calculation for Brake Specific Fuel Consumption (BSFC).

Here is the mathematical formula:

(Target Crank Horsespower x B.S.F.C) / (# of injectors x duty cycle)(do not go more than .85) = lbs per hour per injector

to convert to cc/min from lbs per hour: x 10.5

For turbocharged applications B.S.F.C. should be at .60 or higher
For N/A it should be .50.

This means the engine will use .60 of fuel per hour for each horsepower it produces.

For the MSP we have around .55 B.S.F.C. from the factory. This is insufficient for turbocharged modified cars.

With a .60 B.S.F.C, target crank HP of 230 @ 85% duty cycle, 4 injectors.
You will need approximately 426 cc/ min.

 

[JDM Sam]

How to call bulls*** on someone by just knowing their fuel injector size.

Injector Flow Rate (lbs/hr) x duty cycle / BSFC = HP supported per injector
HP per injector x # of injectors = HP supportable by the injectors

 

[Bala]

So glad I got 550's instead of 440's (target whp: 300) ... and now I know I may need to go bigger. (boom01)

 

[JDM Sam]

To use this formula you want to convert your target WHP to Crank HP.
We have approximately 11% to 15% drivetrain loss. So for this formula you want the target to be around 350 crank hp vs. 300 wheel hp.

 

[Bala]

Sooo ... 350 X .6 X 10.5 / 4 = 551.25 (?) Looks like I'm right on target.

 

[JDM Sam]

No, you missed the 2nd half of the formula in bold.
I edited the formula to make the divided sign easily decipherable.

350 x .6 / 4 x .85 = 210/3.4 = 61.76 lb/hr or 62 lbs injector x 10.5 = 651 cc/min injectors each

 

[Bala]

No, you missed the 2nd half of the formula in bold.
I edited the formula to make the divided sign easily decipherable.

350 x .6 / 4 x .85 = 210/3.4 = 61.76 lb/hr or 62 lbs injector x 10.5 = 651 cc/min injectors each

Cool - thanks. I knew I was missing the duty cycle, just wasn't sure where to throw it in . Alright, so for my initial goals, it looks like the 550's are a little less than I need to stay within the 85% of duty cycle.

 

[orng1]

Wow great thread, I had been doing a lot of searching and had found some of these calculations for fuel but not this easily. 750's will work for you Bala, we should go in on a few together.

 

[JDM Sam]

This is just a simple way to find out what you should be running. There are more calculations besides this to make it even more accurate. Other variables such as fuel pressure can change the flow rate.

 

[baconbitz]

This should be stickied or added to a "Protege Bible" if there is one.

 

[Speed3.5]

This is just a simple way to find out what you should be running. There are more calculations besides this to make it even more accurate. Other variables such as fuel pressure can change the flow rate.

you know, this is excelent.
Where would I/we find more info like this for other parts that're needet to be upgradet properly to support the hp goal ??
Like fuel press etc. ??

just a question , if I get 550's and expect to get 300 to the wheels or ~350 to the crank , would i really reach that goal or the injectors gonna be out of range ??

 

[dirtysouth_msp]

What's up Sam, good info. RC Engineering has a good calculator on their website that makes some of this a lil easier, in case anyone's interested...

http://www.rceng.com/technical.htm#WORKSHEET

 

[JDM Sam]

There you go an even simpler way! No math involved!

 

[kicker22705]

this is good stuff!

 

[magnumP5]

Wow, and I'm at nearly 190whp on stock P5 injectors... I must be pushing these things to still go pig rich at WOT... so it seems 550's are good for about 250whp... just something else to add to my list of things to buy.

 

[yashart_mp3]

Does anyone know the MSP/Protege max fuel pressure at the rail?

 

[JDM Sam]

41 psi.