One of the most common questions we get here at

accurate answer requires a bit of knowledge about the

individual situation. A good rule of thumb is this; if

your target horsepower is within 15% of the stock

horsepower, the stock injectors should be fine.

In most cases, if you are planning greater power

gains, bigger injectors are a good idea, and will

probably be required. How much bigger? The commonly used

formula is B.S.F.C. x Horsepower desired/number of

injectors x injector duty cycle = pounds per hour of

fuel required from each injector.

As an example we will use a 200 hp 4 cylinder engine

with 4 fuel injectors and a B.S.F.C. of .5 (that keeps

the math easy). 200 x .5 = 100. We have 4 injectors, and

we want to limit their maximum duty cycle to 80%. 4 x

.8 is 3.2. Now we divide. 100/3.2 = 31.25. So I would go

with injectors that flow 31.25 pounds per hour for this

project. Of course I am probably not going to find

injectors that flow exactly that number, so I would end

up with something a little bigger.

That's great, but we often run into situations where

larger injectors are not an option, so we have to figure

out what we can really get out of these injectors if we

push the limits a little bit, and how to do that safely.

In order to do that, it's imperative to fully understand

the formula we used.

The beginning of that formula has us multiply our

desired horsepower by B.S.F.C. What is B.S.F.C. and why

do we care? A lot of engine builders don't understand

this subject. Many think it's volumetric efficiency or

have some other erroneous ideas. Many have no clue and

just blindly use the .5 factor for everything.

B.S.F.C. stands for Brake Specific Fuel Consumption.

It is a measurement of ENGINE efficiency. That means it

a measurement of how much fuel an engine burns compared

with how much power actually reaches the flywheel.

To be Continued. Stay tuned for more!

-Greg

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