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