"Do I Need Bigger Injectors?" Using B.S.F.C. to Determine Your Power Needs

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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