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.