A formula can be used to help determine an injector flow rate for a particular engine application. This formula is only as accurate as the input values used in the calculation. It is at this point that you must determine an honest and realistic horsepower estimate. If you input a “wish” horsepower value and reality is nowhere near your “wish,” then the injector selected will be too rich with the result being a poor running engine. Of course dyno data would give you accurate numbers but for most people, that is a luxury.
The following formula works as a good guideline:
Q = (MAXHP x BSFC
)/No. of Cylinders
Q = Injector flow rate lbs/hr
MAXHP = Estimate of the engines maximum horsepower capability
BSFC = Brake specific fuel consumption: use .45 naturally for aspirated engines
and .55 for turbocharged and supercharged engines
Let’s use a naturally aspirated V-8 engine which produces 400 horsepower as an example:
Q = (400 x .45)/8 = 22.5 lbs/hr
We are not finished. 22.5 lbs/hr is the total per cylinder fuel required for the engine at that horsepower. This should not be the static flow value of the injector. This value should be 10-20% below the static flow because the injectors are rarely, if ever, operated at full static flow. Injectors operating at pulsewidths near static flow become unstable since the injector does not have time to fully close before it is beginning to reopen. Therefore, it is recommended to use a safety factor of at least 10% (20% on the conservative side).
Then the actual injector size should be:
22.5 lbs/hr / 0.90 = 25 lbs/hr
22.5 lbs/hr / 0.80 = 28 lbs/hr
Therefore, an injector that flows 25-28 lbs/hr would be ideal for our example engine. There may not be an injector available off the shelf that will flow exactly in that range. If there isn’t, one possibility would be to raise or lower the fuel pressure. Four our example, an injector that flows 30lbs/hr at 39 psi fuel pressure will flow 28 lbs/hr at 35 psi fuel pressure and will meet the engine requirements.
