How to calculate injector size

Calculating Injector Size and Metric Conversions

You will need access to a dyno to properly do this.

Step 1: Determine the horsepower of your engine.
To determine the horsepower your engine is producing at the crank, you should have your vehicle dyno tested.

Step 2: Determine your approximate BSFC or Brake Specific Fuel Consumption.
BSFC is the amount of fuel used per unit of horsepower made by your engine. Again, you can determine this from dyno testing.

Step 3: Determine your injector’s safe duty cycle.
The injector duty cycle is the percentage of time that the fuel injector is open vs. the total time between firing. For most OEM cars it’s .8, but if you’re on this forum it’s safe to assume for this purpose that your SDC is actually .85, as this is pretty standard for most modified and racing cars. If you’re upgrading your injectors to a higher flow rate, then it’s safe to assume that .85 is your number!!!!! SDC=.85

Step 4: Calculating the injector size.
Okay, so now you have the numbers you need. You’ve done the dyno time, and you have your crank-hp, your aprox. BSFC, and your injector SDC. What now? Now you can use this information to calculate the correct size by using this formula:

injector size in lbs./hr.= (max. hp X BSFC) divided by (number of injectors X SDC)

So, if you have a modified Sentra, putting out 150 hp, then the equation might look like this:

(150 X .45) divided by ( 4 X .8 ) = 21.1 lb./hr.

So, what the hell do you do with lbs./hr.??? I mean, everyone else in the world has the sense to use the metric system… but your Nascar fan parts guy at the local Napa doesn’t understand the simplicity of the universal metric system… so he only knows lbs./hr…. well, that’s easy. Use this formula to calculate:

To convert pounds per hour to cc’s per minute (like normal people would):

cc/min = (lbs./hr. X 60) divided by 6.177

so, using the example above, that would be:

(21.1 lbs./hr. X 60) divided by 6.177 = 204.9 cc/min.