RC Engineering, Inc.   
20807 Higgins Court   
Torrance, CA  90501   
Phone     310-320-2277   
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Technical Notes

   Selecting the proper injector size
   Find HP value of selected injectors
   Find fuel flow after pressure change
   Conversion formulas
   Fuel Injector Worksheet (calculate required injector size)

Selecting the Proper Injector Size

In order to select the correct size injector for your application, you might wish to use one of the following formulas.

In most cases a naturally aspirated engine will have a B.S.F.C of .50. This means that the engine will use .50 lbs. of fuel per hour for each horsepower it produces. Turbocharged engines will want to be at .60 lbs. per hour or higher.

Using these numbers as a guideline, you can select the approximate injector size in the following formula for a four cylinder, naturally aspirated engine at 200 bhp.

Most injectors will max out at 80% duty cycle and this is the accepted industry standard.

B.S.F.C is brake specific fuel consumption - How much fuel you are using per horsepower per hour

Injector Flow Formula
The proper injector for this application would be 31 lbs. / hr. or 330 cc / min
(Use the "Fuel Injector Worksheet" to perform these calculations for you.)

Find HP Value of Selected Injectors

To find the HP value of the of the above mentioned injectors use this formula:

Injector HP Formula

Find Fuel Flow After Pressure Change (see worksheet below)

To find the new FUEL FLOW rate after a fuel pressure change:

Flow Change Formula

Fuel Flow After Pressure Change Worksheet

  Old Pressure: PSI
  New Pressure: PSI
  Old Flow Rate: cc/min         
  New Flow Rates: cc/min   or   lbs/hr

Example 1:
A 240 CC injector tested at 43.5 PSI is run at 50 PSI.
Example 1

Example 2:

The horsepower value of this injector would increase by the same rate
(X 1.0721), if all other engine management systems remain unchanged.

Flow rate at old pressure
240 cc/Min divided by 10.5 = 22.86 Lb/Hr

Flow rate at new pressure
257 cc/Min divided by 10.5 = 24.48 Lb/Hr

Example 2

So, the resultant change in pressure would yield the following:
240 cc/Min X 1.0721 = 257 cc/Min
22.86 Lb/Hr X 1.0721 = 24.51 Lb/Hr
36.58 Hp X 1.0721 = 39.22 Hp

Conversion Formulas

To convert cc / min to lbs. / hr. - Divide by 10.5
To convert lbs. / hr to gal. / hr. - Divide by 6
To convert cc / min to gal. / hr. - Multiply by .015873


Fuel Injector Worksheet

Actual Desired Crankshaft Horsepower


Number of (same size) injectors to be used

Brake Specific Fuel Consumption desired
  * See note below.

Maximum Injector Duty Cycle you will see
  Warning: Duty cycle should not exceed 80% (0.80) for these calculations.

System fuel pressure @ fuel rail


(Static flow) injector required @ test PSI of 43.5

  Lbs/Hr      CC/Min

* Brake specific should be .45 to .50 for naturally aspirated engines,
.55 TO .60 for supercharged engines,
and .60 to .65 for turbocharged engines.