Selecting the Proper Injector

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

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

Injector HP Formula


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


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

Flow Change Formula

Example 1:

If a 240 CC injector tested at 43.5 PSI is run at 50 PSI. The result would be:

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
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


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