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Selecting the Proper Injector Size for GAS

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  • 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 / Supercharged  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

The proper injector for this application is 31 lbs. / hr. or 330 cc / min
(Use the Fuel Injector Worksheet below to perform this calculation for you.)

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

The formula to find the new FUEL FLOW rate after a fuel pressure change:

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

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

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