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Injector Selection Guide

Matching the Injector to the Engine

To calculate the injector size for a particular application:

Injector Flow Rate (lb/hr) = Engine HP(1) x BSFC(2)
Number of Injectors x Injector duty cycle(3)

Or

Injector Flow Rate (cc/min) = Engine HP(1) x BSFC(2) x 10.5
Number of Injectors x Injector duty cycle(3)

This worksheet can do the calculations for you:

Fuel Injector Size Worksheet

Engine Horse Power(1):
BSFC(2):
Number of Injectors:
Injector duty cycle(3):

Flow Rate: Lb/Hr     cc/min

Click here to go to our Fuel Injector Page

* NOTE: Unless otherwise noted, all injector flow rates are calculated at a fuel pressure of 43.5 PSI (3 BAR).
(1) Whenever possible, use actual dyno results for engine horsepower.  If no dyno results are available, consult with your engine builder for a horsepower estimate.
(2) BSFC is usually a number between 0.4 and 0.7 for gasoline engines.  Although BSFC can be estimated, it is best to determine an actual number on a dyno.  If a dyno is not available, the following numbers can be used as a rule of thumb:

  • For naturally aspirated engines, estimate BSFC to be 0.4 to 0.5
  • For nitrous engines, estimate BSFC to be 0.5 to 0.6
  • For forced induction, estimate BSFC to be 0.6 to 0.7
  • For rotary engines, estimate BSFC to be 0.6 to 0.7
  • For engines running methanol, double the appropriate gasoline BSFC (e.g. a forced induction methanol engine has a BSFC between 1.2 and 1.4)

(3) Injector duty cycle should be 0.8 (80%) or LESS.  Numbers higher than 80% are generally NOT RECOMMENDED.  If maximum injector duty cycle is not known, use 0.8 as an estimate. Note that some factory engine controllers have substantially lower duty cycle numbers (some 2nd generation Mazda RX7 engine control modules, for example, are limited to 55-60% duty cycle.)Remember that these numbers are ESTIMATED and cannot be considered as accurate as actual numbers achieved on a dyno! Marren Fuel Injection accepts NO RESPONSIBILITY for errors made using estimated, unverified information.  Always verify engine performance information on a dynamometer!

To calculate the horsepower capacity of injectors based on their flow rating:

Fuel injectors max HP (lb/hr)* =
Injector flow rate x Number of injectors x 0.8
BSFC

Or

Fuel injectors max HP (cc/min)* =
Injector flow rate x Number of injectors x 0.8
BSFC x 10.5

This worksheet can do the calculations for you:

Fuel Injector Max Capacity Worksheet

Injector flow rate: lb/hr
Number of injectors:
BSFC:

Fuel Injector Max HP*:

*NOTE:  This formula calculates the maximum horsepower that fuel injectors in proper working order can produce. This formula WILL NOT be accurate if the fuel injectors in question are damaged, clogged, or modified.  This formula will also be inaccurate if the rest of the fuel system cannot produce enough pressure and volume to supply the injectors.  All calculations  should be made at 43.5 PSI unless otherwise noted.

Injector flow rate conversions:

Fuel injectors flow (lb/hr)* =
Fuel injector flow (cc/min)
10.5

Fuel injector flow (cc/min)* =
Fuel injectors flow (lb/hr) x 10.5

This worksheet can do the calculations for you:

Injector flow rate conversions:
Fuel injectors flow "lb/hr" to "cc/min"*: lbhr: -> cc/min:
Fuel injectors flow "cc/min" to "lb/hr"*: cc/min: -> lb/hr:

 *NOTE:  This formula take into account a properly functioning fuel delivery system.  Damaged components, clogged or modified injectors, insufficient fuel delivery, or other fuel system problems that alter the injector flow rates will render this formula useless!


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91 Willenbrock Road Unit B-2
Oxford, CT 06478

203-267-FUEL (3835)

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