TA-07 Flow at Diff Pressures

Tech Corner
Article 07

Injector Flow Rates At Different Pressures

As was stated earlier, once an injector is manufactured, the flow rate can not be altered. Flow rates can only be changed on a limited basis by raising or lowering the system’s fuel pressure set point (the measured pressure with no vacuum or pressure on the regulator to manifold port). By raising fuel pressure, more fuel is forced out the metering orifice per pulse width. Thus, someone who has performed modifications to their vehicle (i.e. low restriction exhaust, improved air induction, increased boost, etc.) may be near the flow limits of their stock fuel injectors. By increasing fuel pressure, a few more lbs/hr of fuel flow may be squeezed through the injectors to meet engine demand.

The following formula can help you determine the injector’s flow rate with a change in the fuel pressure’s set point:

                                 Q2 = {Square Root (P2/P1)} x Q1

                                 Q1 = Original injector flow rate (lbs/hr)
                                 Q2 = Injector flow rate at modified pressure (lbs/hr)
                                 P1 = Original fuel pressure set point (psi)
                                 P2 = Adjusted fuel pressure set point (psi)

For example, a Ford Mustang 5.01 uses a fuel injector rated at 19lbs/hr at a fuel pressure set point of 39psi. What would the flow rate be at 50 psi?

                                Q2 = {Square Root (50psi/39psi)} x 19lbs/hr
                                   = Square Root (1.28) x 19 lbs/hr
                                  = 1.13 x 19 lbs/hr
                                  Q2 = 21.5 lbs/hr

Figure 1 shows flow rate changes due to pressure of some popular injector applications.

It must be understood that raising fuel pressure is fine if you are near the limit of the injector’s capability. Raising fuel pressure to overcome a major fuel deficiency problem is a band-aid solution and should be avoided. High fuel pressures will add more strain to the entire fuel system, including the injector’s internal spring. In addition, high pressures can make some injectors to become unstable, or in some cases, completely shut-off.

Finally, it must be realized that when the fuel system pressure is increased, fuel pump volume is decreased. So if you though you richened your fuel system by raising the pressure with an adjustable regulator, but the lean problem persists or becomes worse, then you most likely have a fuel volume problem. The fuel pump is not pumping a sufficient volume of fuel to feed the system.

In general, fuel pressures should not exceed 65 psi except during boost in special applications such as turbocharged or supercharged engines with high boost pressures.

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