How to Modify Fuel Injectors

There was a day when finding inexpensive aftermarket injectors to support 500-600rwhp were tough. There were few options for high impedance style injectors, other than the Ford Motorsport 42lbs or MSD 50lb units. The aftermarket fuel injector industry has progressed rapidly over the past several years and as a result there are many brands and sizes available for most performance enthusiasts. Larger injectors are necessary to support higher horsepower levels on modified cars. As a rule of thumb the following equations are useful when selecting the right size injector to support the required horsepower level.

To calculate the size for a particular application:

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


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

An old trick that some of the “Tuners” used to do was modify the 30lb/36lb Ford Motorsport units to increase the flow rate. There are still street/strip racers using these modified injectors today with great results for the intended purpose. Generally, 30lb injectors becom 47lb injectors and 36lb injectors may become 66lb / 72lb / 77lb injectors.

Furthermore, there are some original equipment manufacturer bosch style units used by Ford that can also be modified to flow 47lbs. This method of modifying certain Ford/Bosch style injectors will dramatically increase fuel delivery without significantly compromising the injector spray pattern or vehicle emissions. From a very high level, here are the steps to modify fuel injectors;

  1. Remove the pintle cap, o-ring and insulator.
  2. Remove the difusser plate by filing around the weld.
  3. Install new pintle caps, o-rings and insulator.
  4. Have theunits flow tested and ultrasonically cleaned by a professional service shop.

The problem is that the actual procedure is not well known and it is very easy to destroy a fuel injector if done improperly. Care needs to be taken when removing the difusser plate, and the injectors must be flow tested and ultrasonically cleaned (a good practice for used injectors) to verify the flow rate. Once the injector size is known the computer/maf can be calibrated to match the new flow rate to ensure proper air to fuel ratio (this is something that needs to be done regardless of what aftermarket fuel injector is used).

When this procedure is properly performed, these modified fuel injectors work very well and will easily support street/strip cars easily generating 400-600 horsepower. There are many still being used today. It is still an economical way to obtain larger units for those car owners on a limited budget.

Pungky Dwiasmoro Hiswardhani

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