How to Add Propane to Gas for Mileage


Introducing propane into an internal combustion engine as a supplement to gasoline has the potential to increase fuel mileage and lower fuel costs. A propane-assisted gasoline engine may also burn cleaner than a pure gasoline engine. Using propane as a fuel supplement is much simpler than entirely converting the engine to a propane fuel source.

Things You'll Need

  • Drill and tap set
  • Propane tank
  • Propane mounting bracket and straps
  • Miscellaneous high-pressure propane fittings
  • Miscellaneous low-pressure propane fitting
  • High-pressure propane tubing
  • Low-pressure propane tubing
  • Adjustable pressure regulator
  • Electronic or mechanically actuated low-pressure flow regulator
  • Programmable engine control module or custom hard-wired circuit board
  • Laptop
  • Engine tuning software
  • Engine control module adapter
  • Mount the propane tank bracket in the rear or trunk of your car. Strap the tank into the bracket.

  • Run a propane line forward from the tank to the engine bay. It is safest to run the line beneath the vehicle. Many propane tanks now come with a regulator on the tank - if your tank did not, install the high-pressure regulator in the line just outside the tank. You may also desire to install an electrically operated shut-off switch - install it in the line just downstream from the high pressure regulator, and install the switch in or on the dash.

  • Install the low-pressure flow regulator in the engine bay -- on the bulkhead or otherwise. Attach the line from the propane tank to the input of the flow regulator. Modify the regulator to accept mechanical input from a cable attached to the throttle, or if it is an electronically actuated valve, wire it to the throttle position sensor (split the output from the throttle position sensor). You will have to install a programmable electronic control module or a hardwired circuit board to convert the output from the throttle position sensor to electronic commands to the flow regulator.

  • Drill and tap a hole in the intake manifold between the throttle body and where the intake manifold splits to the different cylinders. Install a fitting for the propane to enter the intake manifold. Run a line from this fitting to the variable flow regulator.

  • Program the engine control computer or adjust the physical cable that governs the flow regulator. Set the regulator to open about half way when the engine is at full throttle, and open only slightly when the engine is at idle.

  • Start the car, hook up the laptop to the engine control module, then turn on the propane supply. You will have to change the parameters governing the fuel injectors in the car to provide slightly less fuel overall. Adjust the cable or programming for the propane flow regulator so that the engine smoothly accelerates across the operating rpm range.

Tips & Warnings

  • You may want to install a propane pressure gauge in the dash as well. Use a "T" fitting and run a line into the passenger compartment to the location where you desire the gauge. Mount the gauge in a convenient location in the dash.
  • Propane is a dangerous fuel. Turn on the propane supply only after the engine is running, to prevent build-up of the propane in the intake manifold.
  • Thoroughly clean any debris from the installation out of the throttle body and intake manifold. Debris in either of these devices can enter the cylinders and cause severe damage to the engine.

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  • Photo Credit Propane Tank image by John Walsh from
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