Whether for straight (dedicated or propane-only) or dual fuel (propane or gasoline operation selectable by the operator) use, Impco propane carburetion systems consist of four major components:

  • Fuel tank
  • Fuel lock off
  • Regulator/Converter
  • Carburetor


Fuel Tanks

For road-going vehicles, ASME tanks are required. Lift-truck and other off-road industrial vehicles commonly use DOT tanks, which are portable tanks designed for liquid withdrawal. Barbeque tanks and other tanks for heating purposes, which are designed for vapor withdrawal, are not made automotive use and are dangerous when used for this purpose.

Propane is a liquefied petroleum gas because it is stored under a high enough pressure to condense propane and the other constituent gases into a liquid. The pressure inside the tank varies with temperature from 0 psig at -42°C (-44°F) to 239 psig at 52°C (125°F).

Impco does not make propane fuel tanks. Major fuel tank manufacturers are Manchester Tank and Sleegers.


Fuel Lock Offs

These devices are designed to shut off the flow of fuel to the engine when the engine is not running or is shut off. Impco makes both vacuum-operated and an electric solenoid-operated shut-off devices, which they call Fuelocks. Fuelocks, with their integral fuel filters, receive liquid propane from the tank and filter it before allowing it to continue on to the regulator/converter. Straight conversions commonly use the vacuum Fuelocks, whereas dual fuel systems use the solenoid lockoff.

Impco's vacuum Fuelock is their VFF30 and has an integral filter. It is operated by a vacuum supplied from a nozzle on the mixer body. For dual fuel applications, the Fuelock should be actuated with an electric solenoid rather than vacuum.

For dual fuel applications on carbureted vehicles, Impco has a gasoline solenoid valve (GSV-3) to shut off gasoline flow to the carburetor.



Impco makes 3 regulator/converters, which commonly known as converters. Converters receive filtered liquid propane from the Fuelock at tank pressure and converts it to gaseous propane. It then reduces the pressure in two stages to slightly below atmospheric. The fuel expands 270 times as it changes from a liquid to a gas. The reduction of pressure produces a refrigeration effect and heat must be added to prevent the converter from freezing up. Impco converters use hot water from the engine's cooling system to compensate for the loss of temperature and to aid in vaporization.

The smallest Impco converter is the Model J, which is rated for 100 HP, and is typically installed on the small engines used in lift trucks. The other two converters, the Model E and the Model L, are both rated at 325 HP. The practical difference between the two is in the locations of the inlet and outlet nozzles. The Model L was designed to be a lower cost alternative to the Model E. The Model E reportedly works better at higher power outputs.



The difference between a mixer and a carburetor is simply that a carburetor is mixer with an attached throttle body. A throttle body is the part of the carburetor that contains the butterfly valves. With the appropriate adapters, a given mixer can become the carburetor of several different engines.

All Impco mixers use an air valve design to meter the flow of fuel into the engine's air stream. The air valve design utilizes a relatively constant pressure drop (vacuum) to draw fuel into into the mixer from cranking to full load. The air valve incorporates an integral gas valve that controls the fuel flow relative to the air flow.

Mixtures between idle and full-load are controlled by the gas metering valve's shape. The gas metering valve is shaped to produce a lean mixtures at light loads and increasingly rich mixtures at heavier loads and higher engine speeds. The shape of the gas valve is designed for the optimum mixtures for the mid-size engine between the largest and smallest cubic inch displacement upon which the carburetor will be installed.

For dual fuel applications, a vacuum control solenoid (VCS) is sometimes added to some mixers to lift the air valve to its maximum opening. This minimizes the restriction in the induction system when operating on gasoline.


Selecting the Correct Carburetor Size

The following information has been transcribed (and edited) from Impco Service Bulletin 0100-01.

Air-Flow Capacities. It is important to size correctly the air-flow capacity of the IMPCO conversion carburetor to the engine air-flow requirement. Specifying the correct IMPCO carburetor is vital because a carburetor too small for a given engine limits horsepower. Up to a specific RPM, normal torque is obtained. Beyond that point, as air-flow is limited by the carburetor, torque falls off, with consequent diminishing of performance.

A carburetor excessively large for an engine may cause starting troubles. Idle will not be stable, and fuel mixture will not be consistent. As a general rule, the air-flow capacity of the carburetor should be reasonably close to the air-flow requirement of the engine being converted. However, the type of service the engine performs is a necessary consideration in selecting the appropriate carburetor (or mixer). Keep in mind the following:

  • Engines which are never operated at wide open throttle give the best performance and service with under-carburetion. Services typical of this situation include lift trucks and passenger car applications.
  • Engines with a degree of under carburetion are easier to start and will develop the low end torque required in these types of service.
  • Engines in over-the-road applications can safely be equipped with carburetors delivering somewhat over the air-flow capacity dictated by the engine's air-flow requirement. The larger capacity carburetor will be able to respond to maximum air-flow requirements.

Systems incorporating an IMPCO Commander device built into the vaporizer-regulator are less critical as far as carburetion mixture control is concerned. With the device, the fuel mixture automatically maintains a constant fuel mixture at all times. In a system not using the Commander, the shape of the air-gas valve's fuel cone determines the mixture. No mid-range adjustment is possible.


Calculating Engine Air-Flow Requirements

Determining the specific air-flow requirement for any four-stroke cycle engine requires the application for the following formula:

CFM Required = CID x RPM / 1728 / 2 x VE


  • CID is the cubic inch displacement of the engine
  • RPM is the maximum engine speed
  • VE is the volumetric efficiency (0.85 or 1.00)
  1. Determine the cubic inch displacement of the engine from the identification plate or the user's manual. If the displacement is known in cubic centimeters (CCs), convert to cubic inches by multiplying cubic centimeters by 0.06102. If in liters, convert to cubic inches by multiplying liters by 61.02 (e.g., 2.0L x 61.02 = 122.04 CID).
  2. Multiply the figure by the RPM figure corresponding to the maximum engine speed at wide open throttle (WOT). Use the point at which the tachometer is redlined. If the engine is not equipped with a tachometer, refer to the user's manual supplied with the vehicle or engine.
  3. Divide this CIM (cubic inches per minute) by 1728 to obtain cubic feet per minute.
  4. Divide the result by 2 (for 4-stroke engines).
  5. Multiply the figure you obtain by the engines volumetric efficiency. Use 85% (0.85) for carbureted engines. Due to improved intake manifold design, use 100% (1.00) for fuel injected engines.

For example, a 351 CID engine with a 4000 RPM redline:

Carbureted: CFM = 351 x 4000 / 1728 / 2 * 0.85 = 345.31

Fuel Injected: CFM = 351 x 4000 / 1728 / 2 * 1.00 = 406.25


Turbocharged Engines (with mixer upstream of turbocharger)

CFM Required = CID x RPM / 1728 / 2 x Boost


  • CID is the cubic inch displacement of the engine
  • RPM is the maximum engine speed
  • Boost is % boost pressure + 1

Normal air inlet pressure to the engine is 14.7 psia (one atmosphere). Adding a turbocharger merely serves to increase the inlet pressure. For example, 6 psig of boost equates to 14.7 psia plus 6 psig, or a combined inlet pressure of 20.7 psia (or 140% of one atmosphere) at sea level. Here is how this works starting at the above formula:

  1. One atmosphere equals 14.7 psia.
  2. 6 psig equals 40% of one atmosphere.
  3. Thus, you must multiply the normal CFM by 1.40 to establish the requirement for 6 pounds of boost pressure.

For example, a 351 CID engine with a 4000 RPM redline:

Turbocharged: CFM = 351 x 4000 / 1728 / 2 * 1.40 = 568.75

In selecting the correct carburetor or mixer from the listing, bear in mind that whether the conversion is straight propane or dual fuel (propane and gasoline), all models listed are available for straight fuel or dual fuel applications.


Vehicle Engine Applications

(Wide Open Throttle 1-1/2" Hg Manifold Depression)

ModelRated HorsepowerCubic Feet / MinuteCubic Feet / Hour
50 56 91 5,460
50-500 67 108 6,480
100 106 170 10,200
125 126 202 12,120
175 130 210 12,360
200 172 276 16,560
225 205 329 19,740
300A-1, 300A-20 217 348 20,880
300A-50, 300A-70 270 432 25,920
425 287 460 27,600



Industrial Engine Applications

(Wide Open Throttle 2" Hg Manifold Depression)

ModelRated HorsepowerCubic Feet / MinuteCubic Feet / Hour
50 73 118 7,080
50-500 77 124 7,440
100 123 197 11,820
125 146 235 14,100
200 215 345 20,700
225 237 380 22,800
200D 292 468 28,080
425 333 533 31,980
200T 425 680 40,980
600 600 960 57,800
600T 1000 1600 96,000


Application Examples

Mini Pickup Carbureted 97 CID (1600 cc) turning 5600 RPM at WOT. The formula yields an air-flow requirement of 133.6 CFM. Relating this to the Vehicle Engine Applications listing, you will find that it falls between the Model 50-500 and the Model 100. Keeping in mind that vehicle conversions are more satisfactory with carburetion having over, rather than under, air-flow capacity, the choice should be for the Model 100 with its rating of 170 CFM.

Standard Pickup Carbureted 351 CID turning at 4000 RPM at WOT. The formula yields an air-flow requirement of 345.3 CFM. From the Vehicle Engine Applications listing, this would indicate the use of the 300A-1 or the 300A-20 for a dual fuel conversion. (Or for a vehicle operated at high altitudes or under hard working conditions, consider the 300A-50 or 300A-70 which are normally used for a larger displacement dual fuel conversion.) For a straight propane conversion, there arises the question of selection. Is the Model 225, with an air-flow capacity of 329 CFM sufficient, or will it be better to specify the 460 CFM of the Model 425? Since it is wiser for a vehicle conversion to have over capacity, specify the Model 425.

Industrial Gas Engine Naturally aspirated 817 CID turning at 2000 RPM at WOT. As gas engine of this size is often used in industrial service and may be fuelled with natural gas or propane. The formula yields 401.9 CFM. Be aware that slightly under carburetion is satisfactory for this type of service. From the Industrial Engine Applications listing, select the Model 225 to provide sufficient capacity.

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