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Air-Gas Valve Systems

Because of a mono-fuel conversion's effect on a vehicle's emission controls, mono-fuel conversions are the most common for off-road applications. The least expensive conversions (at least for typical North American vehicles) use Impco or OHG/Streamline carburetors and most any type of engine can be converted with these components. These carburetors use an air-gas valve (gas valve for short) that meters the correct amount of fuel in proportion to air flow. These are very simple systems that work extremely well.

Impco carburetors consist of a mixer (the device that meters the correct amount of fuel to the amount of air passing through it) and a throttle body (the device with the butterfly or throttle valves). Combining the mixer with a throttle body creates a carburetor.

Impco has a number of mixers in a variety of flow ranges. Because the gas valve inside the mixer reaches the top of its travel at the flow rating of mixer, it is important to correctly match the mixer to the engine. Too small a mixer will limit the maximum power of engine and too large a mixer will make for a hard starting and poor idling engine. It takes about 6" WC (Water Column) internal mixer vacuum to lift the gas valve off its seat. This is the same amount of suction that you need to draw water up a 6" drinking straw. The rated flow of the mixer occurs at the point where the gas valve reaches the top of its travel and this requires about 13.8" WC. The fuel mixture will gradually become leaner above rated flow and the driver will notice this as a noticeable drop in power.

For each size of mixer, a number of throttle bodies are available to bolt onto common 1bbl, 2bbl, and 4bbl intake manifold flanges. It is extremely easy to create a properly-sized carburetor for just about any engine. Impco and Gann at one time made adapters to mount mixers onto the throttle body of the engine's OEM carburetor but many of these adapters are already obsolete. If you're lucky, you might be able find old stock still sitting on a shelf and this will save you the cost of installing a universal throttle body. Raso Enterprises has created universal carburetor kits for a variety of engines.

Because Impco is an American company that first started developing its products in the 1950s and 60s, the products are geared toward American vehicles. Although many mixers and adapters have become obsolete, it still has a wide variety of carburetors and adapters that fit 1bbl, 2bbl, and 4bbl American engines. Most American cars use standard SAE flanges, which make it easy to install a variety of mono-fuel LPG carburetors. The easiest carburetor to install is the Model 425 because the Model 425 mixer bolts directly up to a standard Holley square-bore 4bbl throttle body. Mounting the Model 425 to a Quadrajet throttle body is also quite simple with the AA3-86 adapter.

Once the carburetor is selected, it needs to be correctly matched to the converter. The engine's power output depends upon the vaporization capability of the converter. For engines up to 100 hp, the Model J will work fine. For all others up to 325 hp, the Model E or Model L converter will be suitable. We have created basic component packages that make an underhood conversion kit when matched with a carburetor kit:

For example, the most economical way to convert a 4bbl Chevrolet 350 engine would be to use a 425 Quadrajet Carb kit with a BCP-EE components kit. A Model 425-based carburetor (460 CFM) could be used in mild performance applications up to 350 CID. Larger performance engines or high performance 350s should use a properly sized Technocarb 4bbl carburetor.

The fuel economy (read $/mile) of an open loop system is dependent upon engine temperature. Cold engines have rich fuel mixtures because the converter supplies fuel at a constant pressure to the mixer and colder water produces colder, denser fuel vapor. Short trips and/or cold weather operation have a significant detrimental effect on fuel economy. To ensure that the fuel mixture never becomes richer than stoichiometric (chemically ideal), we recommend the use of feedback controllers like the Eee Tee Digital Design FC-495-X or Impco Commander, even with standard and lean open-loop gas valves.