The basic concept of dual fuel (aka bi-fuel) has not changed very much with the introduction of computerized fuel injection systems back in the late 1980s. The propane mixer, which is as the name implies a device for mixing propane with air, is installed upstream of the carburetor or throttle body injection (TBI) unit. An air horn adapter is what joins the mixer to the TBI if the TBI still has the basic dimensions of a carburetor. For other types of throttle bodies, the mixer must be inserted into the air hose which leads from the remote air cleaner to the throttle body. If Gann or Impco do not have the correct adapter, the installer must fabricate one.

With the decreasing populations of non-EFI vehicles and the economics of conversions over the past years, the availability of adapters to mate propane mixers to gasoline carburetors has decreased. Gann Products Company was one of the leaders in producing these adapters but has had to scrap hundreds of adapters to recover the value of the aluminum tied up in them. The low demand has prevented them from developing new adapters and they have instead focused on maintaining their core product lines. Although I have not been able to check the availability of Impco's adapter line, they have a much broader range of adapters than Gann.

Impco makes a wide variety of mixers to suit any engine size imaginable. They also make a number of mixers specifically designed for automotive dual fuel applications: Models 125M-10, 175A-1, 225M-10, 300A-1/-20, 300A-50/-70. Of them, only the 175A and 300A mixers have offset adapters available to remotely connect the mixer to a carburetor air horn. The dual fuel mixers allow the mixer's air valve to be set in the wide open position while operating on gasoline so as to minimize the mixer's air restriction. The air valve could be forced into the wide open position either with a cam operated by a cable or by a vacuum solenoid valve.

With modern computer-controlled engines, the exhaust oxygen sensor provides a correction signal to the computer to maintain a stoichiometric fuel mixture (exactly the amount of fuel to produce only water and carbon dioxide in the exhaust). Dual Curve® division of Autotronic Controls Corporation made a wide range of dual fuel electronic control units (ECUs) to provide the proper ignition timing and fuel mixture while running on propane. Although Dual Curve did have EPA/CARB certification in the past, they did not have certification on most late model applications after Memorandum 1A conclusion. With the demise of Dual Curve, this is now a moot point. The only practical way to convert a TBI engine to propane is with a universal injection conversion kit, which results in a multi-port propane injection arrangement.  Dual Curve products now only available from sources like eBay and Craiglist.

Dual Curve® ECUs were designed to use a feedback mixer (which uses an air valve designed to maintain a constant fuel air mixture) to ensure maximum fuel economy and minimum emissions. The Dual Curve® ECU controls the fuel mixture with a 3-port solenoid valve that mounts on the propane converter and pulse-regulates the vacuum demand from the mixer to regulator. There is no way for this system to make the fuel mixture richer so the instructions for the initial set-up must be carefully followed. The three main functions of the Dual Curve® ECU are fuel & emission control, timing advance, and active diagnostic interface & support. This prevents false codes and allows the original signals to be sent to the computer in the event that the system goes out of range, as well as preventing false block learn, resulting in poor running when switched to gasoline.

Propane systems have the potential to run much leaner fuel mixtures than is possible with gasoline. It was possible to use a standard air valve mixer with a Dual Curve® ECU which would produce a lean mixture under cruising conditions but would never get richer than stoichiometric under load. Tom Jennings originally tried using the lean gas valve but had trouble making his engine run properly in closed loop mode. After switching to the feedback gas valve, he has since had success with his LPG Rambler (which uses the PN 5952 ECU). Dual Curve recommended using only feedback mixers with their ECUs unless one were not available.

To shut off fuel flow in a dual fuel application, an electric solenoid is usually used on the propane system although Impco has a device to allow their VFF30 vacuum fuel lockoff to be used in this application. For carbureted engines with mechanical fuel pumps, a gasoline solenoid valve shuts off gasoline flow to the carburetor. Solid metal fuel lines must be used from the fuel pump to the carburetor to prevent the risk of fire from a ruptured rubber line. For carbureted engines with electric fuel pumps, interrupting power to the fuel pump is sufficient. Depending upon the jurisdiction in which you live, local emission regulations determine whether power to the fuel pump in a fuel-injected vehicle may or may not be interrupted while operating on propane.

It is entirely possible to do a dual fuel conversion without a propane ECU. The installer would simply put a switch in-line with the fuel injected vehicle's electric fuel pump to shut off the fuel pump and open the propane system's electric solenoid valve. The propane system would operate in open-loop mode without any way of correcting the fuel mixture. However, a much worse problem is that the propane system will generate CEL (Check Engine Light) codes and run improperly.  Using the correct Dual Curve controller would allow you to easily pass tailpipe emissions tests.

1972 Chevy Truck TBI Dual Fuel

Bruce Bushnell of Kansas has a beautiful 1972 Chev truck with a 305 TBI out of a 1988 Camaro that he has converted to dual fuel. He has very kindly supplied photos of his propane conversion.

Dual fuel system from driver side

The dual fuel system viewed from the driver side. Not visible is the propane converter-regulator. Impco recommends a using a Model E or Model L converter for a Model 425 mixer. Dual fuel systems require electrically operated fuel lockoffs rather than vacuum lockoffs suitable for straight conversions.

To have the optimum dual fuel operation, he uses a Dual Curve® PN 5920 ECU to interface with the 1988 engine's computer and control the fuel mixture and timing when running on propane.

Dual fuel system from passenger side

The dual fuel system viewed from the passenger side. On top of the stock TBI is a 90° adapter to which an Impco 425 mixer is fastened. Although the adapter shown appears to be a Gann AA9-54 or AA9-55, Impco's PN AA2-40 or AA2-40-2 (1/2" taller) air horn adapter is equivalent.

The Impco mixer uses the same 5-1/8" air horn base as standard 4bbl carburetors so an air cleaner with that size inside diameter must be used. Impco's air cleaner for this application is the PN AF1-15, which is 6-5/8" tall. Any cleaner used on a 425 mixer must be at least 3-1/16" between the air cleaner base and the bottom center of the air cleaner lid. Reusing the PCV collar on GM's TBI systems would allow the use of a 7" air cleaner assembly.

Note that the lack of heated air for the air cleaner could cause a problem for visual emission inspections. The heated air from the exhaust manifold stove helps to vaporize gasoline in cold weather but is obviously of no use to gaseous fuels other than to minimize carburetor icing.