Electronic feedback control systems may be a necessary requirement for conversion certification in your locality. If you do a lot of driving, you may want to do this upgrade to just improve your LPG fuel consumption. Since the fuel mixture is controlled by the shape of the gas valve in the mixer, there is no real way to adjust it as you would a gasoline carburetor. There is an idle mixture control and a power mixture control. The power mixture control has a minimal effect at cruising speeds and affects the full throttle power fuel mixture by restricting the flow of propane entering the mixer.

Electronic control systems typically measure the exhaust oxygen (O2) content and control the fuel mixture by adjusting the output of the propane converter's pressure regulator. These systems are called closed loop systems because the controller continually measures the exhaust O2 and adjusts the fuel mixture. Open loop systems are like old-style carburetors in that the carburetor solely controls the fuel mixture and there is no fuel mixture readjustment as the engine is running. Open loop propane converters produce a low (measured in inches of water) but constant output pressure.

Tom Jennings upgraded his 1963 Rambler to a closed loop system and noticed an immediate improvement in his fuel consumption and drivability. He used the Autotronics 4046 Close-Loop Controller and found that their instruction manual left out a lot of small but important details. If you are considering trying this system, read the Tom Jennings experience first.

We have a closed-loop fuel mixture controller (similar in operation to the Impco ADP and Commander).  An addition feature it has (that the ADP, Commander, and Dual Curve PN5952 do not) is the integral status indicator lamps:

These lamps allow the installer to easily setup the fuel system as well as it provides an “at a glance” view of the current engine condition. Clogged air filters and fuel system problems are easily diagnosed using these indicators. If the controller is unable to maintain the correct air/fuel ratio due to a fuel system fault there is a “Check Engine” lamp that will illuminate to advise the operator that there is a potential emission problem.

Impco Commander

Impco has an electronic feedback control system (Impco Commander Kit) to control the fuel mixture supplied to an engine by their mixers. The propane system is very similar to a conventional open-loop arrangement except for a few differences. The ADP system relies on an exhaust oxygen sensor to monitor whether the fuel mixture is rich or lean and tries to maintain a stoichiometric fuel ratio. A stoichiometric fuel mixture is one where there is exactly the right amount of oxygen to react completely with the fuel. That is, the fuel mixture is neither rich nor lean but ideal.

In addition to the exhaust oxygen sensor, a Commander system must obviously have the Commander electronic control unit (ECU), which ideally needs to be installed in the passenger compartment of the car. The Commander ECU receives the feedback signal from the exhaust O2 sensor and sends a control signal to the feedback converter. The converter is what actually provides the ideal fuel mixture to the engine. The converter does this by adjusting the fuel pressure supplied to the feedback mixer.

The feedback mixer is identical to the standard mixer except for the gas valve cone. The gas valve cone has a different shape than the standard mixer's gas valve cone. Impco would not say how it is different but from a control point of view, it would make sense for it to be shaped to supply a constant ratio of fuel to air. A standard mixer may be retrofitted with a feedback gas valve or a mixer may be ordered as a feedback mixer.

For you of you with ADP system (predecessor to the Commander), Impco was kind enough to provide us with the ADP installation manual in PDF form. As the manual is quite large, Impco has provided it in four parts and you may download it for yourself:

Raso Enterprises Digital Fuel Controller

Our Digital Fuel Controller is designed to work with air valve carburetion, such as IMPCO, Nolfe, OHG, or others. This product is only recommended for older carbureted vehicles (ie, pre-1987) and it works in a similar manner to the Impco ADP system. To convert newer vehicles, use an injection conversion instead.