In 1981, Chrysler started building propane-fueled Dodge Diplomats, Plymouth Gran Furys, and, in Canada, Plymouth Caravelles. These cars were engineered and built at Chrysler, with a factory warranty all engine components, and parts stocked in the regular dealer network.
Cold weather operation was tuned with cold-room testing, trips in northern Canada. Computer-aided crash test analysis was used to select fuel line locations and reduce risk of leakage after a crash. They quickly became the most popular propane vehicles in Canada.
There was a police pursuit version of the propane engine, which added durability and performance features, as well as a taxi-package version.
1986 Propane Dodge Diplomat FiguresPower: 150 hp @ 3,800 rpmTorque: 260 lb-ft @ 1,800 rpmCamshaft driven by double-roller timing chainAir-valve Impco CA225 carburetorImpco EC1 lean mixture controlImpco VFF30 fuel filter/lockoffUS-spec HD5/Grade 1 fuel requiredChrome-alloy steel intake valvesNickel-alloy steel exhaust valves(Nimonic exhaust valves on police pursuit)Viton valve stems seals/shieldsHigh-temp caustic-cleaned collant passages
The sole propane engine was the 318 V8, adapted with a package of special parts and processes including high alloy exhaust valve seats, premium valves, treated coolant passages, and positive valve rotators, which went through 4 million kilometers of durability testing. For 1986, new fast-burn cylinder heads increased power to 150 hp and 260 lb-ft of torque.
Because propane tends to burn “clean,” internal parts tended to wear less, giving propane engines a longer life. The fuel tended to be cheaper than gasoline.
The spark control computer used timing schedules developed through propane testing; a dual pickup distributor provided low-advance timing during starts for cold weather, and higher advance after starting. A high charge density for greater torque was created by eliminating intake air heaters. Extra connections were provided for the vaporizer to avoid cutting coolant flow through the heater core.
The Impco carburetor was based on a standard design, but was specially calibrated; it had the largest Impco vaporizer available (the model E), with a higher capacity than usual for the engine size to increase reliability and maintain power in cold weather. Vaporizers were internally coated with a propane wetting agent and had a high-flow coolant supply system. An automatic fuel primer with a throttle-operated override switch helped starting; cold idle was smoothed out by a thermostat-controlled two-speed idle system.
The fuel tank itself was not in the trunk, as was common in conversions, but under the vehicle; Chrysler claimed similar cruising range as with gasoline. The bottom of the trunk was cut out (by the spare tire), the regular gas tank removed with the strap posts left in place, and two side-by-side propane tanks were put in. The rear anti-sway bar appears not to have been installed on these cars; and the spare tire was kept in a different part of the trunk.
The aluminum-clad tank was build using a flame-spraying processes pioneered by Chrysler Canada, so that the aluminum was mechanically and chemically bonded to the tank. Fuel lines had double-wall steel tubing covered in lead alloy and wire-armored where needed. A standard lockable fuel filler door protected the filler. The tank held 79 liters (17.4 imperial gallons).
The cars’ suspensions were modified to compensate for the added weight, with heavier duty springs and other measures to keep the height correct. Extra structural analysis was designed to avoid problems with metal fatigue, corrosion, and impact damage.
The propane program ended with the M-body cars.
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