Chrysler LA Engine - Technical Information

Technical Information

The Magnum engine is a direct descendent of the Chrysler LA engine, which began with the 273ci V8 in 1964. While the Magnum 3.9, Magnum 5.2, and Magnum 5.9 (1993-up) engines were significantly based on the 239, the 318, and the 360 — respectively — many of the parts will not directly interchange and are not technically LA engines; the only part that is actually unchanged is the crankshafts, and even these have different OEM part numbers.

The cylinder block remained basically the same. It was still a V-shaped, 90-degree design made of cast iron. The crankshaft, located to the bottom of the block by five main bearing caps, was cast nodular iron, and each of the eight connecting rods were forged steel. The pistons were cast aluminum, with a hypereutectic design. Cylinders were numbered from the front of the engine to the rear; cylinders 1, 3, 5 and 7 were found on the left (driver side) bank, or "bank 1", with the even numbers on the other bank.

Coolant passages were located between the cylinders. The rotor-type oil pump was located at the bottom rear of the engine, and provided oil to both the crankshaft main bearings and the cylinder heads. Chrysler's engineers also redesigned the oil seals on the crankshaft to improve anti-leak seal performance. The oil pan was also made from thicker steel, and was installed with a more leak-resistant silicone-rubber gasket.

The most fundamental change made to create the Magnum was the fuel delivery system. This was a high-pressure type system, fed by an electrically-powered fuel pump located in the fuel tank, as opposed to the earlier mechanical, engine-mounted pumps found on most LA engines. Gasoline was supplied to the intake manifold through a pair of steel rails that fed eight Bosch-type, top-fed, electronically-actuated fuel injectors; there was one injector located in each intake runner. As such, each cylinder had its own injector, thus making the fuel system a "multi-point" type. Pressure was regulated by a vacuum-controlled pressure regulator, located on the return side of the second fuel rail. Excess fuel was thereafter delivered back to the fuel tank.

To support the new fuel system, the intake manifold was of a new design. Known colloquially as the "beer keg" or "kegger" manifold, the part was shaped like half of a beer barrel lying longitudinally atop the center of the V-shaped engine block. The intake runners, which supplied the fuel and air to each cylinder, fed each of the intake ports in the newly-designed cylinder heads. The bolts that secured the intake manifold to the cylinder heads were installed at a different angle than those on the older LA engine; they threaded in vertically, rather than at the 45-degree angle of the LA.

Air was provided from the air filter intake to the intake manifold by a Holley-designed, aluminum, twin-venturi, mechanically actuated throttle body, which was bolted atop the intake manifold. Each venturi was progressively bored and had a diameter of 50mm. To this unit were mounted the Throttle Position sensor (TPS), Manifold Absolute Pressure (MAP) sensor and Idle Air Control (IAC) valve. A steel cable connected the accelerator pedal inside the vehicle to a mechanical linkage at the side of the throttle body, which acted to open the air intake butterfly valves inside the venturis. During idle, these butterfly valves were closed, so a bypass port and IAC valve were used to control the intake of air.

The cylinder heads were another fundamental change of the Magnum engine, being designed to meet stricter requirements in both power and emissions by increasing efficiency. These heads were cast iron units with new wedge-shaped combustion chambers and high-swirl valve shrouding. Combustion chamber design was most important in these new heads: LA engine cylinder heads were given a full-relief open-chamber design, but the Magnum was engineered with a double-quench closed-chamber type. The higher-flowing intake ports stepped up intake flow dramatically in comparison to the original LA engine, and the exhaust ports improved cylinder evacuation as well. The shape and porting of the chambers allowed for more complete atomization of the air/fuel mixture, as well as contributing to more complete combustion; these virtues allowed for much greater efficiency of the engine as a whole. The intake and exhaust valves were located at the top of each combustion chamber. The valves themselves had shorter, 5/16" diameter stems, to allow for the more aggressive camshaft. Intake valves had a port diameter of 1.92", while exhaust valves were 1.65". Spark plugs were located at the peak of the combustion chambers' wedge, between the exhaust ports; press-in heat shield protected them from the heat of the exhaust manifolds.

Cast iron exhaust manifolds, similar to units found on previous engines, were bolted to the outboard side of each head. The new cylinder heads also featured stud-mounted rocker arms, a change from the shaft-mounted LA arms. This last change was due to the different oiling system of the new engine, as described in the next paragraph. The valve covers on the Magnum have 10 bolts rather than the previous 5, for improved oil sealing. In addition, the valve covers were made of thicker steel than earlier parts, and were installed with a silicone gasket.

The valvetrain was also updated, although it was still based on a single, center-block-located camshaft pushing on hydraulic lifters and pushrods, one for each rocker arm. However, the cast nodular iron camshaft was of the "roller" type, with each lobe acting upon a hydraulic lifter with a roller bearing on the bottom; this made for a quieter, cooler-running valvetrain, but also allowed for a more aggressive valve lift. Each of the lifters acted upon a steel pushrod, which were of the "oil-through" type. This was another change for the Magnum. Because the new pushrods also served to provide oil to the top of the cylinder head, the rockers were changed to the AMC-style, stud-mounted type. The new rockers also had a higher ratio: 1.6:1 compared to 1.5:1 in the LA engine, which increased leverage on the valves. In addition, the oil boss located at the end of the cylinder head on the LA engine was left undrilled, as it was no longer needed. However, the boss itself was left in place, perhaps to cut down on casting and machining costs, and to allow the use of earlier LA heads.

Engine timing was controlled by the all-steel, roller-bearing timing chain, which was located beneath the aluminum timing cover at the front of the engine block. The timing chain sprockets, one each for the camshaft and crankshaft, were all-steel; the last few years of the LA engine came with nylon teeth on the sprockets. At the rear of the camshaft was cut a set of helical gear teeth, these being used to spin the distributor. Mounted to the front of the timing cover was a new-design counter-clockwise-rotation water pump, with much improved flow. Externally, the accessory drive belt was changed to a serpentine system; this, couple with an automatic belt tensioner increased belt life, reduced maintenance and contributed to lower noise and vibration levels.

The ignition system was also all-new for the Magnum. Controlled by a new micro-processor-equipped Single-Board Engine Controller (SBEC, also known as the ECM, or Engine Control Module), the ignition system included a distributor mounted at the rear of the engine. A 36,000-volt ignition coil, usually located at the front right of the engine, provided electrical power to the center of the distributor cap, where a spinning rotor divided the power to each of the individual cylinders' spark plug wires. Ignition dwell, advance and retardation were electronically controlled by the SBEC.

The SBEC controlled the ignition, as well as the opening and closing of the fuel injectors. During cold startup, wide-open throttle and deceleration, it did this based on "open-loop", pre-programmed operating parameters. During normal idle and cruising, it began "closed-loop" operation, during which the module acted based upon inputs from a variety of sensors. The basic sensors that provided input to the SBEC included the Oxygen sensor (O2), Manifold Absolute Pressure (MAP) sensor, Throttle Position sensor (TPS), Intake Air Temperature (IAT) sensor and Coolant Temperature sensor (CTS). The basic actuators controlled by the SBEC's outputs included the fuel injectors, ignition coil and pickup, and the Idle Air Control (IAC) valve. The latter controlled idle characteristics. However, the SBEC also controlled the operation of the charging system, air conditioning system, cruise control and, in some vehicles, transmission shifting. By centralizing control of these systems, the operation of the vehicle was simplified and streamlined.

Emissions output was controlled by several systems. The EGR, or Exhaust Gas Recirculation system, brought unburnt fuel vapors from the exhaust stream up to the intake manifold, allowing the vapors to be re-used by the engine. A PCV, or Positive Crankcase Ventilation system, introduced oil vapor and unburnt fuel vapors from the crankcase to the intake, allowing the engine to re-use these as well. Furthermore, gasoline vapors that would normally be released into the atmosphere were captured by the EVAP system, to then be introduced into the engine.

In 1996, the OBD-II on-board diagnostics system was introduced on all passenger vehicles in the United States, as per United States Environmental Protection Agency (EPA) regulation. As such, a new engine control computer was developed for vehicles powered by Magnum engines, known as the JTEC. The new Powertrain Control Module was more complex and more intelligent, and added programming meant it could also control automatic transmission and other powertrain functions. With the introduction of the JTEC, the EGR system was dropped from Magnum engines.