Engine - M73 B54 Redesigned
Group 11Engine
Bulletin Number
11 01 95 (2146)
Woodcliff Lake, NJ
September 1995
Product Engineering
SUBJECT:
5.4 Liter V.12 Engine (M73)
MODELS:
75OiL (E38), 85OCi (E31)
General Information:
With the introduction of the 1995 750iL model a revised V-12 cylinder engine is also introduced which is designated M73 B54.
The successful M70 engine has been redesigned with the following objective in mind:
^ Reduce fuel consumption
^ Lower exhaust emissions
^ Lower engine noise levels and minimize vibration
^ Increase engine performance
^ Maintain high quality and reliability standards
The new M73 engine has a displacement of 5.4 liters and develops 322hp (240 kw) at 5000 rpm with a maximum torque output of 361 ft.lb. (490 Nm) at 3900 rpm.
Based on the M70 engine design, the new M73 engine block is also cast from an aluminum silicone alloy called Alusil.
During the casting process of the crankcase, a high concentration of silicone crystals are introduced and maintained in the cylinder walls of the bores. The bores then undergo a special machining, honing and etching process which produces a wear resistant, silicone crystal impregnated cylinder wall surface that assures a long cylinder block life.
Note:
As on the M70 cylinder block, machining of the cylinder bores is not permissible if repairs become necessary.
The cylinder bores have been increased to 85 mm on the M73 as compared to the M70 (84 mm). The cylinder block configuration remains the same - 600 V12.
A forged steel crankshaft with a stroke of 79 mm is used on the M73 engine which is a 4 mm increase compared to the M70.
The front journal shaft is modified/shortened to accommodate a redesigned vibration damper which is now fastened with a single bolt. Main bearing 7 is the thrust bearing which uses 4 semi-circular thrust washers for the axial play adjustment.
The aluminum main bearing shells conform to the triple color code classification and are available in three undersizes.
The M73 pistons area oil cooled using a steady stream of oil supplied to the underside of the pistons via oil spray nozzles which are fitted in the crankshaft main bearing support area of the cylinder block thus reducing thermal loads on the pistons.
The combustion bowl on top of the piston has been redesigned to distribute the combustion forces and temperatures evenly across the top of the piston. The compression ratio is 10.0:1 on the M73 engine.
The new M73 pistons have a thicker fire land and the following piston ring configuration:
(1) - 1.5 mm plain compressor ring
(2) - 1.5 mm stepped taper face compression
(3) - 2.0 mm steel strap oil control ring
The connecting rods on the M73 engine are identical to the M70 engine.
The M73 flywheel is a one piece steel flywheel which incorporates an incremental gear wheel that is monitored by the DME control modules for crankshaft speed and reference point. The vibration damper engine speed/reference monitoring is no longer used on the M73 engine.
As on the M70 engine, the new M73 uses die cast aluminum, cross flow design cylinder heads which include single overhead camshafts (two valves per cylinder) along with hydraulic valve compensators.
A thicker head gasket is also available if cylinder head repairs, i.e., milling the sealing surface, become necessary.
The M73 uses roller type rocker arms which allow for a more aggressive camshaft profile to be used and also reduces the rotating friction in the valve train, thus improving fuel efficiency, reducing heat buildup and increasing camshaft life.
A further weight reduction of the valves, valve springs and upper valve spring plates by approximately 7.5% has reduced the total moving mass of the valve train, thus allowing the reduction of the dual valve spring ratings by 13% compared to the M70 engine. These weight reductions and lower spring rates in the valve train also reduce friction losses and improve economy.
Valve diameters remain unchanged: Intake = 42.0 mm and exhaust = 36.0 mm. Solid steel exhaust valves are now used on the M73 engine instead of the sodium filled exhaust valves used on M70 engines.
Repair valves with oversized valve stems are available if the valve radial play between the valve stem and valve guide is found to be excessive during repairs.
The camshafts are a new design using sintered metal cam lobes (1) which are fused to a hollow metal pipe camshaft body (2). This new camshaft design makes the valve train lighter and less expensive to manufacture.
The M73 uses two new type crankcase ventilation valves which function similarly to the ones used in the updated M42 and all M60 engines. The oil vapors in the crankcase enter the oil vapor separators incorporated in each cylinder head cover allowing the liquid oil to return to the crankcase and the oil vapors to be drawn into the intake manifolds via the crankcase ventilation valves. Since the crankcase ventilation valves are influenced by engine vacuum, the amount of valve opening is varied continually through all engine operating conditions. Under low engine vacuum conditions the valves are fully opened by spring pressure and with high engine vacuum conditions the valves are closed.
In order to meet the strict federal emission requirements, the M73 is equipped with a secondary air injection system which injects filtered air into the exhaust manifolds via stainless steel pipes.
The secondary air pump system reduces the exhaust gas pollutants during the period of operation which is normally high in emission pollutants by helping to oxidize CO and HO content and decreasing the warm-up time of the catalytic converter.
The three control factors of the secondary air pump system are engine temperature, engine speed, and engine load.
Air injection is activated:
At an engine temperature of <-10°C: 10 seconds after the engine starts for 2.5 seconds at reduced pump speed and then 2.5 seconds at maximum pump speed.
At an engine temperature of -10°C to 40°C: 10 seconds after the engine starts for 2.5 seconds at reduced pump speed and then for 85 seconds at maximum pump speed.
At an engine temperature of >40°C: immediately after engine start, the reduced pump speed is used for 28 seconds.
At engine speeds of greater than 2720 rpm and/or whenever the engine load signal (Ti) exceeds 5.4ms, the air pump will be switched off to protect the system from damage due to exhaust back pressure.
AIR INJECTION CIRCUIT DIAGRAM
The new M73 engine uses double-walled steel exhaust manifolds similar to the M60.
The exhaust header pipes are also double-walled and welded directly to the catalytic converters which contain three monoliths each, thus allowing for optimum gas flow with low back pressure. An advantage of the double-walled design is the air insulation barrier between the inner and outer pipes which keeps the internal pipe hotter than the external pipe and allows the exhaust heat to reach the catalytic converters faster. The result of this faster heat transfer is a rapid start of the catalytic action for improved emissions. Other advantages are lower exhaust manifold surface temperatures (reducing under hood temperatures) and reduced exhaust noise.
Each catalytic converter has two oxygen sensors mounted before and after the converter. The additional sensor behind each catalytic converter monitors the converter performance.
The Digital Motor Electronics (DME) system, designated M5.2, is being introduced along with the M73 engine.
This new generation DME includes features already found on the M3.x systems which include;
^ Hot Film Air Mass Sensors
^ Knock Control
^ Hall Effect Camshaft Position Sensing
^ CAN Bus Utilization
^ Fully Sequential Fuel Injection
^ Selective Injector Canceling
M5.2 new features and functions include:
^ OBD (On Board Diagnostics - Version II interface compatibility)
^ Catalytic Converter Operation Monitoring
^ Engine Mechanical Misfire Detection
^ Secondary Air Injection System
^ Control of new smaller, lighter ignition coils
^ Automatic Starter Motor Control
The OBD (on board diagnostics) system is now expanded in the M5.2 Motronic version. Designated OBD-II, this monitoring system now includes provisions for over 200 separate fault codes that relate to the powertrain system. Due to stricter future EPA emissions regulations, the OBD-II fault codes are very specific; for example, there are 37 codes that relate to the oxygen sensor system circuit malfunctions.
Failures of any emission related components or systems are still indicated through the check engine warning lamp in the instrument cluster, and are accessible using the DIS Tester or MoDiC.
Technical Data