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BMW 318/325/M3/525/535/M5 1989-1993 Repair Information



See Figures 1 through 5

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Fig. Fig. 1: Cutaway of the M20 engine

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Fig. Fig. 2: Cutaway of the M30 engine

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Fig. Fig. 3: A M42 engine mounted in the E36 chassis resulting in a 318i

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Fig. Fig. 4: A M50 engine mounted in the E36 chassis resulting in a 325i

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Fig. Fig. 5: The S38 engine with the racing derived individual intake runners and throttle plates

bmw has engineered some of the finest engines in the world. The repertoire of bmw produced engines include automotive, motorcycle, marine, industrial and, of course, aircraft units. The concept of refined, no compromise design has given bmw a reputation for high performance, high reliability and high efficiency engines.

All bmw engines covered in this guide have iron blocks and aluminum heads. Intelligent design has made the iron block lightweight while providing the rigidity that any good motor requires. The aluminum head reduces the weight of the engine while providing good heat conductance. The aluminum head is designed to be very rigid and warp resistant.

The M20 and M30 engines are a single overhead camshaft design with 2 valves per cylinder. The M20 engine uses a timing belt to drive the camshaft, while the M30 engine uses a timing chain for the same chore. These engines share similar architecture with the original Neue Klasse 4 cylinder engines designed in the early 1960's. These engines are still considered a goal of reliability and smoothness for other manufacturers to meet.

The cylinder heads employ valves located in an inverted Vee pattern, allowing the use of hemispherical combustion chambers for optimum, swirl assisted combustion with minimum heat transfer to the cylinder head. This kind of combustion chamber also allows for use of the largest possible valves. The valves are actuated by a single, nitride hardened overhead camshaft. Since they are not located directly below the shaft, they are actuated via light alloy rockers. This makes valve adjustment procedure very simple-the only special tool required is a simple piece of wire. Valve adjustments are made via a unique cam, located in the end of the rocker arm and in effect, pinched between sections of it. Since the adjusting cams are separate from the adjusting lockbolts and nuts, and since the lockbolts are prevented from turning via a flat at one end, it is easy to tighten down on the adjustment mechanism without changing the clearance. This minimizes the time required in performing the adjustment. Other engine features that prolong life are the use of chromium plated rings and valve stems, and forged connecting rods.

The M42 4 cylinder and M50 6 cylinder engines are double overhead camshaft design engines. Introduced in the early 1990's these 4 valve per cylinder designs boosted the performance, fuel efficiency and driveability of the vehicles in which they were installed. These engines were designed to be minimum maintenance engines. The hydraulic valve lash adjusters eliminated the need for periodic valve adjustment and the timing chain eliminated any need to change a timing belt. The ignition system has no distributor so there is no distributor cap or rotor to replace. The M50 has a racing inspired coil-over sparkplug ignition system that totally dispenses with parkplug wires.

The famous M-cars feature two of the most driver satisfying engines ever to make their way from racing car to street car. Both the S14 and S38 are double overhead camshaft design, 4 valve per cylinder engines with adjustable valves. Differing from almost every other production automotive engine, the S14 and S38 enjoy the use of a separate throttle plate for each cylinder. The individual intake runners make a very racy sound which is not surprising since the engines come straight from racing backgrounds. Redlines above 7000 rpm and the willingness to spin at such speeds all day long show the engineering expertise and spirit built into the M-car engines.