If a single engine is the cornerstone of what revived BMW in the 1960s, the SOHC M10 four-cylinder is it. Built since 1963, the M10 was a good engine, but fuel economy and performance were average by 1987 standards. Plus, it was quite heavy due to overengineering—it was expensive to make as well. The final 1987-1988 units were fitted to the 316 (carburetor) and the 318i, both having a 1766 cc displacement with the same engine fitted to the E28 518i.
The all-new 1.8 liter M40 arrived in September 1987 in the facelift plastic bumper Euro E30 318i. The 1.6-liter 316i would not appear for another year. Development was shared with the M70 V12 unit and also shared components such as the con rods, rockers, and valves, among others. Machining took place on the same machines.
While the V12 was all alloy, the M40 used an iron block with 84mm bores, a grey cast iron 81mm stroke crankshaft, and an alloy crossflow head with a belt-driven single overhead camshaft. Hydraulic tappets were used with finger rockers, and Bosch Motronic 1.3 engine management replaced the aging LE Jetronic used on the M10. Weighing 10 kilos less than the old engine, it developed more power—114 bhp at 5500 rpm versus 103 bhp at 5800 rpm—and, importantly, more torque as well—165 nm at 4200 rpm versus 145 nm at 4500 rpm. As a result, the M40 was much more flexible. Fuel economy was much improved, plus it was also smaller and much easier to work on.
In Europe, the M10 carried on until August 1988 when it was completely discontinued and replaced by a 1600cc M40 with 101 bhp and 143 nm of torque at 4250 rpm—that was near as dammit the same as the old M10 318i. Far from being bottom of the range, the new 316i was a car worth having. The new 1596cc engine used the same block as the 1796 cc 1.8 with 84 mm bores but with a shorter 72 mm crank stroke to make it even sweeter.
For 1991, the new E36 3 Series was here, and the M40 engine was fitted to the 316i and 318i, running until 1994.
The 1989 E30 318iS was the higher-performance version of the E30 318i in Europe and Asia, but its M42 engine was used in USA market 318i cars, including four-door models where the M40 was not sold. The new twin-cam 16-valve head was a crossflow job, the camshafts driven by a duplex chain. Coil packs were used for the ignition with a revised Bosch Motronic M1.7 management, and this unit developed 140 bhp at 6000 rpm and a healthy 175 nm of torque at 4500 rpm. The M42 crankshaft was forged steel and designed to take high rpm.
The M42 was revised for use in the new E36 318iS Coupe and USA market 318i saloons—like the E30 318i, America never had the M40. Like the M40, the revisions to use the engine in this all-new car included a new oil pan, inlet and exhaust manifolds, and the engine management. This new version of the M42 used a new dual-length ‘DISA’ inlet manifold where a vacuum-operated flap close to where the upper and lower sections bolt together opens or closes. This alters the intake charge routing from short (high rpm) to long (low-speed torque), thus giving variable torque characteristics of the engine suited to rpm and load. The M42 was also used in the new 318Ti Compact from 1994 and was built until late 1995.
BMW aimed to reduce servicing costs, and so the M43 was born, arriving in mid-1994 using the same block as the M40. The new M43 was effectively an M40 with some M42 technology, such as Motronic 1.7 management. Most importantly, the cam belt was replaced by a single-row timing chain. The previous rocker fingers were replaced by new roller rockers, and this virtually eliminated valve train friction. It gained a similar DISA dual-length inlet manifold to the M42 as well, with power being 105 bhp for the 1600 cc 316i and 115 bhp for the 318i, both at 5500 rpm.
A superb engine, the M43 ran until mid-1998 when it was revised again for the new E46 3 Series. The new M43TU engine was a 1.9-liter thanks to a bigger 85 mm bore and stroke of 83.5 mm. New manifolds and revised management gave it remarkable mid-range torque, and it was built until September 2001 when it was replaced by the all-new Valvetronic N42 engine. The Z3 also used the M43 TU engine from 1998 when it replaced the now-defunct M44 twin cam.
The M44 is the 1.9-liter version of the M42 and had many changes, such as replacing the forged steel crank of the M42 with a gray cast iron one. The M44 used M43 features such as roller rockers, and it was used in the Z3 as well as the E36 318iS and 318Ti Compact models—an excellent engine, it was sadly discontinued in 1999 and replaced with the N42. A lively and economical engine, its demise came because without luxuries like Vanos, it could never meet looming new emissions laws.
The M40 and its offshoots powered a lot of BMWs between late 1987 and its demise fourteen years later. It provided the basis for the immensely successful M47 2.0 diesel, and a racing version—the S42—replaced the legendary S14 in BMW’s E36 3 Series race cars—it was that good.