Thursday, October 17, 2013

Motivation: The BMC 'B' Series Engine

The BMC “B” Series engine was one of the world’s most successful medium-displacement four cylinder power plants with production lasting some 26 years. Designed by the Austin Motor Company prior to their merger with Morris Motors and the formation of BMC, the B-series was one of the firms earliest overhead valve engines.   

Early B-Series Engine
In keeping with the technology of the times, the B-series one-piece crankcase and block was cast iron with a cast iron head.  The camshaft was mounted low in the block. The overhead valves were operated via steel pushrods acting on rocker arms. Originally, the engine was built with a three main bearing crankshaft which gave early B-series engines a free-revving nature, but the lack of crankshaft support led to shaft “whip” and eventual failure. Later engines were supplied with five main bearings.

The standard cast iron head had two siamesed  inlet ports and three exhaust ports, all on one side of the head. 

Its first production application was in the new AustinCambridge saloon with engine capacities of 1.2 and 1.5 liters on offer.  Morris used the B-series in their Cambridge and MG installed it in the ZA Magnette, albeit with twin SU carburetors which gave the sporty saloon 60 BHP  - 10 more than the single carburetor 1.5 liter Austin or Morris.

MG further developed the engine for the MGA sports car and was able to extract 68 BHP with clever tuning.

The B-series went through an evolutionary change in 1961 when the block was redesigned to permit greater bore diameter. The engine received bores of slightly over 76mm which provided 1622cc capacity. Again, careful tuning increased power output up to 83 BHP.
The final evolution of the B-series came when the capacity was increased to 1798cc with the introduction of the MGB

MGA with Twin Cam B-Series
Engineers at MG were responsible for a couple modifications to the B-series, both of which involved overhead camshafts. A twin cam version was introduced in a high specification MGA produced from 1958 through 1960. This modification produced 108 BHP in high compression (9.9:1) form.  The aluminum head was a crossflow design with eight individual ports. The MGA Twin Cam developed a reputation for short service life primarily due to holed pistons. The issue was traced back to inadvertent lean running and was corrected, but the damage to the reputation of the engine was done.

Later, MG engineers designed a single overhead cam head with an eye towards reducing emissions for the North American market in the early 1970s. Eventually  with a few further changes, this engine morphed into the O-series which was used in various saloon cars into the mid 1980s in displacements up to two liters. 

The B-series (in both gasoline and diesel versions)  found a home under the bonnets of literally dozens of models around the world with some of the more unusual being Massey-Harris combines, International  Harvester delivery vans and some Mercedes vans in the 1970s.

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