Drivetrain
Engine: All MGBs (except the V8 version) utilized the BMC B-Series engine. This engine was essentially an enlarged version of that used in the MGA with displacement being increased from 1622 cc to 1798 cc. The earlier cars used a three main bearing crankshaft until mid-1965, when a five-bearing crankshaft design was introduced. Horsepower was rated at 95 bhp on both 3 main bearing and earlier 5-bearing cars with peak power coming at 5400 rpm with a 6000 rpm redline. Torque output on all MGB is good with a peak of 110 lb·ft (150 N·m) Fuel consumption was around 25mpg.. US specification cars saw power fall in 1968 with the introduction of emission standards and the use of air or smog pumps. In 1971 UK spec cars still had 95 bhp (71 kW) at 5,500 rpm, with 105 lb·ft (142 N·m) torque at 2500 rpm. By 1973 it was 94 bhp (70 kW); by 1974 it was 87, with 103 lb·ft (140 N·m) torque; by 1975 it was 85 with 100 lb·ft (140 N·m). Some California specification cars produced only around 70 hp (52 kW) by the late 1970s. The compression ratio was also reduced from 9 to 1 to 8:1 on US spec cars in 1972.
Carburation: All MGBs from 1963 to 1974 used twin 1.5-inch (38 mm) SU carburettors. US spec cars from 1975 used a single Stromberg 1.75-inch (44 mm) carburettor mounted on a combination intake–exhaust manifold. This greatly reduced power as well as creating longevity problems as the (adjacent) catalytic converter tended to crack the intake–exhaust manifold. All MGBs used a SU-built electronic fuel pump.
Gearbox:: All MGBs from 1962 to 1967 used a four-speed manual gearbox with a non-synchromesh, straight-cut first gear. Optional overdrive was available.. This gearbox was based on that used in the MGA with some minor upgrades to cope with the additional output of the larger MGB engine. In 1968 the early gearbox was replaced by a full synchromesh unit based on the MGC gearbox. This unit was designed to handle the 150 hp 3-litre engine of the MGC and was thus over-engineered when mated with the standard MGB B-Series engine. In fact, the same transmission was even used in the 3.5-litre V-8 version of the MGB-GT-V8. An automatic three-speed transmission was also offered as a factory option but proved to be fairly unpopular.
Electrically engaged overdrive gearboxes were an available option on all MGBs. The overdrive unit was operational in third and fourth gears but the overall ratio in third gear overdrive was roughly the same as fourth gear direct. Later cars allowed the overdrive to operate only in fourth gear. The overdrive unit was engaged by a toggle switch located on the dash on 1963–1974 cars and on a gear lever mounted switch on later cars. Overdrives were fitted to less than 20% of all MGBs, making it a very desirable feature.
Rear axle: Early MGBs used the "banjo" type differential carried over from the MGA with the rear axle ratio reduced from the MGA's 4.1 (or 4.3) to 3.9 to 1. (Compensating for the reduction from 15 inch to 14-inch (360 mm) wheels.) MGB GTs first began using a tube-type rear axle in 1967. This unit was substantially stronger being, like the later gearbox, designed for the three-litre MGC. All MGBs used the tube-type axle from 1968.
Brakes: All MGBs were fitted with 11-inch (280 mm) solid (non-ventilated) disc brakes on the front with drum brakes on the rear. The front brake calipers were manufactured by Girling and used two pistons per caliper. The brake system on the MGB GT was the same as the Roadster with the exception of slightly larger rear brake cylinders. A single-circuit hydraulic system was used before 1968 when dual-circuit (separate front and rear systems) were installed on all MGBs to comply with US regulations. Servo assistance (power brakes) was not standard until 1975. Many modern and contemporary testers have commented on the very heavy brake pedal pressure needed to stop the non-servo assisted cars.
Electrical system: The MGB initially had an extremely simple electrical system. Dash-mounted toggle switches controlled the lights, ventilation fan, and wipers with only the direction indicators being mounted on a stalk on the steering column. The ignition switch was also mounted on the dash. Like the MGA, the MGB utilized two 6-volt batteries wired in series to give a 12-volt positive earth configuration. The batteries were placed under a scuttle panel behind the seats making access a bit of a challenge but the location gave excellent weight distribution and thus improved handling. The charging system used a Lucas dynamo. Later MGBs had considerable changes to the electrical system including the use of a single twelve-volt battery, a change from positive to negative earth, safety-type toggle switches, an alternator in place of the dynamo, additional warning lights and buzzers, and having most common functions moved to steering column stalks.
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