Performance engines for the MGA are built to order, combining targeted improvements to the cylinder head, camshaft, and rotating assembly to deliver a meaningful increase in power and torque while retaining the engine's fundamental reliability for road use. The B-Series engine responds well to systematic tuning, MG's own Special Tuning department at Abingdon having published detailed guidance on staged improvements in booklet AKD819, and the components available today reflect that proven approach. The build can be tailored to the car's intended use, road touring, fast road, or competition, and a discussion with the technical team before ordering establishes the right specification, with the engine serial number prefix, chassis number, and a description of the intended use allowing the build, camshaft selection, compression ratio, and supporting modifications to be matched to the car and the owner's expectations.
Stage II Cylinder Head
The core of any performance build is the Stage II cylinder head, which involves gas-flowing and porting the standard cast-iron head to improve the volume and velocity of gas flow through the inlet and exhaust ports, with the combustion chambers reshaped for more efficient burning, the valve seats recut to accept larger valves where the port size permits, and hardened exhaust valve seat inserts fitted for compatibility with unleaded fuel. The head is assembled with new valves, uprated valve springs, and bronze valve guides throughout, and a Stage II head transforms the engine's breathing, the single most effective modification available for the B-Series. The improved airflow is the foundation on which the rest of the build is constructed, and a well-developed head paired with matched carburation and exhaust delivers the bulk of the performance gain that a Stage II engine offers over the standard unit.
Camshaft & Balanced Bottom End
The camshaft determines the engine's character, a mild road camshaft retaining smooth idling and good low-speed torque for everyday driving and touring, a fast-road camshaft extending the power band higher into the rev range and delivering more peak power at the expense of some low-speed tractability and idle quality, and more aggressive competition profiles for sprints, hillclimbs, or circuit racing requiring matching carburation, exhaust, and valve-spring changes to be effective. The reciprocating assembly, crankshaft, connecting rods, pistons, and flywheel, is balanced as a matched set, particularly beneficial on the MGA's three-bearing crankshaft which is inherently less rigid than the five-bearing design used in the later MGB engine, the balancing allowing the engine to rev more freely, reducing vibration throughout the range, and extending bearing life. The compression ratio is set during the build to suit the intended fuel quality, higher compression delivering more power but requiring higher-octane fuel.
Carburation, Exhaust & Cooling
A performance engine must be supported by adequate fuel delivery and exhaust extraction, the standard MGA carburettors being twin SU H4 semi-downdraught units of 1½-inch diameter, adequate for a mild Stage II build with revised needles to match the improved airflow, while a more aggressive build benefits from the larger SU HD6 carburettors of 1¾-inch diameter as fitted to the Twin Cam, or their equivalent, providing the additional fuel and air volume the engine demands. The exhaust system should also be reviewed, an original mild-steel system potentially restricting the engine at higher rpm, with a larger-bore stainless system complementing the improved breathing. An engine producing more power also generates more heat, and the MGA's original cooling system, adequate for the standard output, may be marginal with a significantly tuned engine particularly in slow traffic or warm weather, the radiator pressure cap having been upgraded from 4lb to 7lb during production to address boiling issues, and an aluminium radiator or electric fan conversion being advisable on a performance-engined car. The full package, head, camshaft, balanced bottom end, matched carburation, free-flow exhaust, and uprated cooling, produces a balanced and reliable performance engine rather than one part outrunning the rest, and a rolling-road setup after running-in optimises the result.