It featured a 2 in (50.8 mm) longer wheelbase than the original Austin-Healey 100 and a six-cylinder engine replacing the slightly larger capacity four, and added two occasional seats which later became optional. The body lines were changed to a less rounded appearance, with a wider, lower radiator grille below the air scoop which was added to the bonnet, and a windscreen which was fixed and could no longer be folded down.
There were two model designators, the 2+2 BN4 produced from 1956 onwards and the 2-seat BN6 produced from 1958.
The cars used a tuned version of the BMC C-Series engine previously fitted to the Austin Westminster and which at first produced 102 bhp (76 kW) increasing to 117 bhp (87 kW) in 1957 by fitting a revised manifold and cylinder head. The overdrive unit became an option rather than a standard fitting.
In late 1957 production was transferred from Longbridge to the MG plant at Abingdon.
A 117 bhp (87 kW) BN6 was tested by The Motor magazine in 1959 had a top speed of 103.9 mph (167.2 km/h) and could accelerate from 0-60 mph (97 km/h) in 10.7 seconds. A fuel consumption of 20.8 miles per imperial gallon (13.6 L/100 km; 17.3 mpg-US) was recorded. The test car cost £1307 including taxes of £436.