The 406 ended up being the last Bristol to be powered by the BMW-derived straight-six; but the first of a long line of cars that would use essentially the same body, with a series of running updates. In terms of power, the 406 had no more than the 405, despite its engine being enlarged to 2.2-litres, and performance with this large body was predictably leisurely. The brakes were improved, though, featuring discs front and rear, and the rear suspension got a Watt’s linkage . More would come with the arrival of the V8-powered cars.
info from Octane