The car’s engine was a 1,087 cc four-cylinder unit featuring hemispherical combustion chambers and valves inclined at 45 degrees in a crossflow head. Initially, the engine used a cone clutch, which evolved to a plate clutch paired with a Silent Third gearbox, then a helical dog change box, and finally an ENV pre-selecta epicyclic gearbox. Production of the Mark 1 began in 1927 at Percy Riley’s engine factory, following initial resistance in the main works to the new design. The success of the Mark 1 led to a rapid shift in production, with the factory ceasing side-valve production to focus on the Nine in early 1928, coinciding with updates like the move of the gear lever and handbrake to the centre of the car and the adoption of a Riley steering box, resulting in the Mark II.
Further revisions followed with the Mark III, featuring stronger wheels and a modified rear brake rod arrangement. The Mark IV, introduced in 1929, saw heavier Riley-made hubs and axles, a new cable braking system with larger drums, and the addition of the Biarritz saloon, a de-luxe version of the Monaco. The Plus series saw the fuel tank moved to the rear and the adoption of an Autovac, while the Plus Ultra featured a lowered chassis. A Preselector gearbox was offered as an option in 1934 for an additional £27.
In 1936, Riley entered into an agreement with Briggs Manufacturing to produce a steel body for a new chassis, resulting in the Merlin model, available alongside the Kestrel variant. The Briggs body evolved in 1937 with a large boot extension, becoming the Touring Saloon, and the higher-specified Monaco was added to the range. The final version, the 1938 Victor, featured a further forward engine position for increased interior space, a relocated battery, and smaller diameter wheels.
When compared to its contemporary, the Hillman Minx, the Riley Nine boasted a more sophisticated engine, producing 25 per cent more horsepower from its 1098 cc engine than the Hillman’s 1185 cc unit. The Riley’s ENV preselector gearbox also provided easier gear changes. However, the Riley was nearly double the price of the Minx, and while offering better handling, performance was comparable, with the Minx being slightly faster in a straight line. A 1931 Monaco weighed 916 kg, increasing to 1 160 kg by 1937, and despite twin carburettors, a 1937 Monaco took half a minute to reach 50 mph and barely exceeded 62 mph.
[unverified] A detailed study of the Riley Nine's long-term impact on British motorsport belongs to AT Birmingham's work on pre-1939 Riley motor cars rather than this article's corpus.