Tech feature: a four-valve pioneer

In the 1920s combustion chamber design was still primitive. Side valves had yet to succumb to overhead valves, compression ratios were modest, and specific power output very low.

Triumph was a well-established manufacturer with considerable racing success in the early days, but by 1921 it had chosen to ignore the important racing events (such as the Isle of Man TT); a stance it adopted for many years. Its Model H side-valve 500 just wasn’t fast enough.

This changed in 1921 when Triumph commissioned Harry Ricardo to develop a replacement cylinder head for the Model H. Ricardo, a pioneer in combustion chamber design, provided Triumph with several prototypes before it settled on the four-valve design. Ricardo was aware of the inadequate valve materials available and aimed at reducing thermal stress while improving airflow through increased valve area.

With a four-valve design gas flow could be more efficient and the spark plug positioned centrally for improved combustion. The lighter reciprocating components would also respond quicker, allowing higher revs. These were the same factors that led to four valves per cylinder becoming the standard combustion chamber design decades later.

Ricardo’s design was actually quite conservative. Retaining the Model H 80.5 x 98mm bore and stroke, the new engine’s ports were small, its valves were recessed and each pair of valves was parallel – they were set at 90 degrees to each other with the stems and springs exposed.

The cylinder head had two parallel exhaust ports and was made from cast iron, and a light alloy piston ran in a steel barrel. From the head down the engine was virtually a Model H.

Smaller flywheels improved revving and the constant-loss hand pump lubrication was later changed to dry sump fed by an external oil pump. A three-speed Triumph gearbox was employed, and fueling was by a Triumph twin-barrel carburetor.

The four-valve head provided an immediate improvement, enabling the Ricardo Triumph to set a new standard for 500cc motorcycles. The engine produced 20hp (14.7kW) at 4600rpm, virtually the same as a 1500cc car at the time, and it allowed the Ricardo to reach over 110km/h.

The cycle parts were ostensibly that of the Model H. Not renowned for its handling, the chassis was tested by the more powerful four-valve engine. The wheels were 26-inch items and the braking rudimentary, but the initial Model R Fast Roadster was immediately popular.

In production form it weighed 113kg and lapped Brooklands at 109km/h, while a racing version set the hour record at 124km/h and the flying mile at 141km/h. Three were entered for the 1921 TT but handling deficiencies meant that only one finished.

A year later, the Triumph Ricardo was back with the new lubrication system, stronger valve gear and a new front fork. This time it finished second in the TT and the following year went on to take a number of Continental wins and a gold medal in the ISDT (International Six Days Trial).

After 1924 Triumph switched its racing development to a new model, but the Ricardo (or Riccy) stayed in its range as a sports version until 1928. The reign of the four-valve layout was shortlived, but its time would come again when Honda unleashed its new racers at the beginning of the 1960s.

WANT TO KNOW MORE?

WHY WE’RE WILD ABOUT HARRY
Sir Harry Ricardo was born in London in 1885 and died in 1974. He formed his first company, Engine Patents Ltd, in 1915.

In 1921 Sir Harry’s colleague, Major Frank Halford, raced his own Triumph 500 at Brooklands with a special blend of fuel Ricardo developed for Shell. This encouraged Triumph to engage Ricardo to design a new cylinder head.

The success of the Triumph Model R Fast Roadster led other motorcycle manufacturers to commission designs from Ricardo. The 1934 Harley-Davidson VLD had a Ricardo-designed turbulent head.

When Ducati decided to produce a three-cylinder racing Grand Prix engine in 1971 it turned to Ricardo. Unfortunately this design was a failure and the engine never made it off the test bench.

Undeterred by this failure, when designing the Ducati Desmoquattro engine in 1986 Massimo Bordi also consulted Ricardo regarding the cylinder head design.

Story courtesy of Motorcycle Trader.