I write regarding the article by Nolan Woodbury on the little Benelli, which I enjoyed, and which reminded me of the 250 Four they made in 1939. It was a dohc watercooled supercharged four with cylinders across the frame, with a bore of 42mm and stroke 45mm, giving 52bhp. It must have been mind-blowing back then in 1939 – and still would be today with a top speed of 145mph from a 250. Apparently the works mechanics wrapped the engine in greased tarpaulin and put it down a well, out of the way of the German army as they got close to the factory. It took 30 years for the motorcycle industry to improve on Benelli’s achievement.
I believe the engine still exists in a museum in Italy. Brian Moorcroft, member
There is indeed a supercharged 250/4 in the Morbidelli Museum in Pesaro. Designed by Giovanni Benelli and unveiled at the Milan Show in 1939, the 250 revved to 10,000rpm. The steelsleeved light-alloy cylinders leaned 15-degrees forward; the head incorporated 24mm diameter valves and bespoke, central 8mm Marelli spark plugs. Gears drove the twin cams and the small radiator was fixed to the front frame downtube. The comparatively huge supercharger lived above the gearbox and its drive was taken from the transmission. Fuel was fed by a Dell’Orto carb, through the blower which ran at half engine speed, via a cylindrical intercooler to four induction stubs. This awesome little engine was fitted into the standard Benelli racing chassis of the day, with girder forks and plunger rear suspension. Legend has it that not only was the motor stored in a dry well during the war years, the chassis lived in a haystack under a barn! It’s surprising it survived, as Pesaro was heavily bombed in 1940/41. Then, of course, superchargers were banned from racing in the post-war years so development of the machine ceased. If Frank and I ever get to Pesaro, we’ll seek it out… Rowena