What did Birkigt invent for the SPAD S.VII besides the engine?

The SPAD S.VII was designed around Marc Birkigt's Hispano-Suiza engine, and when the new fighter entered service in September 1916, it arrived armed with a single Vickers machine gun synchronised by a new gear that Birkigt provided for use with that engine. The weapon, a .303 (7.7 mm) Vickers, was mounted above the engine, synchronised to fire through the arc of the propeller.

The firing impulses were transmitted to the weapon by torsion, by means of an oscillating shaft that rotated a quarter turn, alternately in one direction and the other. That solution was mechanically more efficient than the back-and-forth motion of a push rod and allowed higher speeds. The mechanism was recorded in a patent in Marc Birkigt's name titled "Mechanism for synchronizing machine-gun fire", dated 31 May 1917. Its text refers to mechanisms for synchronising, by the action of an engine, the firing of machine guns mounted on an aeroplane or similar, in particular those automatically operated by the recoil of the gun.

The Hispano-Suiza 8a of 150 hp, a 90° V8, introduced the innovative concept of light-alloy cylinder blocks. That engine powered the SPAD S.VII of the Stork squadron ace Georges Guynemer. The first aircraft delivered to a front-line unit was the S.112, flown by Lieutenant Sauvage of N.65, followed by the S.113, assigned to Guynemer of N.3. Francesco Baracca, Italy's leading ace, was delighted with the new model, and his personal aircraft is preserved in Italy.