1991 Formula 1 McLaren MP4/6

This was Honda's third engine configuration in just four seasons, and was an entirely new 60˚ unit with a greater piston area than the outgoing engine and a potentially higher rev limit. It was longer, heavier and thirstier than the V10, but it was hoped that an output reputed to be 720bhp would compensate for that. However, the engine installed in the MP4/5C test mule failed to convince Senna. Concerned that it simply wouldn't be sufficient to defeat Renault's new, improved V10, he was quick to tell the Japanese what he thought of it in his usual candid terms.

Though naturally concerned to hear this, the Japanese stuck to their guns, Akimasa Yasuoka arguing that, "Honda traditionally detunes its engines for the first race of the season; we tend to go for reliability rather than power." In retrospect, it was perhaps right to do so for McLaren's season got off to the best possible start with no fewer than four wins in a row, the increased engine weight partly offset by the latest development of McLaren's six-speed gearbox.

The car itself, whilst looking similar to MP4/5B, was quite different in terms of its aerodynamic profile as designer Neil Oatley and his team had received some valuable input from Henri Durand who had joined from Ferrari in mid-1990.

Numerous changes had also to be made to the chassis, not least in order to accommodate the longer engine and the enlarged fuel cell needed to satisfy its greater thirst. Even with four centimetres added to its length the new tub was much stiffer in terms of torsional rigidity, and consisted of even fewer basic components than before. There were changes to the suspension too, an aspect of the car which had altered dramatically since the year before, with pushrod-activated coil-spring/dampers now mounted on top of the chassis ahead of the cockpit instead of being installed vertically either side of the footwell.

The increased fuel consumption posed challenges of its own, despite plenty of development on the engine management system, Senna twice ran out of fuel (at Silverstone and Hockenheim) but the Brazilian and his MP4/6 nevertheless remained unbeaten up to and including Monaco. Giving McLaren a comfortable lead in the Constructors‘ Cup, the margin thus gained was to prove crucially important as the team's performance began to slip  and Williams-Renault began to gather pace with Nigel Mansell and Riccardo Patrese as its reliability improved.

In Montreal two things quickly became apparent. The first was that the Honda's extra power was simply to offset its greater weight relative to the V10s, particularly when its internal frictional losses continued to rise. The other was that the Williams FW14s, particularly Mansell's, were really getting into their stride.  

Undeterred, Honda continued developing its V12. The Spec 1, which had triumphed at Phoenix, Interlagos and in the Principality, eventually gave way to a Spec 2 variant which was actually introduced ahead of the Monaco circuit and offered better mid-range punch thanks to its new induction system. The friction problems were also addressed with a Spec 3 version at Silverstone. New linked rocker arms were also employed in a bid to reduce roll, and a cockpit-adjustable ride-height mechanism was also added.

The fuel metering issues that made themselves so painfully felt during the British and German rounds were largely to do with Shell's experimentation with different fuel densities and viscosities. At Paul Ricard, another inaccurate readout forced Senna to drive conservatively, although following this Honda's research and development effort accelerated dramatically so that by the time he arrived in Hungary he had a car which could be safely revved to 14,800Grpm, albeit only for short bursts.

In Budapest McLaren regained its form in the nick of time. With a tighter-than-ever chassis, and yet another heavily reworked engine with lighter cylinder heads, camshafts and connecting rods, Senna pulled something out of the bag and pushed the Williams duo back to second and third places.

Despite suffering a gearbox failure, he managed it again at Spa for the Belgian Grand Prix, where he nursed his failing car home and saw his lead over Williams grow enormously following another retirement by Mansell. McLaren’s rival was quickly restored to good fortune, however, commanding the field in both the Portuguese and Spanish races, the last of which saw Senna struggling on the wrong tyres.

At Suzuka the order flipped again, the correct tyres and yet more successful engine development leaving Senna in an unassailable position on 96 points. He returned to Brazil with a resounding third title, while Berger finished fourth with 43 points, having been handed victory by Senna in Suzuka. McLaren again took the Constructors World Championship.