For Honda, which had announced its withdrawal from F1, 2021 was its last chance to win the title. The development team took a gamble and prepared a new power unit (PU) in a short period of time. This was the RA621H.

Almost all the components of the RA621H had been re-designed, and was the ultimate weapon for the team to reach the pinnacle. The concept behind the development of the new framework was to maximize the potential of high-speed combustion, which had become Honda’s biggest advantage, and to make the engine as light and compact as possible to increase its contribution to the chassis. Specifically, the distance between the intake and exhaust camshafts was reduced, as was the angle between the valves. This made the cylinder head side more compact and enabled the creation of optimal combustion chambers with a high compression ratio. The cylinder block was changed was no longer cast but fully-machined from a block of aluminum alloy, which increased strength, downsized the block, and reduced weight, greatly increasing the freedom in designing the chassis and enhancing its dynamic performance.

A new device, the CAC bypass, was developed just in time. This was a new mechanism that improved thermal energy regeneration efficiency, creating a wider margin in the amount of regeneration.

Although they were confident of the state-of-the-art RA621H’s performance, engineers later said that there were some concerns about its durability and reliability due to its very short development and production period. However, despite some initial minor problems, the RA621H completed the season without any fatal problems or defects, and as Honda had hoped, became the ultimate weapon in the battle for the championship.

Once homologated pre-season, the regulations forbid any further improvements or modifications aimed at increasing performance. The RA621H was therefore not upgraded, but a new ESS (battery) developed in-house was introduced in Round 12, Belgium, which improved competitiveness.

In 2021, Yuki Tsunoda began his full-time F1 career with AlphaTauri, becoming the first Japanese F1 driver since Takuma Sato as a Honda driver. After performing well in the 2020 FIA-F2 championship, Tsunoda moved up to F1 as a Honda and Red Bull-trained driver, and in his first season, finished within the points in the opening round.

From the season-opener, title rivals Lewis Hamilton (Mercedes) and Max Verstappen locked horns. Verstappen took his first win of the season in Round 2, the Emilio Romagna GP, and from Round 5 in Monaco, Red Bull Honda won five consecutive races (four by Verstappen). By the middle of the season, Verstappen led in the points, but Hamilton regained the lead with back-to-back wins in the closing rounds. Both drivers were on equal points for the season finale. In the Abu Dhabi Grand Prix, an accident in the closing stage changed the course of the race: Verstappen took advantage, and won the race, and his first F1 drivers’ title.

The dramatic conclusion to the last round gave Honda the title it had longed for. The RA621H, with a completely new frame, prepared in a short period of time, was the driving force behind winning the title, and Honda’s challenge should have come to an end with the victory. What then happened, however, was that Honda continued to supply PUs.