In August 2026 JCB set a new FIA world land-speed record for a hydrogen-powered vehicle, achieving a speed of 406.320 mph. This presentation provides an overview of the development of the 805 bhp hydrogen internal-combustion engines which power the Hydromax vehicle. The Hydromax engine is an in-line four-cylinder medium-duty engine with 109 mm bore diameter and 135 mm stroke. The combustion system is a four-valve pent-roof spark-ignition system with direct injection of hydrogen gas. Water injection is used to moderate knock and pre-ignition at peak BMEP levels up to 45 bar. The development of the engine to achieve rated power of 805 bhp (600 kW) will be described, including 1-D gas-dynamics simulation, 3-D computational fluid-dynamics (CFD), and performance and emissions and mechanical development testing. The development of the combustion, fuel-injection, water injection, boosting and thermal management systems will each be examined, both during testbench development and the record attempt on the Bonneville Salt Flats, Utah.