It was highly unique compared to the designs of rivals Renault, Mercedes and Ferrari who had already debuted power units the season prior, and as such, Honda felt its best chance to make up for lost development time was to go aggressive and radical.

The primary focus points of the unit, at the request of McLaren, were extremely compact dimensions and high operating temperature capability that could function with reduced cooling requirements as to aid aerodynamic performance and centre of gravity targets.

This allowed for a significantly shorter engine compared to the Renault, Ferrari and Mercedes concepts whereby their compressor assemblies all protruded from either ends of the block in varying formats.

The exhaust manifold was a small "log" type design (where one pipe housed ports for each cylinder on either bank), this was very beneficial to packaging requirements and allowed significantly tighter bodywork.

[6] After debuting publicly for the first time in the MP4-30 at the 2015 pre-season test at Jerez, rumours began circulating that the engine was extremely unreliable, heavy on fuel use and significantly down on power.

It was quickly discovered that although the compact nature of the turbocharger assembly was beneficial to packaging, it proved to be vastly undersized and as such, had poor air compression capabilities which resulted in significantly less power and combustion efficiency potential.

Honda's lack of experience in programming and controlling such a complex power unit made identifying specific issues difficult and time consuming.

Honda maintained these were early teething issues, only discovered after mounting the engine to a moving chassis under racing loads for the first time and they would quickly sort these out.

All these new technologies being implemented into one engine meant Honda admitted the entire unit was so experimental that it was a "high risk" move and would take time to realise the full potential of several aspects, but one they believed will ultimately provide much greater performance.

The reasoning put forward for the issue was incorrect fluid dynamic simulation estimates of the lateral loads the 2017 car regulations would produce, which were far higher than expected.

The design of the MGU-H housing for the RA617H saw it cast as part of the cylinder block in efforts to improve strength, however this solution upon malfunction, would often terminally damage the ICE as well.

Honda admitted it was taking longer than hoped to understand how to operate the new engine or come up with a viable long term fix for the vibration and shaft balance issues destroying the MGU-H.

The engine was now being run in a state of tune that produced less power than the RA616H it replaced from the year prior for reliability concerns and even with consistent upgrades being provided through the season, progress was initially slow.

The troublesome MGU-H was totally redesigned with input from Honda's aerospace division HondaJet, lending their significant experience in precision turbofan design.

As such, the shaft had its size and shape completely altered, its bearings improved and received an entirely new structural support mechanism which largely eliminated the resonance/vibration issues that affected the previous iteration.

[4] Now having more real estate on the car available to work with, the air intake funnel was enlarged and shaped more optimally for improved airflow into the engine, the exhaust manifold was further optimised and routed differently to suit the STR13 bodywork while gaining a net performance improvement, the cooling system designed for use in the STR13 was substantially more effective, the engine received structural reinforcements and material changes to better handle the forces experienced by the new generation cars and as such, the power unit increased in weight slightly, although still remained the lightest of the field.

[11] During the Canadian GP, Honda introduced the "Spec 2" version of the power unit which consisted of changes to the ICE and combustion system, solely in pursuit of performance.

[12] This performance gain, steep development curve and trajectory, investment into their facilities and drive to win was enough to convince Red Bull Racing to sign Honda as their power unit supplier from the 2019 season to replace Renault.

With advancing it this far (spark beginning too early) however, to compensate for the burn time, can lead to combustion initially working against the piston movement or pre-ignition (detonation before intended), this causes knock and stress on the system and can also reduce power.

[14] The Spec 3 engine was tested in a car for the first time during practice sessions at the Russian Grand Prix, with drivers reporting feeling a substantial increase in power, however there was a lot of vibration and unusual gearbox shift behaviour present.

It was also the first time Honda had supplied two teams in the current hybrid era of Formula One simultaneously, still continuing their relationship with Scuderia Toro Rosso, powering the STR14.

The RA620H was the fourth and final iteration of the engine architecture Honda introduced in 2017, powering the Red Bull Racing RB16 and the Scuderia AlphaTauri AT01 in the 2020 Formula One season.

The compressor was enlarged further to increase engine efficiency and the turbine housing was now 3D Printed (additive manufacturing) with Inconel, allowing more complex shapes to be created.

This re-routed compressed air has multiple benefits but primarily, it allows additional drive force for the turbine to be stored in cylinders when they are not firing, which has a faux "anti-lag" effect.

This changed the valve angle and shape of the combustion chamber, increasing the compression ratio of the ICE, lowering it's centre of gravity and altering the airflow characteristics.

With the vastly improved thermal efficiency of the new ICE came a trade-off, a reduction in waste exhaust energy output which negatively impacts turbocharger and hybrid regeneration performance.

The key for Honda engineers was to play a balancing act in increasing ICE efficiency and crank power output but without losing too much exhaust heat energy which would harm regeneration potential for the battery.

By filling the spaces between carbon particles with the nanometer-sized, tube-shaped CNTs (with a diameter of one millionth of a millimetre), it is possible to achieve extremely low resistance and enable electricity to flow more freely.

The new E10 fuel blend is more resistant to detonation, allowing engines to be run in a higher stress state if well controlled, so for the RBPTH001, along with significant improvements to all the technologies introduced over the past 6 years to the power unit, the result was a major evolution that made even the regulation maximum compression ratio of 18:1 feel restrictive despite it being an enormously high target when Honda first rejoined F1 in 2015.

So the combustion chamber and mechanism was further developed to accommodate the new burn characteristics and lower calorific fuel, the bottom end internals were strengthened, and the ignition timing map was altered completely from the 2021 engine.