Secrets of Honda’s development revealed: “switching from McLaren to Toro Rosso improved HP”

A technical Summary of the Honda RA618H’s Power Unit development from a Japanese magazine has been published recently, and fortunately for those not fluent in Japanese, a native speaker on Reddit F1 has kindly translated the document – makes very interesting reading!

The article is entitled ‘Developing the RA618H”, the Power Unit used last season with the Toro Rosso team. It seems that development is headed in a much more fruitful direction than during the McLaren years, news that is perhaps something Red Bull Racing fans are particularly happy about.

Below is a direct translation by Reddit user u/FCIUS

Working towards a continued improvement of on-track performance
Honda welcomed the Italian team, Toro Rosso as its new partner for its 4th season as a PU constructor in 2018. The contract with Toro Rosso was finalized quite late, but they still worked towards finding a chassis layout that would allow both sides to maximize performance.

As a result, they were able to settle with a layout that enabled an improved performance from the PU.


Honda began supplying PUs—a combination of a 1.6 liter direct injection turbo engine, MGU-H, MGU-K, and the energy store—from 2015. These PUs were supplied to Mclaren for the first 3 years; the 2015 unit being the RA615H, succeeded by the RA616H, and the RA617H.

Broadly, the ’15 and ’16 designs were prioritizing size zero packaging. Compromises in packaging allowed the RA617H to be shaped more optimally, moving the compressor and the turbine outside of the V bank.

As a result, the MGU-H, previously mounted behind the compressor,was lowered, significantly lowering the center of gravity.

With the compressor and turbine out of the V bank, more space was now available for the induction system, while the combustion process was revamped.

Mr. Maruyama, involved with the design of the PU, explains for the RA618H, “although we partly wanted drastic changes, we decided that an improvement of reliability had to come first, for us to properly develop the PU.”

While he says that most components of the 2018PU were new, in terms of appearance, the RA618H appears to inherit the concepts behind the RA617H.

Although reliability was indeed a focus, that didn’t mean that performance development wasn’t done at Spec 1. Still, as they were planning to increase performance for Spec 2 and 3, the emphasis for Spec 1 was on strengthening the backbone of the PU to accommodate the increased performance.

However, the gains from Spec 3 were greater than expected, and Honda struggled to balance performance with reliability.

“With power output increases in mind, we achieved a level of reliability to accompany those increases for the RA618H,” says Maruyama.

“Because F1 engines are developed with very high precision, the rotation itself is very well balanced. Therefore, the source of vibrations were from the combustion. Stronger combustion also resulted in a stronger vibration force. Therefore, for 2018, we designed the engine with a very high level of safety margin for vibrations, to accommodate power increases. However, the gains of Spec 3 almost completely negated that additional allowance for vibrations.


Maruyama explains, “Combustion dictates how we shape the engine. Without evolution there, we cannot hope for large gains, even with drasticchanges.”

Being adventurous with new technology inevitably comes with technical troubles, and resources must be dedicated into solving them.Honda had tended to operate that way for the first 3 years, but this changed in 2018. Having a new partner in Toro Rosso also influenced their new cautious approach.

“Having a separate division work on evolving the combustion, while drastically improving reliability;” this was the concept for the RA618H.

Once the PU is designed by Maruyama and others, the Research division now handles bench testing, and improving drivability. Mr. Orihara, who handled this process, explains:

“if the engine spins properly from the onset, it won’t interfere with testing, and the team’s development. On the other hand, if we delegate combustion development to a separate division, and gradually introduced those developments on the 2nd and 3rd units, we thought we could increase competitiveness as the season progresses, and even for 2019.”

While drivers were allocated 5 units for the 2016 season, this decreased to 4 for 2017, and 3 for 2018. With 21 races in the 2018 seasons, this meant that there were 2 chances to introduce new specs. While technology was developed continually throughout the season, they were incrementally introduced in Spec 2—deployed at Canada—and Spec 3—deployed at Russia.

To clarify, Honda struggled with oscillation for 2017—how does their struggles with vibration for 2018 differ? In short, while vibration is how we understand it colloquially, oscillation arises from rotation. For instance, at the Russian GP, where Spec 3 was deployed, it was oscillation that surfaced, with drivers complaining of oscillation when shifting up.When shifting up, the engine rotation drops, but if the combustion isn’t properly managed immediately afterwards, the crankshaft, gearbox, driveshaft, and tires fail to deliver power properly transfer power onto the tarmac.

This is the effect of oscillation in a specific sense. While it is possible to replicate this on the bench, at the end of the dynamo is a motor rather than a tire, so the vibration profile differs.

Therefore, only at the track can oscillation be fully assessed. Therefore, at the Russian GP, Honda were once again haunted with oscillation, an issue that had troubled them throughout 2017.Because they had built up knowhow to respond to oscillation issues, they were able to resolve them shortly after the introduction of Spec 3, and Honda are confident that they are set for 2019. Direct injection turbos under limited fuel flow is hardly specific to F1, seen elsewhere in categories such as LMP1, Super GT, and Super Formula. However, shift up oscillation isn’t an issue in those categories. A reason might be different environments.

In a nutshell, there are two ways to increase engine output with limited fuel flow: increase the compression ratio, and making the air-fuel ratio leaner. The latter decreases fuel content, which destabilizes combustion. F1 engines normally rotate at 10000-12500 RPM, but other categories typically see 6000-8000 RPM.

The faster the rotation, the shorter the combustion time, so a slight external disruption can affect combustion,making suboptimal combustion more frequent. To convert combustion energy efficiently into pressure, a shorter combustion period is needed,but this exacerbates the instability of combustion.

Maruyama explains, “whether an engine has a large bore/short stroke, or a small bore / long stroke” affects this too. The longer the stroke, the more robust the combustion. This is because air and fuel can properly be mixed, despite changes in fuel and air intake.”

Shifting speeds are also different. In other categories, the clutch is disengaged before selecting the next gear when upshifting. However,seamless shifting is the default in F1. Because engine rotation drops extremely quickly, the engine must be able to combust properly at thatnew, lower RPM. Combustion control in F1 is therefore done under extremely difficult circumstances.

At the end of 2017, the contract with Toro Rosso was finalized significantly later than usual. PU installation only started after that. Development was done in little time, and Honda asked Toro Rosso for a layout that allows for maximum PU performance.

On the other hand, Toro Rosso wanted to prioritize packaging. The two parties therefore had to find a suitable compromise to maximize performance over all.

Switching from Mclaren to Toro Rosso already improved the HP

Compared to the RA617H, the air filter has been made larger, visible in the side view. The pipe leading to the compressor is curved stronger than the RA617H. The RA618H saw an improvement in the air filter, and the curvature of the pipes. This applied for the tail pipe as well, with agentler curve for the RA618H.

Loss during air intake was reduced, and loss at the exhaust was also reduced. Furthermore, Toro Rosso provided an intercooler with lower pressure loss, so simply switching from Mclaren to Toro Rosso already improved the horsepower.

“We can’t compare the effect of PU layout on the aero and chassis performance between McLaren and Toro Rosso. However, with Toro Rosso, both sides presented their demands for maximum performance, and a balance was sought out between the two sides. As a result, the finalized layout was one that allowed for an increase in PU performance compared to before.”


From the front, the RA617H clearly only has the oil tank on one side, with a CFRP pipe running from the intercooler to the plenum chamber on the right side.

On the other hand, the oil tank was designed for the RA618H to surround the larger compressor. For 2017, the oil tank design was partly the reason behind Honda’s struggles to properly separate air and liquid. Having learned their lessons, Honda requested enough space that won’t compromise the oil tank design.

The intercooler was placed on the left side for 2017, but Toro Rosso preferred a symmetrical cooling system, so it was placed on both sides. This was “just a simple difference in philosophy” (Maruyama).

Energy management was also evolved. Under the current regulations, the key technology to improve performance is fully utilizing the MGU-H for converting heat energy to electricity. Energy sent from the MGU-K to the ES is limited to 2MJ per lap. Energy deployment from the MGU-K is4MJ per lap.

However, there is no limit for the MGU-H, and therefore excess power from the MGU-K is routed through the MGU-H to the ES. This is called extra harvesting. This process was improved for the RA618H, allowing for a “straighter and longer retrieval of energy from the MGU-K” (Orihara).

With the cooperation of technicians from the HondaJet, with their strength in rotational design, the technological level for the PU was boosted from the bottom up. This allowed for greater regenerative energy.

In truth, this came at a significant handicap, as the gear ratio was different with Toro Rosso and McLaren. McLaren’s ratio went to a higher RPM, while Toro Rosso’s were lower.

On the pure basis of capacity, a lower rotation makes regeneration more difficult. However, Honda still managed to increase its energy retrieval for 2018.

For Spec 2, deployed in the Canadian GP, the combustion, exhaust and water pump were improved.

“It’s the first time we’re working together, o we both designed with large margins. We in Honda don’t know how they’re radiator’s coming along, so we played it safe. Toro Rosso themselves fed us numbers that gave them some wiggle room.

“We fixed that at Spec 2, reducing the size of the pump as it was doing extra work.” (Maruyama).

Combustion was also worked on for Spec 3. Honda’s technical director, Mr. Tanabe explains: “We’re pushing from the frontlines to R&D to keep pushing, and implement good ideas quickly. Step 3 was R&D’s response. Step 2 built upon Step 1.

“Of course, that remained true for Step 3, but we also discovered something new, so we largely modified our development schedule. We put everything we had into Spec 3, as the final 2018 Spec PU.”


As a result, the gains from Spec 3 was much larger than that of Spec 2, and any safety margins Honda had preserved disappeared.

The new PU, which will probably be named RA619H, will be a full evolution of all components, while accounting for projected gains over the season. Simply improving the capacity of the ICE is not enough.

“If not for the MGU-H, perhaps the strategy would be to run the PU as lean as possible,” says Orihara.

“However, just improving the combustion decreases the MGU-H recovery, so we need to balance the two. And that balance is technologically difficult. A ICE engineer might want an increase in horsepower, but that doesn’t necessarily increase the total energy recovery per lap.”

“It’s really a well thought out set of regulations.”

The reality is that a horsepower gained from the ICE is not equal to that gained from the MGU-H. On some circuits, the former might be more valuable, while at others the MGU-H might be more important. Improving both the ICE and the MGU-H—which are fundamentally at a trade-off —while finding the optimal balance of the two, is the crux of PU development.

Honda now has much more knowledge, and were able to affirm the potential in the technology that they have been continually developing. They’re ready to make it to the next stage.

5 responses to “Secrets of Honda’s development revealed: “switching from McLaren to Toro Rosso improved HP”

  1. fascinating stuff. a far cry from changing the carb jets, float level, idle mixture and timing 🙂

  2. Pingback: Rumours gain weight on Ferrari & Sauber | thejudge13·

  3. Thanks everyone for the nice comments, although I take zero credit on this one. The Redditor that translated the article did a marvellous thing last week

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