Honda has ruled out a complete overhaul of its power unit concept next year despite a disastrous opening half of the 2017 season with McLaren.

For the first two years of its Formula One return, Honda pursued a unique power unit architecture that saw the compressor and MGU-H nestled in the vee of the engine. This year it has started afresh with a similar layout to Mercedes, but it immediately encountered performance and reliability issues at the opening pre-season test in February.

Honda F1 engine boss Yusuke Hasegawa said the extent of this year's changes was on a par with starting the project from scratch, but he is still confident the new concept has the potential to catch rival engine manufacturers Mercedes, Ferrari and Renault.

"We introduced a new power unit concept this year, so I would almost call this Year One again," Hasegawa told the Honda website. "But our aim is to develop the 2017 concept into the 2018 season and hopefully 2019 as well. So the engine weight, centre of gravity and the combustion concept is all going in the same direction as the other three engine manufacturers.

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"It was good for us to do that. We can modify the specification of some of these parts to catch up with the other three engine manufacturers. Last year the engine concept was completely different, so with minor modifications we could not duplicate the same type of performance. That's why we really needed to change the whole engine concept this year."

Despite more development opportunities opening up this year, Hasegawa said it was clear from the moment it was fired up that Honda's new power unit would face reliability issues and suffer a deficit in performance

"When we fired up the complete engine for the first time, we could see it wasn't delivering the durability or performance in accordance with our expectations. We also found many minor issues. So we needed to modify tiny bits.

"After resolving these smaller elements, we started to test the full concept at the start of this year -- call it Spec Zero as it was the initial one -- and before the first winter test we confirmed that it ran on the dyno.

"But of course at that moment we knew that the power was not delivering to our target. Then, at the Barcelona test, we found more issues on the car, such as the oil tank issue. It was a car-related issue. This is not a complete engine issue, but of course it is very important.

"Many items we could not test on the dyno, so it is normal that we need to check some functions in the car. The oil tank is one of the biggest items, so we have a rig for the oil tank but we cannot recreate the same types of G forces and conditions as in the car. Of course, by design we have to consider the actual car situation in theory, but sometimes it is not always the same situation so that is why we had some issues with the oil tank first.

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"The second issue was down to the vibrations. On the dyno, the model is stiffer and heavier, so it doesn't create any synchronised vibrations, but on the car -- with the gearbox and the tyres -- there is a much lower level of inertia. Low inertia does not always create vibrations but it's completely different from the dyno and that's why we suffered a huge vibration on the car. Of course, we were aware some level of vibration would come in the car but it was much bigger than we expected."

It is not yet clear who will be running Honda's power unit next year, with McLaren reluctant to confirm its engine partner for 2018 and Sauber's new deal also in question. But Hasegawa is confident Honda has a strong basis on which to develop its power unit concept for next year, regardless of where it ends up.

"We don't stop developing, we need to keep updating. Of course the performance and results are the most important things but it's all learning for the future too. Compared to last year we needed to modify the engine concept, but next year we will keep the same concept.

"It's good that we can use the same concept because this year's development and improvement is directly connected to next year. So that means we don't need to stop the current development, and from that point of view we have already started next year's design."