HRC vice president Shuhei Nakamoto says Honda's biggest mistake of the 2015 MotoGP season came long before the first racing lap.

The pivotal moment occurred at the opening Sepang test in February, when Honda rolled out revised 2015-spec engines following complaints at November's Valencia test.

The feedback in Malaysia falsely suggested the 'difficult' engine character had been tamed, when in fact the extreme heat and humidity dulled the aggression.

Honda riders - led by reigning double champion Marc Marquez and team-mate Dani Pedrosa - went on to battle unpredictable rear grip during braking plus wheelies on corner exit for much of the season.

"In MotoGP the results were not as good as expected. The main issue is the characteristic of this year's engine," Nakamoto confirmed. "I remember last year here [at Valencia] Dani and Marc were not happy with the new engine. Not happy at all. The power is more than before but the character is more difficult.

"In the winter we tried several things and brought the new engines to Sepang. Most riders said 'yeah, this was acceptable'. Marc and Dani did long runs, the test was okay and so at that time we were happy.

"This was our biggest mistake for this year. Because as you know Sepang is very high temperature and high humidity. Usually in this kind of conditions the engine loses power. Therefore less aggressive.

"So we decided the engine specification at Sepang 1 - we didn't test at the normal conditions, meaning ambient temperatures of 20 something. When we got to Qatar the rider felt again this aggressive feeling."

With no engine design changes allowed during the racing season, Honda was forced to try and tackle the problem indirectly.

"We changed the throttle body, exhaust pipe and Marc changed to the 2014 chassis [at Assen]. Anyway our HRC engine for this year was not friendly. That means Dani, Marc and also Cal and Scott were struggling to make a good lap time, because of the engine characteristic."

MotoGP champion in both his previous seasons, Marquez typically refused to back off in the face of such hazards - causing successive falls at Mugello and Catalunya. The Spaniard claimed just one win on the 2015 chassis, then four victories after reverting to last year's frame.

But it was far from a perfect solution, Marquez being caught out again at Silverstone and Aragon, plus a second clash of the season with Rossi at Sepang.

"For Marc, he won two titles already but is still young," Nakamoto said. "He wanted to win more races, so he tried very hard and crashed many times. He needed a little bit more experience to understand to win the championship. So on the hardware side and rider side we learned a lot of things this year."

While Marquez later confessed his championship hopes were realistically over at Catalunya, Pedrosa's chances were gone at round one. The Spaniard slumped to sixth place at Losail then revealed he was withdrawing from racing to try and seek a cure for arm pump problems.

"For Dani, last year the arm pump was better but still he had some issues. Usually Dani likes a light-handling bike, which means he preferred to use the 320mm brake discs. But because of the arm pump sometimes he had to use 340mm, even though he doesn't like," Nakamoto said.

"At the end of last season, Dani went to several hospitals to get an idea of how to fix everything. All of the doctors said 'you don't need surgery', but again at Qatar his arm pump was very, very bad. His arm was a strange shape.

"We spoke with Dani and decided it may be necessary to have surgery, then he went to see another doctor and this doctor said they can do another surgery, but maybe it is 50-50 whether they can fix it or not.

"We and Dani decided to do this and he missed several races but the surgery was a success. As everyone knows, for Dani the second part of this season was like a comeback. He showed how strong he is. He won two races, for four races he was on the podium and I was very happy to see him on the podium.

"But for Marc, HRC made a mistake and the rider made a mistake. And Dani didn't know the situation - let's say he was not lucky. So we lost all three championships [riders', constructors' and teams'] but fortunately we have learned a lot of things in this year."

While HRC have been left in no doubt about what needs fixing for the 2016 bike, the task has been complicated by the simultaneous change from Bridgestone to Michelin tyres and introduction of a new single ECU software.

"The main items we are testing [at Valencia] are a new engine, new software and the Michelin tyres. We are still using this year's chassis, just we put the new engine because we don't want to make a misunderstanding again," Nakamoto said.

"If the chassis is the same it is easier for the rider to notice the engine difference, but at the same time with the new electronics and the new tyres it is not easy to understand if the new engine is okay or not.

"It feels different in some areas but both Dani and Marc couldn't say if it is better or not. Some area is maybe to do with the engine software. Our software engineers do not have enough experience [with the 2016 ECU] to fix everything. I hope the new engine works well but is too early to say.

"We are collecting a lot of data and we will also test at Jerez at the end of this month because we don't have enough time to set-up all the new software here. This is not just about the hardware and software, but also the engineers. So we need testing. This is our situation."

Marquez was fastest at the Valencia post-race test with Pedrosa just 0.120s slower in third. However both confirmed it wasn't possible to evaluate the new engine until the 2016 software is set-up and 'in sync' with the RC213V.

MotoGP will no longer hold two of its three pre-season tests at Sepang, with Phillip Island replacing the second Malaysian visit.