Honda has found further performance improvements for its Formula 1 engine but is yet to decide which elements it is confident enough to introduce, says project leader Yusuke Hasegawa.
The Japanese manufacturer has struggled with a lack of power and unreliability this season, having introduced a new concept over the winter.
But Honda has retained belief in the concept and has made progress with performance and reliability after it introduced its 'spec 3' update during the Azerbaijan GP weekend.
It hopes to be in a position to introduce 'spec 4' in one of the early races after the summer break, but will not do so until it is confident in the update.
"We know the target, which other teams have already achieved," Hasegawa told Autosport.
"We know most of the teams are using the same concept of our engine but obviously we don't know the exact solution, the exact design, the same level of performance we can achieve.
"But we know the direction and we know some elements to achieve that performance, but we still have not concluded what element we have to introduce.
"[We are focusing] mainly the combustion but also every area, such as fuel consumption, auxiliary parts - everywhere we're trying to find more horsepower."
Honda plans to continue with the same concept next year and has several research programmes focused on 2018 running alongside its normal development for this year.
Hasegawa admitted Honda underestimated the challenge of changing concept but felt it has made progress this season.
"From the concept point of view, yes, we underestimated it," he said.
"Although it took a bit of time to stabilise, we understand it was a stage we had to overcome - it was very tough.
"We thought we had enough preparation time, which is why we started this year's engine development in May, but one year is not enough.
"Theoretically speaking, we have already achieved some good steps.
"We are in the middle of our target but we need more steps to catch the top runners."