Honda’s most recent upgrade has failed to propel Toro Rosso up the grid – they have not scored a single point in the four races since taking the new spec.
The junior team could test 2019 parts during races over the remainder of the season as Red Bull try to hit the ground running next season.
Daniel Ricciardo took victory in China and Monaco, the latter a particularly fine performance, while Max Verstappen won in Austria – results that Horner says would have gone unchanged if they were powered by Honda.
“We’d have still won the grands prix we won,” said Horner. “Honda are within one per cent of our measurement of where we currently are.
“There’s still a significant chunk to get to Ferrari and Mercedes but having the full focus of an OEM [original equipment manufacturer] like Honda behind us, combined with the fact that with Renault we were becoming more and more the customer as inevitably their focus becomes more centred on their own team, it was absolutely the right timing to go this different path.”
Horner and Red Bull have grown frustrated with an inability to guide Renault’s engine plans since they brought a works team back into F1, and the boss says a closer relationship with Honda will pay dividends.
“With Honda, we will have the ability to have the discussions in advance to try to optimise the integration between engine and chassis.
“They are earlier on the curve and they have the resource and the capacity. One of the biggest issues that probably Renault have struggled with is probably the financial commitment to the R&D process.
“Mercedes have spent a lot of money, and invested heavily, as have Ferrari.
“These power units are extremely complex and you can see now that there are still incremental gains being made with the introduction of each power unit.”