Honda is continuing to evaluate whether its latest third-generation engine will be race-ready in time for the company's home Grand Prix in Japan on Sunday.
The 'Spec 3' upgrade to its RA618H power unit had been expected to make its maiden race outing last weekend in Russia.
But after running it in Friday practice at Sochi, Toro Rosso subsequently opted to revert to an older specification for the rest of the weekend. It meant the team incurred heavy grid penalties for the race.
Honda said this has been a precautionary move after finding some "unsuitable behaviour" during the initial track sessions. Reports suggest that the the unit suffered from higher oscillations than had been expected, affecting driveability.
Honda is now putting the upgrade through further tests before deciding whether to use it at Suzuka.
"We need to evaluate the test data," Honda F1 technical director Toyoharu Tanabe told Motorsport.com. "We plan to have a dyno test with the new spec and then we will try to optimise our calibration.
"If the level of optimisation is suitable for the race application, we will decide yes or no," Tanabe added. "It's not very easy but not impossible to optimise the calibration."
Tanabe suggested that being forced to roll back on the upgrade in Sochi had proved beneficial in some ways as Honda had been able to do like-for-like comparisons.
"It's actually a negative, but to us is a positive," he said. "We found some areas we need to improve for race application.
"There are many areas we got information from this weekend at the same track," he added.
In its brief time in service in Sochi, the new unit got good reviews from drivers Pierre Gasly and Brendon Hartley who said it provided a much-needed boost in power.
"Everyone is quite excited about the step that Honda made," Gasly had reported. "It’s really positive.
"We need to see how much we can extract from it, - reliability, driveability - to see if everything is fine when we turn everything up, but for them it’s a good step."
“We definitely had a step in performance," agreed Hartley. "It was a really, really promising day from Honda’s point of view ... I think everyone is quite excited about the step that Honda made."
Team boss Franz Tost insisted that the latest engine development would put Honda ahead of Renault in the engine pecking order.
“Honda is making big steps forward. It is in front of Renault," he said. "[It is the] complete package."
That assessment will be music to the ears of his colleagues at the senior Red Bull team, which switches from Renault to Honda engines in 2019.
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