Despite introducing Honda's latest-spec engine in Sochi, thereby incurring grid penalties, Toro Rosso will use the previous-spec for the remainder of the Russian GP weekend.
The move confirms that that latest-spec unit is essentially a 'Suzuka special' an upgraded unit introduced for Honda's home race, which takes place at Suzuka next weekend.
Both Toro Rosso drivers were delighted with the upgrade when they used it in Friday's sessions, Pierre Gasly declaring: "Sometimes you bring updates and you put it on the car you don't really see the gains... with Honda it's not the case.
"It's really positive," he added. "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 and it was a really, really promising day from Honda's point of view," added teammate Brendon Hartley. "I think everyone is quite excited about the step that Honda made today.
"Introducing a new PU means a lot of mapping and a lot of things to get under control," he admitted. "There were not so many dramas and we definitely had a performance gain."
Confirming the decision to revert to the old-spec, Honda boss Toyoharu Tanabe said: "Having studied all the available data after FP2, we realized that, although the performance was encouraging, there is still some work to do in some areas such as calibration and matching the PU to the chassis.
"We therefore decided to revert to the older specification PU on both cars for the rest of the weekend.
"We believe we can complete the work before the Japanese GP, so that we can run the new specification PUs on both cars from then on," he added.
Along with the engine, Gasly's is also reverting to his older-spec components, meaning in Japan he will have access to the all-new power unit he ran yesterday, though Hartley will run a new turbocharger, MGU-K and MGU-H for the remainder of this weekend.