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Toro Rosso's late switch to Ferrari engines for the 2016 Formula 1 season has led to "many compromises" on its STR11, according to Carlos Sainz Jr.

The deal for the Italian team to replace Renault with 2015-spec Ferrari power units was only completed at the start of December, with Sainz admitting Toro Rosso did not expect to have its car ready for this week's first test.

But after completing 55 laps on the opening day before a gearbox problem curtailed his running, Sainz said he was still pleased with the start.

"There are many compromises, both in the aero, and a lot of parts not being here yet," said Sainz.

"I don't mind; I've driven on the first day of testing at Barcelona, and two months ago the team were telling me this was probably not going to happen.

"It's been a record-breaking pre-season for Toro Rosso, to change the whole bodywork of the car to adapt to the engine and be here was a record.

"We achieved something like a big team would, and you could see the happy faces in the team when we went out at 9am."

Sainz added that the step from Renault to Ferrari was significant in several areas, not just the amount of power produced.

"It's a good step, it's what we expected," he said.

"A very decent step on power, also on the driveability and easier software to handle for the driver - less buttons to switch to maintain the battery at a certain level.

"With the radio clampdown for this year, this Ferrari engine compensates for that and helps you."

Red Bull team boss Christian Horner believes Toro Rosso will be capable of defeating the Renault-powered main team early in the year.

But Sainz expects the year-old Ferrari engine to slip down the order as the season progresses.

"As we saw last year, the Mercedes and Ferrari engines were developed in-season quite a lot so we should drop back," Sainz added.

"But reliability was our most limiting factor last year and now we have an engine that has already raced for one year it should compensate for that."