Toro Rosso will only have small upgrades through 2015 F1 season

The Italian team brought what its technical chief James Key described as an "almost completely brand new" car to the third winter test at Barcelona, with new aerodynamic packaging, cooling and suspension.

That contributed to a strong start to the season for Toro Rosso, with Sainz scoring points on his debut in the Australian Grand Prix and Max Verstappen, 17, becoming the youngest F1 points scorer in Malaysia.

While most teams usually bring a big upgrade package to the first race of the European season in Spain next week, Sainz suggested Toro Rosso will not be doing the same.

"We are obviously bringing some aero upgrades, but not a lot," he told AUTOSPORT.

"The biggest aero upgrade was obviously in test three in Barcelona.

"Now every upgrade will be much smaller."

Verstappen missed out on his second points-finish of the season when his Renault engine failed with four laps to go in China.

And Sainz believes the power unit, which has also proved troublesome for senior team Red Bull, is the key area where a step forward needs to be made.

"The biggest improvement that we need now is on the engine side," he said.

"We need to improve that, as it is the area where we gain the most performance."

After two races, Toro Rosso was fifth in the constructors' championship ahead of Red Bull, but it has since slipped down to seventh.