Toro Rosso's year-old power unit is the least powerful engine on the grid, according to Mercedes' data.

Toro Rosso is the only team running a 2015 power unit this year, with all the other outfits benefitting from upgrades from their engine manufacturers over the winter. Much of Toro Rosso's impressive start to the year has been attributed to its Ferrari power unit having more power than Renault and Honda, but data gathered from GPS readings by Mercedes suggests all the 2016 power units are already ahead of the year-old Ferrari.

"You may not know this, but the engine with the least power on the grid is actually the Ferrari in the Toro Rosso," Mercedes technical boss Paddy Lowe told ESPN. "The Renault and the Honda are not so far behind the Ferrari Mercedes, so there is not a lot of difference now between the power units."

The current engine formula is based around efficiency, with manufacturers tasked with extracting as much power as possible from the restricted 100kg/hour flow rate of fuel to the engine. Mercedes has recently trumpeted its achievement of utilising more than 45 percent of the potential energy in the fuel, compared to 29 percent in the V8 era, but Lowe says rival manufacturers are not only a couple of percent behind at most.

"Even the worst engines in Formula One, and people complain like hell about them, are still fantastically efficient by any absolute standard. They just happen to be one or two percent worse than the best engines."

The news also underlines the remarkable job Toro Rosso has managed with its car this year after Max Verstappen finished sixth in last weekend's Bahrain Grand Prix. The Bahrain International Circuit is a power sensitive circuit where fuel usage is critical, but Toro Rosso still managed to hold off more powerful opposition.

After qualifying in Bahrain, Verstappen said he could see a big difference between his car's power output and that of the 2016-spec engine.

"If you are behind a factory Ferrari you can see that they have more power, yeah. But we have to be happy with what we have because it's very reliable and for us that is a good start."