Using new F1 engine at Monza a risk, Mercedes boss Wolff admits

Nico Rosberg's 2015 F1 title hopes were badly damaged at Monza after a coolant system leak led to damage on his revised unit on Saturday, and the old unit put back in to replace it then failed three laps from the end of the race.

With Lewis Hamilton taking victory, Rosberg is now 53 points adrift in the world championship.

"We brought that phase four engine because we wanted to understand if that direction of development was the right one," said Wolff.

"It was a bit of a risky call and we saw what happened to Nico.

"That was the result of that engine - the reliability runs were on quite a high mileage but they weren't finished yet."

Wolff insisted the engine modifications were more about Mercedes exploring a development direction for 2016 than a straightforward power increase.

"You have clearly seen that Ferrari made a step up with the engine and they've come closer, but as we've said before our engine is a new development direction," he said.

"It's not a pure performance factor but there are other things behind it.

"It's a different direction, but if we were to pursue that direction we have an estimate that we would gain more performance in the mid to long term, so it's not a short-term gain.

"It's not a major step up from the last phase."

He stressed the early introduction of the engine was a gamble worth taking in anticipation of a tougher fight in 2016.

"In hindsight, yes we lost a car and Nico lost valuable points, but this is a competitive championship, it's going to be one next year," Wolff said.

"So I think the earlier you can understand which direction you need to go development wise, the better it is."

Mercedes is not yet sure whether the engine damaged on Saturday in Rosberg's car can be reused.

"We don't know yet whether it is terminally damaged," Wolff said.

"To make the point, it wasn't an engine failure we had, it was a leak in the cooling system which led the engine to fail."