Williams facing F1 engine dilemma regarding upgraded Mercedes

Mercedes spent seven development tokens on the power unit revisions it brought to the Italian Grand Prix, with just factory team drivers Lewis Hamilton and Nico Rosberg getting the upgrade for Monza.

The German firm's customer teams - currently led by Williams - remain unsure when they will receive the new version of the engine.

Williams fitted a third fresh engine each to Valtteri Bottas's and Felipe Massa's cars for Monza, giving them one more penalty-free change before the end of the season.

Head of performance engineering Rob Smedley wants that to coincide with the switch to the upgraded Mercedes, but he admits there is no firm indication yet of when or if Williams will receive it.

"Obviously the works Mercedes team has had a power unit upgrade which we would dearly love in the back of our car as well," he said.

"That seems to have been a good step - Lewis was certainly going very quickly.

"We've got a new power unit to take before the end of the year.

"Whether that is the new specification I can't really tell you, but it's under discussion at the minute.

"If we get it before the end of the year, that would be a great advantage to us."

Williams is currently on target with its engine usage strategy for 2015, having mirrored the works Mercedes squad in taking new engines for Canada and Italy.

"We're pretty much right on what we set out at the start of the year," Smedley said.

"We've had very few hiccups in that. We're sensibly pushing the boundaries.

"We're on a strategy at the minute where we won't have any penalties to the end of the year if nothing changes from here on in."

Massa agreed that the new Mercedes could be a major asset to Williams.

"It looks a really big improvement," he said. "I have no idea when we're going to have it. I hope we will have it."

Fellow Mercedes customer team Lotus is also awaiting news from its supplier on when or if it might get the upgrade.

Deputy team principal Federico Gastaldi told AUTOSPORT: "I don't know to be honest. We are waiting to hear from them.

"It's not in our hands. We need to understand when they are ready."