Oct.9 (GMM/Inautonews.com) Drivers for Mercedes’ customer teams are beginning to speak up about the issue of engine parity.

Williams and Force India, both powered by Mercedes, are receiving fresh units from Brixworth for the Sochi weekend — but they are not the latest specification.

That is despite the fact that, earlier in 2015, Mercedes chiefs insisted that all of the German marque’s F1 customers enjoy full technical parity with the works team.

“Well, for sure we have a very good engine,” Williams’ Felipe Massa said. “Maybe not the best — Mercedes has the best,” he told F1’s official website.

Reports earlier this week suggested Mercedes had not released the latest specification to its customers because Petronas was struggling to produce enough of the relevant fuel.

“At the moment all we are hearing is they (Mercedes) are checking the reliability to see if it is working well,” Sergio Perez, who drives for Force India, said.

“We are not planning to get the full (new) spec for the rest of the year, that’s what I heard from the team. Hopefully for next year we get it,” he added.

The Mexican said he and Force India are not “too concerned” they are now out of step with the works Mercedes team, but he answered “yes” when asked if he thinks customers should receive identical equipment.

Valtteri Bottas, meanwhile, backed Mercedes’ decisions with the customer team programme, arguing that “If they (Mercedes) thought the new engine was the best option for us in the fight with Ferrari, they would have given it to us”.

“For sure, Mercedes is choosing the best option for us and they have their reasons. I don’t know if it was for reliability or for something else, but I believe them,” the Williams driver added.

However, the Finn acknowledged that the specification being run by the works Mercedes team is superior.

“I did notice a slight difference in maximum speed,” said Bottas, recalling an on-track battle with Nico Rosberg, “but you never know what mode they were on.

“Also, you can’t forget that the cars are different too.”

As for whether the current engine disparity is fair, Bottas smiled: “Life is not always fair, especially in formula one.”