SOCHI, Russia (REUTERS) - Mercedes' decision to limit their latest Formula One engine to works team drivers Lewis Hamilton and Nico Rosberg is due purely to technical and logistical reasons, according to motor sports director Toto Wolff.

Since last month's Italian Grand Prix, won by Hamilton, Mercedes's factory team has had a different specification power unit to their customer teams Williams, Force India and Lotus.

"The decision we took is to do an R&D (research and development) exercise in order to learn more for next year and you can't do this kind of exercise with a customer team," Wolff told reporters at the Russian Grand Prix.

"If you go into a development direction, you can't make eight engines because it could be the case that it was the wrong direction we went in."

Mercedes have a policy of treating their customer teams equally but, with only four races remaining after Russia, have decided it is not feasible to make the latest upgrade available to all.

The works team can clinch their second successive constructors' title in Sochi this weekend if they score three points more than Ferrari.

However, Williams are defending third place from Red Bull and attacking Ferrari while Force India and Lotus are fighting a close battle for fifth.

Finland's Valtteri Bottas, who set the fastest lap in Sochi last year, said he was still happy to have a competitive engine in his Williams.

"Even though it's not a new step, it's a very good engine, and it is competitive and we can get good results with it," he told reporters. "Life is not always fair, and nor is F1... I still think we have the best possible engine for us to try and get maximum points for the rest of the year."