Head of Mercedes-Benz Motorsport Toto Wolff believes that offering the German power unit's latest spec to its customers would not be in their best interests.
Mercedes' engineers exploited the outstanding allowance of tokens to upgrade the engine just before the Italian Grand Prix, but restricted its availability exclusively to the factory team.
The decision ruffled the feathers of its customers, among which Williams and Rob Smedley who contended that an immediate widespread use of the power unit would accelerate its development for 2016.
But Wolff disagreed, refusing to assign the engine to Williams, Lotus and Force India until satisfactory assurance over reliability is achieved.
"The engine is very much an R&D engine," said the Mercedes boss.
" We decided on trying out the direction that we believe could be beneficial for next year but we are not 100% sure. In order to do that exercise you can only concentrate part wise and logistically on the two engines."
While he understands his customers' stance, Wolff disagrees with the notion that a new engine is one which necessarily offers better performance, also insisting that logistic considerations are currently a natural barrier for extending supply.
“When we had the problem with Nico's engine in Monza we weren't even sure we had the spare parts to give him another engine. This is why we decided to do it."
"From phase 3 to phase 4, the main thing it is not a performance step up. Of course people always say that engine must be much better, but that is not the case and it is just a development direction for next year.
“We can probably run it a bit harder for a bit longer, but it is not the miracle step up and we just can't supply it logistically.”
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