Is the Cosworth engine useless?

[csirl]

Looking at last seasons results, you'll see that the 3 x Cosworth teams occupy 3 of the last 4 places in the constructors championship. Williams, who are a vastly experienced and competent outfit are the worst of the established teams by a considerable margin. The race pace of the 2 new teams using the Cosworth is considerably worse than the new team using a Renault engine. This cannot be purely coincidental. Makes you think whether Marussia and HRT would be a lot closer to the pack if they had any other engine and so would have a better chance of survival long term.

Why are the Cosworth teams so off the pace? Does the engine need to be re-benchmarked?