United States Grand Prix: Renault to take upgraded engine

Red Bull have threatened to pull out of F1 if they cannot get a satisfactory engine

Red Bull and Toro Rosso will have an upgraded Renault engine available for Sunday's United States Grand Prix.

Renault say the changes - which concern the internal combustion engine - will improve "power and efficiency".

However, both teams will have to accept grid penalties if they want to upgrade in Austin, as they have already exceeded the season's allocation.

It is unclear whether Red Bull will use a Renault engine next season after a recent breakdown of their relationship.

Red Bull have been heavily critical of the French company over the past two years as it failed to produce an engine to keep up with Mercedes and Ferrari under F1's new turbo hybrid rules.

A split has yet to be finalised, but it would leave Red Bull without a confirmed engine partner for 2016.

Mercedes have already been crowned champions this season, but both Renault-powered teams still have points and positions to fight for in the constructors' championship.

Red Bull are fourth - 71 points behind third-placed Williams - while Toro Rosso are seventh and 21 points behind Lotus.

Speaking about the upgraded engine, Renault's director of operations Remi Taffin said: "The principal changes involve the internals of the ICE (internal combustion engine) to give improved power and efficiency.

"We know that introducing the new PU (power unit) will incur a grid penalty so the decision to use will be made in full consultation with the teams."