Nico Hulkenberg has admitted that returning engine supplier Cosworth's V8 unit feels "better" and "stronger" than the Toyota power plant.
The German rookie was Williams' test driver in 2009, meaning that the last two Formula One cars he drove were respectively Toyota and Cosworth powered.
Williams has this year switched to Cosworth, the independent British supplier supported by the FIA to also power F1's new teams Virgin, Lotus, Campos and USF1.
Asked about the Cosworth engine by reporters at Jerez, 22-year-old Hulkenberg said: "It is good; I'm positively surprised about it."
Asked to compare it with Toyota's engine, he added: "It feels better, it feels stronger."
Thursday was otherwise not a great day for Williams, after a hydraulic leak preceded a driveshaft problem aboard the new FW32 car.