Jenson Button hailed Honda's latest power unit as a good step forward for McLaren after completing 121 laps and finishing fifth fastest on the second day of Formula One's final pre-season test on Wednesday.
"I would say that it's the biggest improvement I have felt with the power unit over the last 14 months," said the 2009 world champion after the session at Barcelona's Circuit de Catalunya.
"The improvements are good but we are still a long way off the other guys and you can see that in the speed traps," added the Briton. "But you can definitely feel the difference on the initial part of the straight."
Former champions McLaren have not won a race since 2012 and endured their worst season in 2015, the first year of a new partnership with Honda whose V6 turbo hybrid engine was no match for Mercedes or Ferrari.
Button said the Japanese manufacturer had made big improvements in the hybrid energy systems that cost the team heavily last year.
"We have really pushed the performance with the power unit," said the Briton.
Finland's Valtteri Bottas put Mercedes-powered Williams on top of the timesheets on the supersoft tyres on Wednesday, ahead of triple champion Lewis Hamilton on soft tyres in the works Mercedes.
Bottas said Mercedes, who have done eyecatching amounts of mileage in testing so far, were still in a league of their own but Williams, third last year, could hope to be on a par with Ferrari or Red Bull.
Hamilton was happy with his morning work before handing over to German team-mate Nico Rosberg in the afternoon.
"I got my laps in and I've got an understanding where the car is good and where the car isn't good," he told Sky Sports television. "But generally testing has been great."
Denmark's Kevin Magnussen was third fastest for Renault and Ferrari's Sebastian Vettel fourth.