Reigning triple MotoGP champion Marc Marquez made his public debut with the 2017 Honda engine during the start of post-season testing at Valencia on Tuesday.
The Spaniard said the engine was the only new component on his all-black bike.
While no details have been officially revealed, the revised V4 sounds like it uses a 'big-bang' firing pattern, rather than the even-firing 'screamer' previously used. Honda's change of approach means only KTM now runs a screamer engine.
Although the new engine had already undergone private testing, it proved to be far from 'plug and play' on Tuesday, with a 'lot of work' needed to adjust the powerplant to the standard electronics.
Marquez set the fourth best lap time, just 0.172s behind new Yamaha signing Maverick Vinales, but made clear he would have been faster if he remained on his 2016 machine.
"I did the first run with my current bike. To understand the track and everything. Just four-five laps. Then I started to work with the new bike," Marquez said. "I know that with the current bike I can be faster, like in the GP, but we just concentrated on the new bike.
"Looks like for riding [the new engine is] a little bit easier. Easier when you open the gas and control with the throttle.
"But in the end, at the moment, still we need to adjust the electronics to this engine. It is the most important thing because all the torque maps, all the engine brake maps are made for the 2016 engine. So sometimes you are riding and it feels unpredictable. We need to readjust everything, now they have a lot of work to do.
"The character of the engine I think is quite [good]... But the most important is to be faster; at the moment today we were not so fast!"
The softer engine character also had little effect on the RC213V's biggest problem of corner exit wheelies: "At the moment no difference."
Marquez added: "Tomorrow we will concentrate all the day also with the new bike. Maybe at the end of the day we will make some back-to-back to understand everything well."
Team-mate Dani Pedrosa, recovering fitness after last month's Motegi accident, was tenth fastest.
"I was trying to save energy for tomorrow and did very short runs. I just focussed on trying the bikes and not much into the settings," Pedrosa said.
"I tried the new engine. As you expect, we cannot speak a lot. So the feeling was okay. The bike was running. We had no issues and can do many laps. One of biggest points of the new engine is the endurance and reliability of the engine, also the performance of course.
"So [we are] comparing both bikes and trying to get the feeling. Also a lot of work in the box with the data for the engineers."
By Peter McLaren