Jolyon Palmer admitted the difference between the Renault Formula 1 engine and the Mercedes the team used last year was clear during Barcelona testing this week.
Palmer had a taste of the Mercedes power unit last season in a number of grand prix first practice sessions in his role as reserve with Lotus.
With a Renault engine now in the back of the car following the manufacturer's acquisition of Lotus over the winter, Palmer has been able to draw comparisons.
"There's a difference," said Palmer. "It's not as powerful as the Mercedes engine, you can feel that.
"There are obvious benefits in that the driveability is good. The Renault engine guys have been working really well. We've been working with them over the winter.
"They've a good handle on procedures, and as I say the driveability of the engine, but at the moment they are definitely behind Mercedes on power."
He is hoping for "a clean run" when he resumes testing next week following a troubled opening two days.
After managing just 37 laps on Monday amid software issues, Palmer could only add 42 on Tuesday due to a turbo failure, so completed a lowly 79 overall.
That left him at the foot of the week's mileage table, while his team-mate Kevin Magnussen had the third highest tally as he covered 264 laps on his two days.
"I was getting a feel for the car, without setting any kind of time, and then it all came to a stop, which is a shame," said Palmer of his truncated Tuesday.
"We didn't really have any issues, then suddenly we had a big one.
"There are still positives as we at least had the car running to do some sort of baseline set-up work, to get an understanding of it from [Monday], which was a bigger disaster.
"At least we did some more relevant laps, but yeah, 79 laps is not ideal for half of my pre-season testing.
"I targeted a lot more than that for two days. I hope to do a lot more next week."
With Renault now behind the Enstone-based organisation, Palmer hopes its resources will ensure quick fixes can be found for the kind of problems he has encountered.
Last year the takeover led to a financial impasse between Lotus and Renault regarding the development of the car.
"If we've problems they're easier to fix because we've the support of Renault now," said Palmer.
"We're not so cash-strapped and looking for alternative solutions. We've a strong company behind us to push."