Kevin Magnussen hailed Porsche's 919 Hybrid LMP1 car as "the most advanced race car in the world" following his first test at Barcelona.
The former McLaren Formula 1 racer joined GP2 star Mitch Evans and Le Mans LMP2 class winner Oliver Turvey for the first of two days of testing with Porsche on Tuesday.
"This is the first race car I have driven for months," said Magnussen, who last competed in the F1 season-opening Australian GP standing in for Fernando Alonso.
"It's an amazing car with amazing power - it's easy to feel that this is probably the most advanced race car in the world at the moment."
The Dane, who will leave McLaren at the end of the year after spending 2015 as reserve driver to Alonso and Jenson Button, said there were elements of driving a closed-cockpit LMP1 car he still needs to adjust to.
"I have a few things to work on, especially getting on top of braking," he said.
"As you can't see the front wheels here, you don't really know when you're locking up, so that's something to get used to.
"There is also a lot less visibility compared to a single-seater, but the first impression is really good and I enjoyed it a lot."
Porsche played down the possibility of the test leading to any drivers landing a drive with the squad for 2016.
Team principal Andreas Seidl said: "Everybody involved in this test will learn and benefit a lot, and that is what this is for.
"With regard to anything else in the future, we have no announcements planned, especially as we are very happy with our works drivers."
Evans, a protege of current Porsche driver Mark Webber, made his debut at Le Mans this year in LMP2, and he said the 919 was a big step up.
"It's a great opportunity for me to test a Le Mans-winning car," he said.
"A few things are completely new to me - I'm not used to power steering, this type of tyre and have never raced a four-wheel-drive car.
"But the car is very stable and gives you a lot of confidence. I'd like to thank Porsche for the opportunity."
Turvey added: "It is phenomenal to drive this car, the amount of power is really impressive.
"It's a pleasure to drive and I'm very grateful for the opportunity to test here in Barcelona."
Porsche factory driver Timo Bernhard is also in action as a benchmark driver at the test, which continues on Wednesday.