While Johnson declared the V8-powered 2013 McLaren to be "mind-blowing" in braking and acceleration, Alonso found the Hendrick Motorsports number 48 Chevrolet a blast from the past with its V8 engine and steel brakes.
"Literally on the first outing, my helmet was trying to leave my head, and I was staring at the microphone in my helmet it was so high," Johnson told ESPN at the Sakhir desert circuit south of Manama.
"I was like, 'I don't want to stop but I think I should'.
"I got my helmet more under control, and then it was really my eyes trying to find their way far enough ahead and far enough round the turns..."
The fun swap came a day after Alonso's last Formula One race in Abu Dhabi.
The American returned the favour with some set-up laps in his NASCAR.
"To be honest, I was supposed to only drive the NASCAR, but it made sense if I was doing the installation lap (in the F1 car) just to make sure the car was feeling OK," Alonso said of his swift return to the F1 cockpit.
"I found myself in the cockpit at 11 a.m. this morning after retiring yesterday," the Spaniard said.
Both drivers said they were impressed by the other's performance.
Alonso set a benchmark time of one minute 40.204 seconds in the F1 car and Johnson lapped in 1:40.462.
"I think he was really gaining time every run he was going out," said Alonso. (Reporting by Alan Baldwin, editing by Ken Ferris)
(This story has not been edited by Devdiscourse staff and is auto-generated from a syndicated feed.)