The 30-year-old Frenchman, who is hunting a drive in the series after being dropped from his role as Toyota's official Formula One test driver, was with Forsythe Pettit Racing again and did not disappoint at all.
The best of his 183 laps came in 50.209 seconds - over half a second faster than he had managed on day one and two tenths of a second quicker than his nearest challenger.
"Everything is good. For the first time in the car, I'm quite happy," said Montagny. "Everybody is very motivated inside the team. I was at the factory working with the engineers, the mechanics and everything was fine but you never know until you get in the car.
Montagny tops opening day at Sebring
"Obviously things are going well. We did everything we wanted to do. So it's cool, I just hope I will stay in the car for the championship!"
Leading the chase was Britain's Justin Wilson, who was again impressive on his second day in the car after signing for Newman/Haas/Lanigan.
His team-mate Graham Rahal - the youngest man to stand on a ChampCar podium - was quickest during the morning runs, but dropped back to third during the afternoon.
Franck Perera was fourth fastest for Conquest, despite having yet to sign a deal with the team, while PKV's Oriol Servia - who has signed for the season - and Alex Tagliani - who has not - followed him.
Pacific Coast's Alex Figge and Simon Pagenaud - who made it an all-French line-up at Conquest during the weekend, were seventh and eighth, with the latter finding more time during the day than any other driver.
"We are getting to where we need to be," said Pagenaud. "Sebring is not my favourite track -it doesn't really show what the car can do. However, I'm pretty happy to see how things were going today.
"We made a lot of improvement compared to yesterday when we were struggling a little. The car was doing what I wanted at the end of the day, it was just unfortunate that there was so much grip on the track, it was almost not drivable, but I'm happy and the team is doing a really good job."
2003 champion Paul Tracy and former series runner-up Bruno Junqueira were ninth and tenth quickest for Forsythe Pettit and Dale Coyne as one second covered the top ten drivers.
Ernesto Viso - who is looking to replace Britain's Dan Clarke at Minardi USA - ex-Formula One racer Enrique Bernoldi, Mario Moraes and David Martinez were also in action.
The test continues for one more day.
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