And it changed for the better, even though neither man won the inaugural NASCAR event at Montreal in August 2007.
"It was a fairy tale come true for me," said Carpentier, who finished second that day. "I wouldn't be here today if it wasn't for that race. I quit racing at one point and was working on my farm [in Canada]."
Both drivers believe they can contend for the victory in Sunday's Corona Mexico 200. And both proved their road-racing skills in the wild Montreal race.
Carpentier's second-place showing was a bit of a gift, thanks to a confrontation up front between Amrbose and Robby Gordon.
Gordon, angry that Ambrose had bumped him earlier, punted Ambrose on the overtime restart when Ambrose was on his way to a victory. Gordon was black-flagged and Kevin Harvick was declared the winner.
"I look back at it and realize it was a mess," Ambrose said. "But I think I handled it well and made the best of a bad situation. A negative became a real positive for me.
"That broke me out in the American racing scene. It was unusual, but you have to run with it when things like that happen. It showed I could get the job done."
Ambrose, a Tasmanian who came to the U.S. in 2006 to race in NASCAR, was the talk of NASCAR after the crazy finish in Montreal.
His conflict with Gordon was short-lived. Gordon offered Ambrose a ride in a Sprint Cup car the following week for the road race at Watkins Glen, but rain canceled qualifying and ended Ambrose's chances of making the field.
Carpentier, a former CART and IRL driver, got a ride in the Cup event at Watkins Glen, which eventually led to a full-time Cup ride with Gillett Evernham Motorsports. But it wasn't a consideration until Carpentier opened some eyes with his run in front of the home folks at Montreal.
"I met George Gillett five years earlier because he was friends with Bobby Rahal and came to a race," Carpentier said. "We talked for five minutes and that was it. But Monday after the Montreal race, Ray called."
Carpentier finished a respectable 22nd at the Glen and earned a test with the team at Kentucky Speedway.
"It went really well, but I didn't hear anything for a couple of months," Carpentier said. "I was about to sign a contract with Fitz Motorsports to run a full Nationwide season when George called and said, 'Don't sign it.' They sent me a full contract and I just signed it right then and there. That was it."
It wouldn't have happened without a major investment Carpentier made to race in his home event at Montreal.
"It took a year and a half to get the money to do Montreal," Carpentier said. "The whole thing was like $250,000. That was everything we had, but it paid off."
Carpentier now is paying his dues as a Cup rookie. His No. 10 Dodge ranks 41st in owner points, meaning he has to qualify on speed at each event.
"I don't like that too much," Carpentier said. "I don't get much sleep the night before qualifying. It's pretty stressful. It didn't show in the results because we got a penalty in the pits, but last Cup race at Phoenix is the first time I felt I could run on pace with the leaders. It's getting better."
Ambrose, 31, still is hoping to make the jump to Cup. But you have to grade his Nationwide performances on a curve. His No. 59 Ford with JTG Racing is a Nationwide-only team at a disadvantage by trying to compete with Cup teams in the series.
"The more you do this the more you realize it," Ambrose said. "We race against full-fledged Cup operations or at least teams affiliated with a Cup group. They get engineering resources and technology to tap into. We don't have that. But we still feel we can win a race before the year's out."
Ambrose, whom his fellow competitors fondly nicknamed Kangaroo Meat, isn't complaining. He's thrilled to be part of NASCAR.
"It's been a fantastic experience and I feel I've achieved a lot," he said. "But I still want to get a win and have a chance to move up. I hope I can hang around long enough for that to happen. I want to be a contender and I don't feel I'm quite there yet."
Terry Blount covers motorsports for ESPN.com. He can be reached at email@example.com.