ST. PETERSBURG, Fla. -- Graham Rahal has been touted as the next big thing in American open-wheel racing since he was 16.

Last year, at 18, the son of longtime racing star Bobby Rahal, came oh 'so' close to winning several races in the Champ Car World Series, but was denied each time.

Sunday, at the still-tender age of 19, Rahal broke through, winning the Honda Grand Prix of St. Petersburg. It was his first IRL IndyCar Series race, and he won by outracing Helio Castroneves, a two-time Indianapolis 500 winner and winner of the past two races in the streets of St. Pete.

``At the end of the race, with Helio behind me, I knew he has won a lot of races and has a lot of experience, but I knew we had the pace to beat him,'' Rahal said, grinning happily after enjoying a victory celebration that, of course, included no champagne for the underaged driver. ``I kept telling myself that.''

Rahal came back from a spinout early in the race to become the youngest winner in major open-wheel history. At 19 years, 93 days, Rahal broke the age record set two years ago in Sonoma, Calif., by another driver from a racing family, Marco Andretti, who was 19 years, 167 days old.

The win was also a crowning moment for the former Champ Car teams that only last month became part of a unified American open-wheel series under the IRL banner.

His father, co-owner of the rival Rahal Letterman Racing team, watched from the top of his team's pit box. The younger Rahal, the top rookie in Champ Car in 2007, took the lead by passing Ryan Hunter-Reay, his father's driver, on a restart on the 65th of 83 laps.

``It was tough after getting hit by Will (Power) and with the rain,'' the winner said. ``But we were pulling away from Helio while I was saving fuel. It's not like we just lucked into one.''

The race was slowed by periods of rain and cut short of its scheduled 100 laps by a 2-hour time limit.

``He drove beautifully and, when he had to go fast, he did,'' the elder Rahal said. ``I'm so proud of him. To come back and not get depressed after he got turned around by Will (Power), that was a great job.

``I don't know if I expected him to win this year at all,'' the 1986 Indy 500 winner added. ``I hoped he would, but this is a tough crowd with guys like Castroneves, (Tony) Kanaan, (Scott) Dixon and (Dan) Wheldon out there. And now you multiply that with the guys from Champ Car.''

The younger Rahal, who missed the season opener on the oval at Homestead after crashing in testing earlier that week, appeared headed for an easy victory as he built a lead of more than 4 seconds in his Newman/Haas/Lanigan Racing Dallara. But the last of six full-course caution flags came out for a three-car incident on lap 76, giving Penske Racing's Castroneves another shot at the leader.

``It was a great day for motor racing,'' said Castroneves, who also raced against the elder Rahal. ``I was going for it. We tried everything and we couldn't catch him. But, you know what, second is good enough.''

The nine former Champ Car teams in Sunday's 25-car field acquitted themselves well on the more familiar territory of a street circuit after being overmatched on the Homestead oval, a type of track where most of them have little or no experience.

This was a particularly satisfying weekend for most of them, considering they have had their new Dallaras for less than a month and have had almost no testing and few spare parts.

``We didn't have a backup car and we didn't have any spare parts to put it back together,'' Rahal said. ``If I had hit the wall again, I would have missed this race, too. But the team just did a great job preparing both of our cars and Justin (Wilson, who led laps and finished ninth) had a great weekend, too.''