Several Sprint Cup Series teams had their pre-season testing at Daytona International Speedway cut short on Friday due to a multi-car wreck during the afternoon session.

The accident occurred when Marcos Ambrose spun around in a pack of 18 cars after Dale Earnhardt Jr. was attempting to bump draft Ambrose on the backstretch at this 2.5-mile superspeedway. Twelve of the 18 cars riding in a pack were involved in the accident.

The other drivers in the incident included: Jeff Gordon, Kasey Kahne, Jamie McMurray, Joey Logano, Kyle Busch Greg Biffle, Carl Edwards, Regan Smith, Aric Almirola, who is Ambrose's teammate, and reigning Sprint Cup champion Brad Keselowski.

"I guess I just got caught on the edge of the bumper there, and with the shaped noses and the tires, just spun me out," Ambrose said. "It was hardly even a bump. It was just enough at the wrong angle, wrong time and just went for a spin. I feel bad for everybody because quite a few cars were torn up there."

Earnhardt Jr. did not suffer any significant damage to his No. 88 Hendrick Motorsports Chevrolet and will continue with his test session, which concludes on Saturday.

"We were just out there running around, and I felt like Marcos was backing up to me in (turns) one and two to get a run down the back," Earnhardt Jr. said. "I was just going to give him a push down the back straightaway and see if he could get the lead. I was trying to eventually get the lead myself. We got off the back straightaway and were just kind of pushing him along there and our cars sort of just didn't match up very well. I got him hooked into the fence."

Teams are at Daytona this week to familiarize themselves with NASCAR's new sixth generation or "Gen 6" race cars for the 2013 season and prepare for the Feb. 24 Daytona 500. Most of the teams involved in the wreck did not have backup cars since the redesigned vehicle is currently in short supply. Keselowski was one of those drivers who was forced to head home earlier than expected.

"I saw cars smoking and wrecking in front of me," Keselowski said. "I think I ran into the back of the 43 (Almirola) and someone ran into me. That is just the way this deal is. It is unfortunate but sometimes you have to wreck them to learn."

Each of the three auto manufacturers in NASCAR's top series -- Chevrolet, Ford and Toyota -- have designs on the new cars that are different from the most recent ones used in the sport from 2007-12. Drivers have noticed a significant difference in the designs while bump drafting.

"Definitely the drafting is not like it used to be," Earnhardt Jr. said. "You can't really tandem certain cars. Certain cars don't match up well. Our bumpers on the Chevy have a little bit of a point. It makes it a little bit of a challenge to get into guys and kind of help them. We definitely weren't doing that in the corner at all, because it was pretty hairy trying to do it on the straightaways."

During the first day of testing on Thursday, most drivers did single-car runs while some did up drafts of up to four cars. There were no incidents during the morning or afternoon sessions.