An odd and controversial video has been cropping up in the online world of auto enthusiasts.

The five-minute spot, made by a company called powerTV, shows a group of car customizers rebuilding a Ford Mustang with a high-performance engine made by General Motors Corp. The video, posted to a host of auto fan sites by a blogger named PowerMelissa, has all the hallmarks of one made by die-hard auto enthusiasts.

But this particular production has riled scores of bloggers, who say powerTV is shamelessly using their Web sites as a form of free advertising for big-name clients like GM, something both the automaker and powerTV deny. At least a dozen auto fan sites have banned powerTV videos altogether. The situation exemplifies the type of messy debate that's playing out in cyberspace as companies big and small turn to increasingly unconventional forms of advertising in their quest to get a younger tech-savvy crowd to pay attention to their products.

"There's just no rule book for this like there is with other types of media," said James Lawrence, who founded powerTV in 2006.

PowerTV, a fledgling Murrieta, Calif. company with eight employees, runs an online video-sharing network for the automotive market and creates promotional videos for its clients. Lawrence said the company made the Mustang video purely to stir up debate and promote its own Web site. For Mustang fanatics, the idea of swapping a Corvette engine into the iconic pony car is nothing short of automotive sacrilege.

GM has employed powerTV to create productions, such as a recent employee training video, Lawrence said. And, naturally, he said, his company will do what it can to make GM happy. But he said the automaker doesn't pay powerTV to advertise on the Web.

"In our world, does the fact that GM is a client of ours affect what parts we'll put on our vehicles? Of course," said Lawrence, who added that his company now asks permission before posting such videos. "When you're dealing with the Internet, there's definitely a further blurring of lines."

GM spokesman Tom Henderson said the automaker's relationship with powerTV is entirely unrelated to the video in question. The automaker does, however, recognize the value in courting the crowd that cruises such fan sites.

"In the past year or two, we've really begun to focus a lot of what we do on engaging enthusiasts," Henderson said. "PowerTV helps us reach our consumers."

The new-fangled frontier of cyberspace is unquestionably playing a larger role in how old-school companies reach the coveted young and hip niche. The automakers' spending on Internet advertising skyrocketed by 67 percent during the first nine months of last year, according to the latest data available from Nielsen Monitor-Plus. The surge comes as auto companies pare back on more traditional forms of marketing, including print ads and TV spots.

The payoff can be huge for companies that manage to strike a chord with younger consumers.

Toyota Motor Corp.'s Scion brand, for example, was propelled to popularity by its nontraditional -- and oft-copied -- guerilla marketing campaigns that brought its cars and ads to art galleries, rock concerts and street corners.

"The lines are definitely getting blurred," said George Kang, a senior analyst for Edmunds.com, a consumer Web site. "The pure enthusiast says, 'I don't want to get covert information.' But another school of thought is that consumers are able to make the decision as to whether they're getting good information."

Not only are the rules still relatively undefined when it comes to online marketing, Kang said, but the effectiveness of the Internet, especially when it comes to so-called stealth marketing, is still unproven.

"The question is, if you are going to generate all this buzz, so what?" he said. "Does this help sales?"

It's a sensitive issue for the enthusiast sites, most of which specifically prohibit companies from posting advertising content without paying for the right to do so.

"It sounds trivial to some people, but to true enthusiasts, it's a slap in the face," said Tim Beggs, moderator of StagNet.com, a longtime Mustang fan site. The forum was among those to ban powerTV.

"You pour your heart into something and try to make a product nice and strong, and someone comes and litters on your front door. You do take exception to them."

PowerTV's Lawrence said his tiny company made some missteps, namely posting promotional material without permission, in trying to establish a name. Another one of powerTV's high-profile clients, parts supplier Tenneco Inc., asked Lawrence to stop posting videos because the company feared an image problem, he said.

Complications are bound to arise, Lawrence said, as the world of automotive media becomes more interactive and user-driven.

"We showed a little bit of disregard for those communities by not asking them if they could post our material. We definitely learned our lesson."