Steve McQueen's final custom car up for auction

However, when that car once belonged to none other than Steve McQueen, those excitement levels have been known to go off the charts.

And that's exactly what auctioneers Mecum are anticipating when McQueen's 1976 Porsche 911 (930) Turbo Carrera goes under the hammer on August 13-15 as part of the Mecum Daytime Auction in Monterey, California.

The McQueen factor, as it's known in auction house circles, is something unquantifiable. If the iconic Hollywood star owned it, came into contact with it or endorsed it, its value can rocket. Such is the aura that surrounds the man and his mythology. For example, TAG Heuer reissued the ‘Monaco' chronograph because McQueen sports one in the publicity poster for his 1971 passion project film, "Le Mans."

But McQueen was as consummate a racing driver and mechanic as he was a leading man, and what makes this particular piece of memorabilia so important is that it was the final car he ordered, specified and customized before his untimely death, aged 50 due to complications from mesothelioma.

"This car is rich in history," said McQueen's son Chad. "This is the last of the McQueen cars, really. That's known. That's real."

The car was specified with dual side mirrors -- believe it or not, a very unusual request in the 1970s -- a sunroof and a limited-slip differential for better cornering grip and control at the limits. The star understood what happens when a rear-engined car is fitted with a turbocharger. However, he also had a couple of other special features added.

"My dad had little things done to it," Chad says. "Like on the dashboard, he had a switch put on so it would kill the rear lights in case he was being chased on Mulholland Drive."

This fear of being caught by the cops while on a nighttime cruise also goes to explain why he specified overtaking mirrors for both sides of the car.

McQueen was the quintessential bad boy who turned his life around -- spectacularly -- so it's also fitting that the institution he credited with giving him a firm hand when he needed it most, the Boys Republic reform school, will be receiving a portion of the sale price, when the car goes under the hammer. While there is no official estimate given for this car, it is being speculated that it could fetch about $1.5 million.