When Magnussen and Grosjean tried NASCAR

Neatly parked on the start/finish stripe of the Circuit of the Americas is the shape of a Mustang. Under the hood is a 5.8-litre V8: pure American muscle capable of producing 725 thunderous horsepower. When it fires up...

Mechanics from Stewart-Haas Racing, breath condensing in the morning chill, huddle around the steel-framed beast. It is unseasonably cold here in Texas, but finally the numb-fingered posse persuade the Ford to rumble into life.
 Hot, blue smoke billows out of the pipes positioned just ahead of the right-rear fender.

The guttural roar tempts both Romain Grosjean and Kevin Magnussen out of the relative warmth of the Haas F1 garage to inspect their sister team's racing machine. On hand to give them a guided tour is NASCAR team owner and three-time Cup series champion Tony Stewart, aka Smoke. Both F1 drivers are grinning from ear to ear at the prospect of wrestling the powerful but heavy Mustang around the fast sweeps of Austin's quick circuit, a day before practice begins for the United States Grand Prix.

Since the ambient temperature stubbornly refuses to budge from 6C, Grosjean is hidden under a hood and multiple layers. "I remember in 2012 it was freezing here," says the Frenchman. "I wanted to rent a Harley Davidson to come to the track as I thought that would be much cooler than a rental car. Then I saw the weather was lower than 10C and thought better of it..."

Beside him, perched on the pitwall, is the Cup car's steering wheel. It's lean but wide - and delightfully sparse, unlike the multiple dials and rotary switches on an F1 car's futuristic-looking equivalent. Grosjean is surprised at the simple thing he picks up, with just one button for the pit-to-car radio. "That's your scream-at-everyone button," laughs Stewart.

Since this is a Mustang specially modified to enable passenger rides, both F1 stars get a seat fitting alongside the driver. Grosjean climbs in through the window, Dukes of Hazzard style, and notices two grab-handles on either side of the seat. Romain takes hold of them and giddily says to the Stewart-Haas mechanic: "It's like Space Mountain! Whooo-hoooo!"

Magnussen is equally excited but more measured in his enthusiasm. "It's such an iconic-looking race car," he says. "I've been seeing them on TV my whole life and I've always wanted to have a go in one. It's amazing they haven't changed the basic concept of these cars - they know what works and they've stuck with it."

Since Stewart has never driven Austin before (Sonoma and Watkins Glen are currently the only road courses on the Cup schedule), he's keen on some reconnaissance laps in a course car. Magnussen joins him for the ride, as does F1 Racing.

"The last thing Romain asked me, traumatised me. He asked how to do a donut and I thought 'oh my God' - I didn't want F1 or the FIA getting mad at me. All I said was, 'first gear, wide open and crank it.'" Tony Stewart

"Wow, it's much tighter than I expected," says Stewart as we navigate the esses on the opening sector of the lap, which are based on Silverstone's flowing Maggotts-Becketts-Chapel complex. His only experience of the layout until this morning had been drawn from lapping it in the video game iRacing and watching YouTube clips. Downforce-endowed single-seaters revel in the fast direction changes but heavier, less aerodynamically complicated machinery needs to be wrestled. Later in the lap, as we tackle the twisty section at Turns 14 and 15, the NASCAR champ offers K-Mag some insight into what he might expect driving the Cup car.

"Because of the weight [1,500kg, which is twice as heavy as an F1 car] and lack of downforce you won't be able to arc wide as much into the tight corners," says Stewart. "It is a much straighter brake because the car acts more like a pendulum. When the weight goes one way, it tends to keep going."

Stewart slows as we pass some workers rolling green paint onto the asphalt on the outside of the corner. "Am I the only one who thinks you shouldn't be painting a race track the day before the event starts?" he says quizzically. "Were those guys booked yesterday, busy painting houses, so could only do today?"

As Stewart goes round for another lap, Magnussen points out the bumps that are soon to become such a talking point (the track will now be resurfaced before next year's grand prix), and they discuss the extent to which it's possible to ride the kerbs here. The inner radii of many of these are guarded by steeper orange-coloured 'sausages' and the NASCAR man is interested to know about the damage they can cause to the underfloor of an F1 car. As the two pros swap notes, you sense their anticipation building. They can't wait to attack this 3.425-mile lap at speed.

After the recce, it's time for Stewart to get behind the wheel of the real thing and he takes each Haas driver for a lap. On the door frame, stickers have been applied with the national flags of each driver and, simply, 'Smoke' - in honour of the champ. In the limited time allowed, there follows an interesting twist where Magnussen takes Grosjean for a spin - then they swap.

"The last thing Romain asked me, traumatised me," says Stewart. "He asked how to do a donut and I thought 'oh my God' - I didn't want F1 or the FIA getting mad at me. All I said was, 'first gear, wide open and crank it.'"

Sure enough, at the end of his run, Grosjean nails the throttle approaching the finish and spins the Mustang into a series of loops, with smoke billowing from the screaming Goodyears. Both drivers leap out with huge grins on their faces.

"That was so much fun," smiles Grosjean. "To have Tony Stewart drive you around and teach you about one of the best NASCAR cars out there was awesome. It was a great experience and very different in one way to F1 and similar in another. For example, turning into Turn 1 you get a bit of oversteer, then understeer. But the experience was brand new, that's what was so exciting, a little bit like a new girlfriend... maybe I shouldn't say that as I've been married for seven years..."

Grosjean explains that both he and Magnussen tackled the first sector of the lap as they would in an F1 car. But then they realised Stewart was piloting the Mustang differently.

"In the esses, Tony was preparing the car for the exits, while we both drove straight to the apex and saw whether we could make it turn. We both soon realised that wasn't efficient - we carried more speed in but it wasn't the fastest way."

Despite the Cup car's deliberate simplicity (limited electronics, basic tech) Grosjean loved the gearchange. "I thought it would be really tricky, but it was like a knife through butter."

Magnussen is equally effusive and relishes the chance to one day race a NASCAR Cup car - not just on the road courses, but on the perilous banked Superspeedways such as Daytona.

"It was such a cool experience to get in one of those iconic cars," he says. "I don't think it would be easy to jump in and be on it straight away, but I'd love the challenge. F1 is great. It's perfection, efficiency, high grip and big forces. But NASCAR is more... cool. Badass. Fun."