Before the iconic F1 supercar, McLaren built a turbocharged Ford Mustang

It’s arguably the rarest Ford Mustang of all and it paved the way for the birth of Ford’s in-house performance arm, Special Vehicle Operations, known the world over as SVO.

Meet the 1980 M81 McLaren Mustang, a crazy collaboration between McLaren, of Formula One and Can-Am racing fame, and Ford.

The story goes that when Ford wanted to create a lighter, more powerful and altogether sharper version of its four-cylinder Fox Body Mustang, it enlisted the help of McLaren Engines, the U.S. arm of the famed F1 constructor.

The third-gen Mustang was available from dealerships with a choice of a 2.3-litre inline four, a 2.8-litre V6 and a 4.9-litre V8. There was also the option of a turbocharged version of the 2.3-litre ‘Lima’ inline-four, that was almost, but not quite, as powerful as the 4.9L V8, 97kW against 103kW.

Ford needed to make the four-cylinder Mustang more desirable, and not just an economical version of its fabled muscle car. Enter McLaren which had, in the 1960s and ’70s, built a solid reputation in the US thanks to its all-conquering exploits in the wild Can-Am racing series.

Starting life as a regular Fox Body Mustang, Ford’s newly-minted SVO department added ludicrously wide IMSA-style wheel arches, a menacingly large bonnet bulge, period hot BBS alloys, larger brakes, adjustable Koni shock absorbers, and heavy-duty sway bars front and rear.

SVO stripped out the interior, replacing the standard fit bucket seats with racy Recaros, added Stewart-Warner instruments and gauges, and a bolt-in roll cage. Finished in a striking coat of Bittersweet Orange paint (a close match for McLaren’s famous Papaya Orange), the Mustang certainly looked the part.

Enter McLaren Engines, which took the factory-fitted 2.3-litre turbo and tore it apart. The team at McLaren then balanced and blueprinted the engine, ported and polished the head, and added a new variable turbocharger that could pump out from 5 to 11PSI, adjustable from the cabin.

McLaren’s engine workover saw power increase from 97kW to 130kW while torque, even with turbo boost dialled right down to 5PSI was rated at 210Nm.

The end result, the striking orange car was one helluva tough four-cylinder Mustang.

With the prototype finalised, Ford planned to build 250 cars of which 249 would go on sale to the public at the then astronomical $US25,000 ($AU37,200). That translates to around $US92,000 ($AU137,000) in today’s money.

And that was the McLaren Mustang’s downfall. With a list price almost five times more than the donor car (a new three-door Mustang sold for $US5616 in 1980), buyers weren’t exactly knocking down the doors of Ford dealerships.

Just 10 cars were produced, including the original prototype. It’s the car in the accompanying photos, and it’s now being offered for sale in the US.

A star attraction of Mecum Auction’s Indy 2023 sale, the 1980 Ford M81 McLaren Mustang Prototype is listed with an estimate of $US75,000-$100,000 ($AU111,000-$148,000).

The prototype was used as a promotional vehicle by Ford, touring dealerships throughout the US when it was released in 1980. With an indicated odometer reading of just 556 miles (894km), the McLaren Mustang is being sold with a number of extras including the original Firestone HPR radial tyres fitted in the factory when new. There’s also the original bill of sale, and a McLaren poster only available to dealers back in the day.

Featured on the cover of the December 1980 issue of Motor Trend magazine, a copy of which is included in the sale, the McLaren Mustang may at once be the rarest Mustang and cheapest McLaren money can buy.

This one-of-a-kind prototype goes under the hammer in May. You can check out he full auction listing here.