Mercedes-Benz SL 73 AMG: The Rare Sleeper That Shared a Massive V12 with the Pagani Zonda

Though it still helps develop the most powerful models in the Mercedes-Benz lineup as a subsidiary of the German manufacturer, AMG used to employ a slightly more outlandish formula back when it was an independent Mercedes-Benz tuner.

Among the company's favorite recipes was cooking up rabid Mercs powered by humongous, thoroughly beefed-up engines.

It all started in 1971 when their engineers took a damaged 300 SEL 6.3 and converted it into a race car. Equipped with a V8 that was enlarged from 6.2 to 6.8 liters, which enabled it to make nearly twice more power (422 hp, to be exact), the car nicknamed "Red Pig" crossed the finish line second at the 24 Hours of Spa endurance race, putting AMG on the map.

From then on, Mercedes-Benz enthusiasts traveled to Affalterbach to get their rides tuned. During the following decades, AMG became the world's most renowned Mercedes tuning house, enjoying a close relationship with the Stuttgart-based automotive giants.

Working their magic on the R129 SL-Class

At the Geneva Motor Show in March 1989, Mercedes-Benz unveiled the new generation of its famous SL model.

Designed by the legendary Bruno Sacco and based on a modified W124 chassis, it picked up where its R107 successor left off, enjoying strong sales worldwide from the get-go.

Initially, its most potent trim was the 500 SL, equipped with a 322-hp M119 DOHC V8. With potential buyers demanding more, the short-lived, 389-hp, V12-powered 600 SL (later rebranded SL 600) joined the range in 1992. In addition, those who wanted something more unique could get the 500 SL 6.0 from AMG, which offered 385 hp in a more bespoke package.

AMG continued offering a range of tuned SL models available through Mercedes-Benz dealerships, but none were particularly insane in terms of power.

That changed in 1995 and 1996 when the SL 70 and SL 72 AMG models hit the streets. Boasting enlarged, V12s capable of either 496 or 503 hp, these cars were sensibly more potent than the stock SL 600, which made 100 hp less.

But from 1997, there was an even more insane offering for the wealthiest of AMG customers that year: the mighty SL 73.

The largest engine ever fitted into a production Mercedes-Benz

For the SL 70, 72, and 73 models, AMG took the all-aluminum DOHC 6.0-liter M120 V12 of the SL600 that served as a base for all three cars and turned it into a thoroughbred performance engine codenamed M297.

In the SL 73 AMG, it was enlarged all the way to 7.3 liters, which enabled the naturally-aspirated unit to make 525 hp and 553 lb-ft (750 Nm) of torque.

To this day, the largest displacement engine ever fitted into a production Mercedes-Benz, the M297 was more powerful than the one used by the Lamborghini Diablo SV (510 hp) or the Formula 1-derived Tipo F130B (512 hp) at the heart of the Ferrari F50.

In 2002, this particular unit - albeit pushed to 547 hp - became the heart of the Pagani Zonda S 7.3. With constant improvements, it continued to power the Italian supercar until it was discontinued.

It's also interesting to note that a 6.9-liter version of the M297 was used for the ultra-rare Mercedes-Benz CLK GTR Strassenversion.

Supercar-worthy performance in a drop-top sleeper

With this insanely powerful V12 under the hood, the SL 73 could sprint from 0 to 100 kph (62 mph) in 4.5 seconds and achieve a top speed that of 200 mph (321 kph). These figures made it one of the fastest production cars around at the time of its release.

Like its AMG-badged, V12-powered siblings, the SL 73 started life as a stock SL 600 equipped with the AMG Styling Package that was shipped to Affalterbach for conversion. Its engine was removed, disassembled, tuned, then put back together by hand.

AMG engineers also tuned the suspension and braking systems, fitted a set of the company's famous alloy wheels, then finished the car with optional extras demanded by each buyer.

Remarkably, once the SL 73 was finished, it looked nearly identical to any other R129 SL equipped with the AMG Styling Package. Only the wheels and badges found on the trunk lid indicated that this was the meanest R129 around.

Rare and highly-collectible today

One of the last high-performance beasts conceived before AMG became a Mercedes-Benz subsidiary, the SL 73, was produced from 1997 to 2001.

Since the cars were essentially conversions, the company didn't keep an exact record of the units that left its headquarters, but multiple sources state that between 50 and 80 of them were sold.

Today, the SL 73 is highly sought-after by Mercedes-AMG fans who enjoy driving one of the coolest 1990s convertibles. Those who own one rarely part ways with their prized possession, so the chances of finding one at an auction are pretty slim. The last SL 73 AMG that went under the hammer at an RM' Sotheby's auction in 2019 was scheduled to sell for £240,000 (approximately $300.000), but the highest bid did not meet the seller's reserve.

You can learn more about this monster and see it in action in the YouTube video below by Retro Car Reviews.