Hidden behind a modest exterior sits a powertrain packing a fierce punch. Some know these as "wolves in sheep's clothing". Here are 10 legendary sleepers from the past, in no particular order...

1. Mercedes-Benz 300SEL 6.3 (W109)

Rudolf Uhlenhaut (the man responsible for the 300SL Gullwing) was sitting at his desk, when suddenly the almighty growl of a V8 flying past his office prompted him to get up. Curious as to what it could be, he asked Mercedes test engineer Erich Waxenberger to pop by for a chat. It turned out that what Uhlenhaut heard was a secret, after-hours project Waxenberger was working on, without consent from his superiors. The project? A seemingly innocuous 300SEL (essentially an S-Class) with a 6,3-litre M100 V8 sitting under the bonnet. The donor? Merc's very own 600 limousine. Uhlenhaut was so impressed by the car he signed it off for production. Thus, the 300SEL 6.3 was born. Sharing its beautiful bodywork with lesser stablemates, the 300SEL is the original sleeper. A subtle "6.3" badge on the bootlid was the only clue as to how potent this Benz was. Unveiled in 1968 at the Geneva Motor Show, the V8-powered W109 was a breath of fresh air, injecting a much-needed element of performance into the brand's mainstream models. Peak outputs of 184 kW and 500 N.m of torque were enough to propel the luxury sedan to 100 km/h in just 6,6 seconds (quicker than an E-Type Jaguar) and on to a top speed of 220 km/h. Air suspension, ventilated disc brakes and many other luxuries were standard on the 6.3. New, it cost U$16 000 (a 911 would set you back a more reasonable U$7 200), meaning the 300SEL 6.3 maintained a low production figure of just 6 526 units.

2. BMW M5 (E28)

The year 1984 saw the beginning of a long and rich legacy. By fitting the 3,5-litre straight-six M88/3 engine from the M1 sportscar into a 5 Series, the first M5 was created. Along with the 210 kW straight-six, the handsome E28 received subtle exterior embellishments, with a set of 16-inch BBS Cross Spoke alloy wheels and a centrally mounted rear exhaust being the most notable changes. Look a bit closer and the lowered ride height and M5 badging gave passers-by slightly more insight into what this special BMW was all about. With more power than a Ferrari 328, the M5 made light work of handing out a hiding to almost every dedicated sportscar of the era, bar a number of supercars. Inside, it was business as usual. A driver-focused interior featured fuss-free, clear dials with a high-quality centre console angled towards the driver. Offered only as a five-speed manual, 6,1 seconds is all the M5 needed to reach 100 km/h, before hitting a top speed of 251 km/h. Interestingly, the E28 is one of the rarest M cars ever to be produced. With 2 241 examples produced over four years (including the 96 assembled in Rosslyn, South Africa) the original M5 remains a rare sight on the road today. Replaced by the equally lovely E34 M5, the E28 can be credited for starting the high-performance executive sedan trend.

3. Mercedes-Benz 500E/E500 (W124)

Seeing the success BMW was enjoying with the E34 M5, Mercedes-Benz decided it was time to make a fast sedan again. The last one, the 6,9-litre W116 S-Class, had been out of production for nearly a decade. Using the W124 platform, Mercedes planned to create an M5 rival, with its own twist. Due to Benz's own engineers being fully occupied with the development of the W140 S-Class, Mercedes contracted the work out to Porsche, which had to figure out a way to fit the 5,0-litre M119 V8 into the compact W124 shell. Due to production issues, Porsche would end up building the 500 for Mercedes-Benz, too. Thanks to the complicated production process, each 500E took around 18 days to build. The year 1991 saw the V8 W124 introduced to the motoring press, who dubbed it "the wolf in sheep's clothing" due to the remarkable performance and relatively subtle exterior. Aside from a "500 E" badge affixed to the rear and the flared wheelarches, the new performance Benz looked as dignified and reserved as its lesser stablemates. Courtesy of 240 kW and 480 N.m of torque, the 500E could sprint from a standstill to 100 km/h in just 5,9 seconds and on to a top speed of 260 km/h. A different breed from the M5, the 500 E-Class was offered as an automatic only, with luxury and comfort being as important as driving dynamics. Some 10 479 examples were produced, with a handful of customers handing their examples over to AMG, who bored the engine out to six litres, creating the very rare E60 AMG.

4. BMW 540i (E39)

Arguably one of the most elegantly styled BMW sedans, the E39 is widely acknowledged to be one of the most complete cars the Bavarian automaker has ever made. So impressive, that the straight-six 528i even made off with the 1997 South African Car of the Year award. As lovely as it was, the 142 kW 528i was adequate for cruising to the country club and back. For the executive on the move, there was the 540i. The wonderful 4,4-litre V8 produced 210 kW and 440 N.m when new, providing superb acceleration. In our May 1997 road test, CAR achieved a 0-100 km/h time of 7,14 seconds, with 80 to 100 km/h dispatched in a mere 1,96 seconds. When cruising, the almost-silent V8 manages to waft the 5 down the road swiftly but with grace, while the Steptronic five-speed gearbox slurs the changes smoothly. Prod the throttle and that all changes, the M62TUB44 firing the sports sedan down any straight that takes its driver's fancy. A superbly built and luxurious interior had all the typical BMW touches, such as the driver-facing dashboard and uncluttered dials. At R350 000, the E39 540i didn't come cheap, especially considering the 528i retailed for a more palatable R251 000. However, the best doesn't come cheap, and according to testers at the time, the 540i "really is the consummate luxury sports saloon".

5. Jaguar XJR

Making use of gorgeous design cues from the past, the X308 XJ was introduced in 1997. Compared with its imposing German rivals, the XJ sported lithe, elegant exterior styling that gave it a sportier image than the Teutonic limos of the time. To match this athletic persona, Jaguar fitted a 4,0-litre supercharged V8 under the bonnet of the big cat, creating the XJR. To give Jag's first V8 some extra oomph, the AJ26 powertrain was fitted with a supercharger. This resulted in 276 kW and 525 N.m of torque, sent to the rear wheels. Leaping to 100 km/h in just 5,3 seconds, the XJR was one of the fastest saloons in the world. Classy good looks, an opulent leather/burr walnut-trimmed interior and a wonderful V8 went a long way in making the XJR such a desirable sedan. 

6. Volvo V70 R

In a tradition started by the 850 T5.R, fast Volvo station wagons have gained a large following from those requiring a car as quick as it is practical. Throw in a touring car that raced in the mid-90s British Touring Car Championship and your fast station wagon now has racing pedigree. While the original, boxy V70 R is remembered fondly, it's the second-generation estate that makes for the ultimate Swedish sleeper. Equipped with a 2,5-litre turbocharged five-cylinder pushing out 221 kW, this subtle family shuttle could hit 100 km/h in 5,9 seconds. Courtesy of all-wheel drive, the V70 R was a great performer around bends too, with high-performance Brembo brakes proving brilliant stopping power. Although an automatic option was made available, it's the six-speed manual that is highly coveted now, its slick operation enhancing the driving experience further. Supportive bucket seats kept passengers firmly in place, with the harsh ride often sending a jolt or two into the cabin. The V70 R and its sedan sibling, the S60 R, were one of the first mainstream vehicles to be equipped with adaptive suspension. Three modes (comfort, sport or advanced) changed the handling characteristics of the car. Discontinued in 2007, the V70 R still remains an excellent performance family car.

7. Mercedes-Benz S600L (W220)

Available since 1998, the W220 received a welcome facelift in 2003. Not only were a number of quality issues addressed, but the big Benz received updated styling and a new engine line-up. The previous 270 kW 5,8-litre V12 made way for the sublime M275, a 5,5-litre twin-turbocharged V12. Power now sat at 368 kW, with 800 N.m of torque available from just 1 800 r/min. Despite weighing in at a hefty 2 060 kg, the 600 could lunge itself to 100 km/h in 4,8 seconds and on to its electronically limited 250 km/h top speed. Without a limiter, the big V12 will whisper its way to 300km/h; supercar territory. Despite the power on tap, the silent 600 never makes its hasty progress known to passengers, who sit in the absolute lap of luxury. Should you feel the need to hustle the S600L through a set of bends, the standard ABC air-suspension was designed to eradicate body lean, allowing the two-tonne saloon to corner flatly. If ever you roll up to a set of lights next to one, just remember: this heavy, automatic Mercedes will blitz the quarter mile in 12,5 seconds. You've been warned.

8. Mazda6 MPS

Tested in the March 2007 issue of CAR, the 191 kW performance sedan impressed the team with its numerous abilities. Priced at R319 900, the MPS undercut performance rivals from BMW and Alfa Romeo by a considerable sum. Sadly, this wasn't enough, and the Mazda sold in very small numbers. The 2,3-litre turbocharged four-pot was good for a 0-100 km/h sprint time of 6,39 seconds, making it faster than its European rivals, too. Still, much of the MPS appeal was in the subtlety of its appearance. Here was a handsome, if somewhat anonymous, family car that had the ability to outrun a Nissan 350Z, a far more expensive and focused sports car. Four-wheel drive made the Mazda great fun to drive in all weather conditions. As one tester remarked, "handling across the board is commendable, with excellent turn-in". If you found yourself getting a little carried away, standard stability control was there to get you out of sticky situations. Being a poor seller, the 6 MPS remains a rare sight on South African roads, which is a pity. 

9. Volvo S80 V8

As discreet as an English butler, the S80 V8 blends in with very subtle yet classy styling. A variety of engine options were made available, with the turbodiesel variants being the popular choice. For the first time ever, Volvo offered its executive S80 with a V8. Built by Yamaha, the 4,4-litre V8 produced 232 kW of power and 440 N.m of torque. Performance was strong, with the S80 V8 capable of reaching 100 km/h in 6,93 seconds, when tested by CAR in the March 2007 issue. The halo S80 sends its power to all four wheels via a six-speed automatic. Aside from the subtle "V8" badging on the grille and bootlid, the S80 appears to be just another Volvo. This, of course, is part of its sleeper charm. "The transverse-mounted V8 emitted a bark that more than once caused those nearby to do a double take, actually confirming that a classic V8 soundtrack was emanating from a Volvo". A textbook sleeper car, the S80 V8 manages to hide its bite under a very modest exterior.

10. Audi S6 V10

Not one to be left out, Audi joined in on the V10 revival of the mid 2000s with the third-generation S6. A 5,2-litre 10-cylinder managed to produce 320 kW and 510 N.m of torque. Despite tipping the scales at 1 910 kg, the S6 managed a 0-100 km/h run of just 5,1 seconds. A ZF-sourced six-speed automatic gearbox was the only available transmission choice, with a Tiptronic mode allowing for some control over the gears. While the more expensive RS6 variant was considerably faster and more dynamic, the widened wheel arches and body kit of the RS6 marked it out as a full-fat performance sedan (or wagon). The subtlety with which the S6 goes about its business is what marks it out as an excellent sleeper. From afar, the styling can be mistaken for that of an A6 with a few S line adornments. Today, the C6 S6 and RS6 are remembered for the unique V10, something no other midsized Audi has offered since.

Original article from Car