Once the Audi TT RS leaves production after this model year, just two cars will come equipped with five-cylinder engines, the Audi RS3 and (not-sold-in-America) RS Q3. Figure those two will be around for a few more years, and whenever they're replaced, the five-cylinder automotive engine will officially meet its end.
Since its introduction in 1974, the passenger-car five-cylinder has always been an odd duck. Mercedes was the first, creating a 3.0-liter five-cylinder diesel for its 300D sedan. In an SAE paper from 1975, Daimler engineers Kurt Obländer and Manfred Fortnagel outlined the company's reasoning behind a five-cylinder: "With time, the demands of modern traffic called for a continuous increase in performance of diesel engines," they wrote. "Until 1974, this demand was met by increasing average piston speeds and displacement of four-cylinder engines in connection with progress in diesel combustion. Considering wear, fuel consumption, noise and vibration problems, speed and cylinder displacement had reached an upper limit, so that a further sizable raise of performance of naturally aspirated engines was only possible by increasing the number of cylinders."
Mercedes already had a four-cylinder diesel, so why not just add one cylinder? That ensured that a number of major parts, like pistons, rods, and valves could be shared with the four-cylinder, dramatically saving costs. While Mercedes offered six-cylinder gas engines at the time, the company deemed a six-cylinder to be too costly and too heavy. (Mercedes did eventually introduce a straight-six diesel in the mid Eighties.) So it went with five, and created a distinctive engine whose longevity became legendary.
The OM 617 five-cylinder diesel had the same nearly square bore and stroke ratio as the 2.4-liter four-cylinder OM 616 unit, and weighed in at 515 pounds—68.3 heavier than the 'four. Power rose from 62 to 77 hp while torque rose from 101 to 116 lb-ft. These are not big numbers. But in R&T's January 1975 road test of the 300D, we wrote that it was "substantially faster, smoother, and quieter than previous [Mercedes] diesels." Still, it only managed to do the 0-60 mph run in 20.3 seconds, but the fuel economy figure of 24.5 mpg was a lot more impressive. That was a big number for a heavy, well-equipped luxury car of the day. Mercedes had a relatively large following of diesel customers in the U.S., and with the country still reeling from the fuel crisis, good economy with relatively good performance held a strong appeal.
That Mercedes five-cylinder has an idiosyncratic clattering idle, but the engine, as with all five-cylinders, is very smooth. When using the typical 1-2-4-5-3 firing order, primary forces—those created by pistons moving vertically—are perfectly balanced, as is the case with inline-four and -six cylinder engines. Secondary forces, created by pistons speeding up as they reach top dead center and by slowing down as they hit bottom dead center, are also perfectly balanced too, like an inline-six but not an inline-four. Unlike either inline-four or -six, the 'five does have rocking couples, which result in the engine oscillating back and forth along its length. This is why five-cylinders typically use a balance shaft. Engineers also strengthen transmissions and engine mounts to deal with these vibrations.
Power delivery is smooth, though, with a cylinder firing for every 144 degrees of crankshaft rotation, which in turn means that there's 36 degrees of overlap between each power stroke. In a four-cylinder, each power stroke occurs with 180 degrees of crankshaft rotation, which means there's no overlap, while in an inline-six, ignition occurs every 120 degrees, meaning there's 60 degrees of overlap. The characteristic and unusual warbling exhaust note of a five-cylinder is borne from these unevenly spaced exhaust pulses, which occur every 72 degrees.
While Henry Ford himself had experimented with gas-powered five-cylinder engines since the Thirties, and others tried too, it wasn't until the 1976 introduction of the second-generation Audi 100 (called the 5000 in the U.S.) that it became a reality. As a 2018 Hemmings article pointed out, carburetion issues were the biggest impediment to gas five-cylinder development. Unless someone were to invent a five-barrel carburetor—which could only be used with a five-cylinder engine—metering out fuel evenly between the cylinders was virtually impossible. Audi actually did produce a small number of carbureted five-cylinders, but the vast majority, including all sold in the U.S. were fuel injected.
The best engine for a car is not the best engine on a dyno bench. Packaging—where stuff goes—defines everything. For Mercedes, this wasn't a huge concern, as the W115 chassis was already sold with six-cylinder engines (oddly codenamed W114) and used a conventional rear-drive layout. Things were different at Audi. It used (and uses) a unique longitudinal engine, front-wheel drive layout—a layout that dates to when DKW used compact inline-threes—where the engine naturally is ahead of the front axle line. Audi wanted to compete with Mercedes and BMW's inline-sixes, but making a longitudinal-engine, front-drive, inline-six car was a nonstarter: It would've been too long at the front and too nose heavy. So under the leadership of VW Group scion Ferdinand Piech, always a man who loved an engineering challenge, it got to work on a five-cylinder.
According to a 1978 SAE paper written by Audi engineers, a V-6 was considered, but deemed unfeasible as it wouldn't be as smooth as an inline-five and it couldn't share many components with VW/Audi's four-cylinders. (Audi did eventually go V-6 in the Nineties and has stuck with the engine layout since.) Audi's first five-cylinder was really just a longer version of its 1.6-liter four-cylinder, now displacing 2.1 liters. It also shared some components with the 2.0-liter four-cylinder used in the then-new Porsche 924. This engine helped put Audi on the map, and when turbocharged to hell and back in the company's Group B rally cars, it became an icon. A chirping, snorting, 500-hp icon. In the Quattro road car, the turbo-five helped establish Audi as a technological leader.
Honda introduced a five-cylinder in 1989 with the Acura Vigor, though only stuck with the engine for around a decade. Like Audi, Honda's 'five was mounted longitudinally, rather unusual for the brand, which almost otherwise exclusively used transverse engines.
Mercedes, being a maker of longitudinal-engine rear-drive cars, never saw the need for a gas-powered five-cylinder, as packing inline-sixes wasn't an issue. But others did. For transverse-engine front-wheel drive cars, the five-cylinder presented a neat solution. It's not that much longer than an inline-four cylinder, but crucially, it's much narrower than a V-6. Plenty of automakers built (and build) transverse V-6 cars, of course, but sometimes you just need a bit of extra room.
That was the case with Volvo, which brought out a transverse five-cylinder for the 850 in 1991. Volvo made five-cylinders a centerpiece of its lineup, offering them in many iterations and in basically everything they sold up until 2016, when it went to an all four-cylinder lineup (foreshadowing). Ford also notably borrowed the Volvo five-cylinder while it owned the Swedish brand for its second-generation Focus ST and RS, with its strongest showing in the RS500, where the 2.5-liter five-cylinder made 345 horsepower. Separately, Ford also made a five-cylinder diesel, which was used in the Transit van up until very recently.
In the Nineties, the five-cylinder grew. Fiat brought one out, while Mercedes still offered five-cylinder diesels, and Audi's five-cylinders found other homes throughout the VW Group. VW even made a VR-5 based on its narrow-angle VR-6, a vee-type engine with just one cylinder head. Some Volkswagen models like the Beetle, Golf, and Jetta got a transverse inline-five in 2005, too, though this engine is not remembered among the five-cylinder greats. GM even made a five-cylinder for the first-generation Colorado and Canyon pickups and the Hummer H3, though it was never replaced.
Perhaps the greatest modern five-cylinder is Audi's current unit. While other VW products continually used 'fives, the engine type was actually dropped from the Audi brand in 1997, but for the 2009 TT RS, the brand created a new, bespoke transverse five-cylinder. Turbocharged, it made 340 horsepower in that first TT RS, and now in today's TT RS and RS3, it generates a punchy 400 horsepower. It's smooth as anything too, while providing character you just don't get in a four-cylinder.
The thing is that the technology that helped birth the most special five-cylinders, turbocharging, also killed this engine type. Modern turbocharging technology means engineers can get big numbers and excellent fuel economy and emissions out of a blown inline-four. So why then go for the cost and complexity of a five-cylinder? Consider too that ever stricter safety standards make packaging more difficult, and so the inline-five's extra length became an issue. Audi's application of a five-cylinder is very niche, only offered in high-margin cars that can more easily offset the extra expense of a fifth piston. Audi even worked on a 400-hp version of VW's EA888 four-cylinder, so it's not beyond its capability to do with four what it does with five.
The original five-cylinders were intended to be something of a compromise, more powerful than a 'four, smaller and more economical than a 'six. Today, it's just a throwback. A purely irrational choice for its unique character in sound. There isn't any logical reason for its continued existence, which is why five-cylinders are dead in everything but these Audi RS models. The 'five separates them from the pack, tying them to Audi's history. Yet as Audi makes the transition to EVs, the five-cylinder is not long for this world.
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