Straight From the Grave: An Iconic Engine Design Makes a Comeback [Inline 6!]

Once upon a time, the straight six was a dominant engine design. Jaguar put them in its best cars, Jeep built its late 20th century reputation upon them, and almost every humdrum family car or pickup truck in America had one as standard—then they nearly died out.

For years, V6 engines were killing off the straight six, also known as an I6 or inline six, and the entire design appeared destined for oblivion. But Mercedes-Benz has performed a resurrection. It brought back the straight six in the form of the M256, an entirely new engine design, to replace many of its V6s.

In the end, it was the engine’s low development costs—not the inline engine's inherent smoothness— that gave this old motor design a stay of execution. Already puttering around international markets since 2017, M256-powered Benzes arrive in the U.S. for the first time in the CLS450 sedan and AMG CLS53 grand tourer coupe later this year.

If Mercedes' newfound interest in straight sixes sticks, it could be the beginning of a comeback.

“Straight” refers to the arrangement of cylinders in the engine block, and “six” refers to the number of cylinders. “V” was the arrangement that eventually superseded it, where two banks of cylinders shared space like interlocking fingers. Mercedes built its lineup heavily on straight sixes from 1924 to 1943, until the business of making Nazi war wagons got in the way. And then from 1951 and all the way through 1998, Mercedes always had at least one straight six in production.

Powerful V8s took over the American auto industry from the 1950s to the 1970s as cars grew monumentally large and gas was cheap, but if you ordered a family car, a pickup truck, or a pony car in base trim, it probably came with an I6.

But cars started shrinking in the 1980s, and engine bays got smaller as crumple zones and underhood electronics competed with engines for space. V6s, because they stagger cylinders in two opposing banks, are shorter than I6s. Engines had also become more expensive to develop, and with a V8 in most companies' lineups it made more sense to share costs by chopping off two cylinders to create a V6, and the shorter V6 fit inside engine bays more easily. Making It Fit Mercedes-Benz Sechszylinder-Benzinmotor M256 Mercedes-Benz six-cylinder engine M256. Daimler AG

Short hoods are still an issue, but Mercedes has a few tricks to shorten the M256 enough to cram it inside today's snub-nosed cars.

On a conventional engine, engine power runs the hydraulic power steering, alternator, air pump, and air conditioning compressor, and it powers them through a system of rubber belts on pulleys that hang off the front of the engine. All of it takes up a lot of valuable underhood room between the engine and the front grille. The M256 does away with belts and pulleys, and instead runs accessories such as air conditioning and the alternator through a 48-volt electrical system called the Integrated Starter-Alternator (ISG). image The 5 millionth Jeep 4.0L straight six. Wikimedia CommonsCZmarlin

It made the engine shorter, so Mercedes had an easier time fitting the M256 into its cars. ISG also incorporates an electric auxiliary compressor alongside a conventional turbocharger on the CLS53. Depending on power demands, the compressor can either help spool the turbo or provide boost directly to the engine. The combo cuts out turbo lag, the delay you feel between pushing the gas pedal and receiving the boost of power.

Today’s six-cylinder engines are often derived from other engines because you can easily lob off or add cylinders to an existing design. So that beefy V8 can be trimmed down to a V6 or a lean I4 can add some muscle and become an I6. For example, General Motors Vortec V6 was just a small-block V8, lightly pruned. This approach avoids redundant design, and more importantly, added costs.

These more-compact V6 designs had been displacing I6s for 20 years. When Mercedes ended production of the W140 S-Class in 1998, the M104—the company’s last I6—died with it. By then most car makers had long-since replaced straight six gasoline motors in their light passenger vehicles.

Entering the 21st century, hardly anyone made inline sixes. Jeep killed off its AMC-derived 4.0L I6 after 2006 in favor of V6s. General Motors, unique for its time, created an I6 in 2002 as part of its new Atlas engine family, which lasted until 2012. Only BMW continued to build much of its lineup on the straight six. Smaller Engines, More Power image Mercedes-Benz 2019 CLS450 sedan Daimler AG

But the demand for bigger engines that consumed the 1980s and ‘90s have been tempered by engineers squeezing more power out of less cylinders. Because of growing climate change concerns, automakers are under pressure to design more fuel-efficient engines, and V8s tend to take in and burn more fuel than smaller-displacement four- and six-cylinder engines. It also helps that today's sixes can out-muscle a lot of V8s from just ten years ago. Smart Your Engine Infiniti QX50 How Infiniti's Variable-Compression Engine Works 4-cylinder engine for chevy silverado and gmc sierra Four-Cylinders Are Coming to Full-Size Trucks image How a Knocking Engine Also Mean Auto Innovation

So rather than design a clean-sheet V6 to replace its outgoing and dated V6s, Mercedes created a modular engine family based around half-liter cylinders, which could be built into 2.0L fours (M254) and 3.0L sixes (M256). The M256 makes 362 horsepower and 369 foot-pounds of torque in the CLS45, and 429 horsepower and 384 foot-pounds of torque in the CLS53—those are V8 numbers.

It’s this industry-wide engine downsizing that could be giving straight sixes new life. Jaguar and Land Rover, owned by Indian Tata Motors Limited since 2008, are co-developing a new I6 to replace their V6s for the same reasons as Mercedes.

Mercedes' return to its dormant, dusty roots as a straight-six car company is more than a tentative step. The automaker has invested a significant amount of money into an entirely new engine family to run the majority of its lineup for years to come. So for now, the straight six isn't going anywhere.

My son has a 2017 Chevrolet Colorado and I think it has a inline 6?

Straight 6 engines are also great workhorses. My wife had a 4.0 Liter Jeep Cherokee and I hauled a Chevy Blazer 400+ miles with it on a route that took it through some of the Tennessee mountains....highway mileage went from 22 unloaded, to 19 at double weight (Blazer actually weight more than the Jeep and tow dolly added a few hundred lbs.).

I had a V-6 3.7 Liter Jeep Liberty that boasted about 20 more HP than the straight 6...pulled a 1200 lb trailer over same rout and highway mileage went from 24 to 15...

82 posted on 09/07/2018 2:55:27 AM PDT by trebb (So many "experts" with so little experience in what they preach....even here...)

I had a ‘96 Cherokee Country with the 4.0 High Output I6. Couldn’t kill that motor if I wanted to. Smooth, great power curve, all the low-end torque I needed. Still regret selling it when it turned 15 and ran like a top!

83 posted on 09/07/2018 3:05:13 AM PDT by usconservative (When The Ballot Box No Longer Counts, The Ammunition Box Does. (What's In Your Ammo Box?))

I still have my 2003 GMC Envoy with that same 4200 I-6 under the hood. Just replaced the coil packs, spark plugs, fuel filter, transmission cooler lines and upgraded the air intake at 110,000 miles. Runs like a champ and will push you back in the seat accelerating hard off the line from all the low-end torque that motor has. Great engine and transmission on this thing. Can't kill it.

My oldest son drives it when he's home from college. Whines about the gas mileage all the time but still can't keep his foot off the gas pedal .....

84 posted on 09/07/2018 3:15:47 AM PDT by usconservative (When The Ballot Box No Longer Counts, The Ammunition Box Does. (What's In Your Ammo Box?))

Yeah, I’ve been looking around the internet since this thread and there are a lot of fans of that atlas/vortec 4200 straight six. 270-290 hp and tons of low rpm torque depending on the year. I have an envoy too, so far all I’ve had to do besides simple maintenance is replace the coil pack, knock on wood.

Freegards

Two V-6’s make a V-12 which is balanced but would make a huge engine.

Yes, it does................

86 posted on 09/07/2018 6:14:08 AM PDT by Red Badger (July 2018 - the month the world learns the TRUTH......Q Anon.......Timelines change. Aug 16)

I had a great BMW with a straight 6 when I was stationed in Germany. Bought it used for $3,000 in 1994, drove it 3 years all over Europe, and sold it for $2,500 in 1997 to another soldier when I PCS’ed. One of the best purchases I ever made. It was a sweet ride on the autobahn up to 130 mph before I chickened out!

There are some nice tweaks you can do to the computer like turn off throttle management for example that’ll really boost the acceleration. I added a cold air induction kit to mine which gave me 5-10 extra ponies and improved gas mileage too. There are some nice tuning tweaks out there and last I looked a few years ago a few places that will custom tune a chip for you that’ll boost mpg and performance.

89 posted on 09/07/2018 3:29:25 PM PDT by usconservative (When The Ballot Box No Longer Counts, The Ammunition Box Does. (What's In Your Ammo Box?))

It’s definitely something to think about. There’s a lot of stuff on the internet for this motor, a lot of fans.

Freegards

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