Posted on 09/06/2018 1:25:26 PM PDT by Red Badger
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.
What Is a Straight Six?
“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 “V” was the arrangement that eventually superseded it, where two banks of cylinders shared space like interlocking fingers.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. 
Animation of an inline straight-six engine. Wikimedia CommonsMichael Frey
Yes it was....................
But yeah... a workhorse of a small truck engine that just couldn't keep up with modern emissions requirements.
Nobody informed these guys:
Wish I still had my straight 6 ‘51 Chevy.
I had two Plymouth 225 slant 6 engines...put around 250,000 miles on them and the engines still ran good. The rest of the car died. Transmissions were good too. 1965 and 1967 Valiant and Belvedere. Single barrel carburetors...good gas mileage. Super easy to work on. Frequently putting ball joints on the front suspensions for those cars.
I put a new long block in my 75 Chevy C-20 pick up
about 10 years ago and it’s still going strong.
292 Six inline.
“Powerful V8s took over the American auto industry from the 1950s to the 1970s as cars grew monumentally large and gas was cheap,....”
Here’s an example: Premium gas for my ‘69 GTO was $0.379 when it was new in 11/68. Now a gallon of premium is about $2.80. Adjusted for inflation, that 1968 gallon of premium gas cost $2.70 in today’s dollars (hard to believe, I know, but it is true: https://www.bls.gov/data/inflation_calculator.htm). Note, however, that my GTO never got over 15 mpg, whereas today’s rough equivalent - a Camaro with a “big” (by today’s standards) V-8 probably gets about 22.5 mpg. Thus, the price-adjusted cost of gas per mile is about 1/3 LESS right now than in the pre-OPEC “glory days” of the late ‘60s.
These are the good old days - especially with how performance cars accelerate and handle.
My dad bought a 1950 Pontiac Silver Streak Straight 8 new. He got an automatic because my mother assured him she would learn to drive an automatic (never happened). However, I learned to drive in it. It had decent power and was a tank. While driving down a narrow street, someone opened a door in my path. I didn’t realize I had knocked that door off until checking the rearview mirror a few moments later .
It lasted 14 years. By then it was only getting 8 mpg.
It's a 2015; just got it, to replace a 2005 that I put 480,000 miles on.
Maybe you should look at a better class of FWD cars ...
Well the inline six I drive everyday has somewhere between 600 and 650 hp. I don’t know how much torque. (Yeah it’s in the 03 Pete I drive. A Cat C15, model 6NZ. Probably the best truck engine Cat ever made. Most days it will average just over 7 mpg, which ain’t bad for a big truck)
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Never will I spend money on one of those blister bombs.
The Maz cost $1000 and had only 40,000 on the odometer.
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Smooth, torquey, reliable, long lasting, and plenty of power as long as the power to weight ratio was reasonable.
They sound pretty good too. The BMW Z4 has them (among many other models) and bone stock they sound mean. Makes it a low cost of ownership too. I had a straight 6 in a 1970 Camaro back in the day. Kept running and running and running.
I learned to drive in a Checker Marathon. Since then, I’ve been behind the wheel of ... smallest was a Prius (rental car) ... largest was a 25,000lb commercial truck (GMC chassis). Everything has its place. For snow, I’ll take the front drive.
everything has its pros and cons:
https://www.carthrottle.com/post/v6-vs-straight-six-the-pros-and-cons/
Had two Plymouths and a Dodge rot and fall apart around their 225 slant sixes. That torsion bar front suspension just wasn’t made for the pothole filled streets of Fun City in the 60s, 70s and 80s. The low distributor would sometimes fall afoul of the rinse in a car wash. (LOL)
A lot of cruising boats used them also. Not new, but as replacement engines. They could just hum along, day after day.
It is a straight six with 120° crank throws. The heads and valve train have evolved but BMW has been producing a straight six of the same basic design for automobiles continuously since 1933.
IMHO, the two best automotive engines available today are the Dodge Ram/Cummins Diesel I-6 and the Subaru Boxster H-4. I own one of each so I’m a little prejudiced... I do not have any experience with the Subaru Boxster H-6 but I suspect that is a good engine too.
I read an article in Popular Mechanics years ago that discussed engine designs and which were the smoothest. I-3, H-4, I-6, V-8, V-12, V-16, etc. were all inherently smooth. I-4 was awful and required a counterweight on the crank to prevent the engine from vibrating itself apart. Put two I-4’s side-by-side to make a V-8 and that balanced it out. V-6 is a poor design that requires counterweights as well. Two V-6’s make a V-12 which is balanced but would make a huge engine.
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The motor pictured is not an “L6.”
It is an “I6.”(overhead valves)
L6 is a flathead.
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