The Forgotten Inline Engine: GM’s 4.2-liter Atlas I-6 - SUV VEHICLE

The Forgotten Inline Engine: GM’s 4.2-liter Atlas I-6


General Motors has a long history of making innovative strides in engine development. The Chevrolet small-block V-8, for example, began life in the 1950s and soon became the standard for high horsepower in a small package, a legacy that continues into today’s fifth-generation GM V-8s. Even GM’s lineup of V-6 engines made waves throughout the industry, ranging from the 60-degree V-6 that powered nearly every GM car from 1980 through 2010, to the twin-turbocharged V-6 powering the Cadillac ATS-V.

However, GM has a lesser-known engine family that deserves admiration for its outside-the-box thinking and outstanding technological advancements, the Atlas engine family. That Atlas family consisted of three main engines; the front-running 4.2-liter inline-six, the 3.5-liter five-cylinder, and the 2.8-liter four-cylinder. All three shared the same basic architecture and a wide range of parts, though it was the 4.2-liter that led the Atlas program. Here’s a closer look at the technical aspects and history of GM’s Atlas 4.2-liter inline six.

In order to give you the most up-to-date and accurate information possible, the data used to compile this article was sourced from various manufacturer websites and other authoritative sources, including Motor Trend.

Vortec 4200 Technical Specifications


Standard on Trailblazer and Trailblazer EXT


4.2-liter, DOHC inline six-cylinder w/cast aluminum block

Displacement (cu in/cc)

256 / 4195

Bore & stroke (in/mm)

3.66 x 4.01 / 93 x 102

Cylinder head material

cast aluminum


dual overhead camshafts, variable cam phasing – exhaust cams

Ignition system

coil-on-plug, dual platinum electronics

Fuel delivery

sequential fuel injection, electronic throttle control

Compression ratio


Horsepower (hp/kw @ rpm)

275 / 201 @ 6000

Torque (lb-ft/Nm @ rpm)

275 / 372 @ 3600

Horsepower (2004-2005)

275 @ 6000

Torque (2004-2005)

275 @ 3600

Horsepower (2006 and up)

291 @ 6000 rpm

Torque (2006 and up)

277 @ 4800 rpm

Engine weight (complete)

470 pounds

Recommended fuel

unleaded regular

Maximum engine speed (rpm)


Emissions controls


Estimated fuel economy (mpg city / hwy / combined)

TrailBlazer 2WD: 16/22/19

TrailBlazer 4WD: 15/21/18

TrailBlazer (Vortec 4200 I6)

The 4.2-Liter Atlas I-6’s History

To discover the unique details of the GM Atlas engine, otherwise called the Vortec 4200, we reached out to GM and found Tom Sutter, the Assistant Chief Engineer for the Atlas. Sutter has been involved with engine programs for the last 30 years, ranging from Oldsmobile’s Quad Four to Cadillac’s current V-Series mills.

Looking Towards Other GM Engines For Inspiration

During development, Tom Sutter reverted to his time developing the Oldsmobile Quad Four, GM’s first four-valve per cylinder engine, which was essentially a small-block Chevy inline-six engine. A similar design was sketched for the Atlas and eventually became an integral part of the project.

Twin overhead camshafts were used to control the exhaust and intake valves separately.

Furthermore, GM’s first stab at variable valve timing was put into place on the exhaust side. This allowed the GM inline-six engine to breathe deeper during high-throttle situations, resulting in higher power without the fuel consumption penalty being present throughout the rev range. GM’s new innovative valve cover kept things quiet under the hood, which was built using a plastic-like composite material.

GM even used its relatively new manufacturing method for the Atlas engine block and cylinder head. Called the Lost Foam Casting technique, it basically uses a pre-shaped block of Styrofoam buried in sand. Molten aluminum is then poured in, filling the voids left by the melting foam. The result is a super detailed engine block that requires very little rough machining.

Its Impact On Motorsport

The GM Atlas engine also had a short-lived racing career. General Motors contracted with Falconer, a specialty engine builder, to construct a 5.0-liter, high-performance version of the Atlas inline-six.

It produced roughly 600 horsepower at 7,500 rpm and 450 pound-feet of torque at 6,000 rpm, but shared almost no parts with the production engine.

Herzog Motorsports ran the trucks fitted with the GM Atlas inline-six, with the first race truck being the Vortec Trailblazer that ran in the Baja 500 and Baja 1000 off-road races in 2000. The one-off vehicle was built with a tube-frame chassis and covered with a carbon-fiber body. Driven by Larry Ragland, the 81 Vortec Trailblazer only competed in those two Baja races, before retiring to Ragland’s garage.

The GM Atlas Engine’s Diverse Potential

The 4.2-liter Atlas engine was a groundbreaking engine for GM. It featured an all-aluminum construction, dual overhead cams with variable valve timing on the exhaust side, four valves per cylinder, a coil-on-plug ignition system, a high compression ratio of 10:1, and its cylinder heads featured GM’s then-prevalent “Vortec” engineering designed to maximize airflow.

Each of these mid-sized SUVs shared the same cutting-edge architecture, including the industry’s first fully hydroformed frame in a mid-size SUV. Introduced for the 2002 model year, the GMT360 platform sold a couple million examples worldwide before ending production after 2009.

Pushing The Industry Forward

GM also broke ground in the SUV segment by using a dual overhead camshaft with roller-followers to control the four valves per cylinder. The design was chosen thanks to the large volume of air four valves can move, while the roller-followers provided a smoother operation with less friction than conventional lifters, adding to the engine’s efficiency.

Thanks to the number of valves, their sizes were kept small to help with cooling. Large coolant chambers were routed around the valves and the center-mounted spark plugs, further helping the engine run cooler. Because of the efficient cooling and the decreased risk of detonation, the Atlas inline-six was able to run its high compression ratio without the need for premium fuel.

The Further In-Depth Craftmanship For The Atlas Inline-Six

As well as using dual overhead camshaft, GM’s Atlas inline-six engine featured additional elements that ultimately led to its success. A new engine comes with new challenges, leading to GM introducing all-new components to extract from its large fleet of SUVs at the time.

As mentioned before, the Vortec 4200 was the first GM truck engine to feature variable valve timing. This early adopter only had variable valve timing on the exhaust side, however, but allowed for 25 degrees of exhaust cam phasing. This kept torque levels at near-peak levels throughout the rev range and allowed a higher maximum horsepower without killing fuel efficiency or hindering drivability at lower RPM levels.

A First-Of-Its-Kind drive-by-wire System

The Chevy Atlas engine was also controlled by a drive-by-wire system, another first in a GM truck application. The system used an electronic throttle with no mechanical linkage between the gas pedal and the throttle body, controlled through the PCM and was opened via an electric motor mounted inside the 77-mm diameter throttle body.

The innovative intake manifold featured a 180-degree bend, bringing air into the throttle body just above the engine and wrapping it around into the cross-flow cylinder head.

The intake manifold was kept short in order to accommodate easy access to the spark plugs, adding a much-welcomed touch of practicality.

Its Unique Mounting System

The GM Atlas inline-six also utilized an innovative mounting system for its accessories. Each component besides the power steering pump was mounted directly to the block. This includes the alternator, A/C compressor, and idler pulley. GM says this helped eliminate play in the belt-drive system since each component was perfectly positioned in relation to the block.

Key Takeaways:

  • The General Motors Atlas inline-six engine was first introduced in 2002, being used for vehicles such as the Chevrolet Trailblazer and GMC Envoy.
  • The Atlas engine was used in off-road race trucks, which competing in iconic races such as the Baja 1000.
  • Smaller versions of the Atlas engines were introduced after the I-6’s success, including the Vortec 3500 and the Vortec 2800.
  • Production of the GM Atlas engine ended in 2009, due to the size and costs of the engine not being a logical business decision moving forward.

The End of an Era

A Black 2003 Chevrolet Trailblazer
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The Vortec 4200 was a huge leap forward for GM powertrains. But despite this, the innovative inline-six didn’t enjoy a long life. It was the cancelation of the GMT360 platform in 2009 that spelled the end for the Vortec 4200.

The engine simply wouldn’t fit in other applications. “The length of the engine and its structure definitely played a part in that,” Sutter said. “The engine doesn’t work in a transverse application, so it was limited in the vehicle choices.”

The inline-six would theoretically have fit in GM’s full-size trucks like the Chevrolet Silverado and GMC Sierra. However, it was logistics that prevented that from happening. Those trucks used GM’s 4.3-liter V-6 as their base engine, which was based on the GM small-block V-8.

What Were The Other Atlas Engines?

Vortec 3500 Inline Five
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Two years after the Atlas inline-six, GM introduced a smaller Atlas engine, named the Vortec 3500. Instead of six cylinders, the Vortec 3500 reduced the size down to only five, with a displacement of 3.5 liters. It used the same 3.7-inch cylinder bore and 4.0-inch piston stroke as the Vortec 4200, along with the same rods, pistons, and valves.

The Vortec 3500 was used in GM’s compact pickups, these being the first-generation Chevrolet Colorado and GMC Canyon. Both trucks were introduced for the 2004 model year and used the five-cylinder as the extra-cost engine option over the Vortec 2800 four-cylinder. The straight-five produced 220 horsepower and 225 pound-feet of torque.

Chevy Colorado Inline-five
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Speaking of the Vortec 2800 engine, this four-cylinder Atlas engine also used many of the same parts, including the rods, pistons and valves. A big reason for this was to keep costs in check, but also helped attract customers after the success of the Atlas I-6 engine. Standard for the Chevrolet Colorado and the GMC Canyon, the Vortec 2800 would produce 175 horsepower and 185 pound-feet of torque.

Despite having a fairly short lifespan of only seven years, the GM Atlas engine, particularly the first inline-six iteration, set the stage for further innovations under the hood of countless cars and trucks. Despite not being as spoken about as engines such as Chevrolet’s big-block V-8s or the LS7 Crate engines, the Atlas’ contributions to the industry can’t be forgotten.


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