The Forgotten Inline Engine: GM’s 4.2-liter Atlas I-6
The story of GM’s high-tech straight-six and why it diedby Mark McNabb, on
General Motors has a long history with 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 is impressive, ranging from the 60-degree V-6 that powered nearly every GM car from 1980 through 2010, up 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 inline family.
That Atlas family had three main members, 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.
The 4.2-liter called the GMT360 platform home. This included the Chevrolet Trailblazer, GMC Envoy, Buick Rainier, Oldsmobile Bravada, Isuzu Ascender, and Saab 9-7X. Each of these mid-sized SUVs shared the same 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.
The 4.2-liter Atlas LL8, otherwise called the Vortec 4200, 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.
This combination allowed for the production of 1.06 horsepower per cubic inch – a total of 270 horsepower at 6,000 rpm. Torque was rated at 275 pound-feet at 3,600 rpm, but 90 percent of peak torque was available between 1,600 and 5,600 rpm. These stats far exceeded every comparable V-6 on the market at the time, including GM’s own 4.3-liter Vortec V-6.
We decided to take a closer look at the Vortec 4200 and its forward-thinking design. 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. Sutter was able to give us a deeper insight into the Atlas program, so keep reading for more.
Continue reading for more information.
It all started back in 1995 when GM began researching and planning its next mid-size SUV. The Atlas program got its start around the same time, with GM sold on the benefits of an inline engine. Along with being naturally balanced, an inline engine only has one cylinder head, which lowers cost, and its design could be altered to include four- and five-cylinder variants.
During development, Tom Sutter reverted to his time developing the Oldsmobile Quad Four, GM’s first four-valve per cylinder 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 engine to breath deeper during high-throttle situations, resulting in higher power without the fuel consumption penalty being present throughout the rev range.
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. Fine-tuning polishes everything up to exact tolerances. Features like coolant passages, oil galleries, and oil drain-back passages were cast directly from the foam, eliminating the need to drill into a pre-formed cast block. The same is true for the cylinder head, which came straight from casting with its intake and exhaust ports already formed.
GM used a premium aluminum called A356 as the primary material for casting. The aluminum is very similar to the aluminum used in wheels, but is heat-treated to increase strength. The same aluminum and heat-treating were also used for the cylinder head.
More Technical Details
The Vortec 4200 featured a deep-skirted block that added strength to the crank area. A bearing beam, or “ladder,” connected the seven main bearing caps longitudinally, further stiffening the block. The bearing beam sat sandwiched between the main caps and main cap bolts. The beam also helped reduce vibration and noise, while reducing the chances for an oil leak.
The long crankshaft presented its own set of inherent challenges. Because of its length, torsional vibration from end to end had to be mitigated, otherwise, the crankshaft would be susceptible to twisting. Sutter and his team devised a dual-frequency harmonic damper that attached directly to the front of the crank to counteract the effect. Large, 70 mm-diameter main bearings were also used to minimize crankshaft movements.
GM also broke ground in the SUV segment by using dual overhead cams 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. Both cams were operated via a timing chain with a hydraulically operated tensioner. When running, oil pressure is sent to the tensioner, which keeps the chain tight throughout its lifetime.
And 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 Vortec 4200 was able to run its high compression ratio without the need for premium fuel. Regular grade gasoline was recommended.
As mentioned before, the Vortec 4200 was the first GM truck engine to feature variable valve timing. This early adopter only had VVT 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 cam phaser mounted at the front of the exhaust camshaft controlled the system. Engine oil flowed into the phaser and operated a helical spline and piston. The cam would then rotate relative to the cam sprocket, creating the different camshaft profile, which changed the timing of the exhaust valves opening.
GM says the result was a smooth idle, a 25-percent reduction in hydrocarbon, and a 40-percent reduction in NOx gasses. This alleviated the need for a separate Exhaust Gas Recirculation system, or EGR.
Keeping things quiet was GM’s innovate valve cover. Constructed from a plastic-like composite material, the valve cover was held in place with 21 grommets and fasteners that work with the main silicone seal to keep the cover “floating” atop the head. This was designed to quell vibrations to the components mounted above, including the individual ignition coils.
The Vortec 4200 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. The water pump and oil pump were integrated directly into the front of the block. GM says this helped eliminate play in the belt-drive system since each component was perfectly positioned in relation to the block. Even the starter motor had its own mounting spot within the bell housing flange at the rear of the engine. The steel water pump pulley was used to power the clutched cooling fan to pull cool air through the radiator.
And speaking of those individual ignition coils, the Vortec 4200 came with six of them (Obviously). This coil-on-plug design helped eliminate wires while providing 25 millijoules to each plug for a hot, fast burn. The factory plugs were rated to last 100,000 miles before replacement. What’s more, the individual coils could better control spark timing. Camshaft and crankshaft position sensors ensured the Powertrain Control Module could keep track of precise timing information.
A single stainless steel fuel rail delivered fuel to the intake port-mounted injectors. The system operated at 350 kPa and used Multec II injectors that were more precise than past injectors. They also had a reduced risk of fuel leakage, which would lead to the cylinders getting soaked with fuel when not running. Pooling fuel will seep past piston rings and contaminate the engine oil, so this was a huge advantage.
The Vortec 4200 was 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. The throttle was controlled through the PCM and was opened via an electric motor mounted inside the 77-mm diameter throttle body. The innovative intake manifold featured an 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.
Trailblazers, Envoys, and other GMT360 vehicles also necessitated some innovative engineering to solve the issue of where to place the 4WD front differential and axles. Mounting the components under the engine like previous SUVs would push the engine too high. This would mean the entire front clip would need to be redesigned. However, Sutter’s team devised an ingenious method of mounting the front differential directly to the driver side of the engine’s oil pan and running the passenger side axle shaft directly through the sump. This necessitated a highly strengthened oil pan and a cylindrical pass-through for the axle shaft. Despite this, the oil pan still held seven quarts. “It was a packaging thing,” said Sutter. “We had to find a way to run the 4WD system, yet still meet the engine’s oiling needs.” Impressively, the oil pump moved 11 gallons a minute at 2,500 rpm while maintaining a maximum pressure of 65 psi.
The Atlas’ Smaller Variants
Introduced two years after the Vortec 4200, the Vortec 3500 used nearly all the same parts and construction practices as its six-cylinder brother. Hinted by the name, the Vortec 3500 displaced 3.5 liters between five cylinders. 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 saw service in GM’s compact pickups, 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 at 5,600 rpm and 225 pound-feet of torque at only 2,800 rpm. That represents a mere 50-horsepower and 50 pound-foot of torque decrease over the I-6.
The 2007 model year saw significant changes to the Vortec five-cylinder. Its displacement jumped to 3.7 liters thanks to a larger 3.76-inch bore. The four-inch stroke remained untouched. The increase in displacement resulted in power increases as well, boosting horsepower to 242 at 5,600 rpm and torque to 242 pound-feet at 4,600 rpm. These matching horsepower and torque figures would mirror changes taking place with the Vortec 4200 for the 2004 model year.
Just as with the five-cylinder, the Vortec 2800 used many of the same parts and manufacturing procedures. The same stroke and bore were present, as well as the same rods, pistons, and valves. This commonality throughout the Atlas family was certainly a big attraction, both for ease of manufacturing and keeping costs in check.
GM’s 2007 refresh of the Atlas family brought changes to the Vortec 2800 as well. Its displacement increased to 2.9 liters, again thanks to very slight bore increase. Power levels were slightly boosted, giving the four-cylinder 10 more horsepower and five more pound-feet of torque. Despite the long list of similarities between the I-4, I-5, and I-6, the smaller engines were built at the Tonawanda Engine Plant in Tonawanda, New York. The I-6 was built at GM’s Flint Engine South plant in Flint, Michigan.
Part of GM’s sizable investment into the Atlas project afforded room for future advancements. This included the larger displacement version of the four- and five-cylinder engines. There were also plans to introduce variable valve timing on the intake side, making it GM’s first fully variable-timed engine. “We considered variable valve timing on the intake side, for sure,” Sutter said. “The entire engine line was definitely set up to add technology to better power and efficiency.” Though Sutter didn’t specify, a modern take on the engine could include direct fuel injection, cylinder deactivation, and even turbocharging.
GM even experimented with adding boost for a high-performance Chevy Trailblazer variant. Rather than the 6.0-liter small-block V-8 that would eventually power the Trailblazer SS, Chevy bolted on a pair of turbochargers to the 4.2-liter.
The skunkworks project kicked off shortly after the Trailblazer’s launch in 2002. Engineers added twin Aerodyne turbos tuned to generate 8.5 pounds of boost. An air-to-water intercooler kept temperatures down, and a slew of new internal parts kept the Vortec 4200 humming along. The compression ratio was dropped from 10.1:1 to a milder 8.5:1, oil jets were added to help cool and lubricate the undersides of the pistons, and 2.0-mm shorter connecting rods with thicker piston pins increase the engine’s durability. A high-flow oil pump, fuel injectors, and exhaust system rounded out the updates. Amazingly, the block, crank, head, head gasket, cams, and valve sizes all remained untouched.
The package generated an impressive 400 horsepower at 5,200 rpm and 400 pound-feet of torque at 2,500 rpm.
Controlling the power was GM’s Hydra-Matic 4L60-E four-speed automatic transmission with slight modifications to handle the extra power. A beefier transfer case borrowed from the Cadillac Escalade’s AWD system offered better on-road control than the Trailblazer’s stock two-speed transfer case.
Testing revealed the twin-turbo Trailblazer could hit 60 mph in 5.35 seconds before running the quarter-mile in 13.91 seconds at 97.4 mph. Larger brakes borrowed from the Trailblazer EXT and sticky BFGoodrich tires helps shave 12 feet of its 60-to-0 distance for a total of 120 feet. As Truck Trend pointed out in its 2002 review, these performance stats put the twin-turbo Trailblazer squarely against the then-current BMW X5.
GM canned the project, however, electing to use the 6.0-liter LS2 V-8 instead. The Trailblazer SS debuted for the 2006 model year and had 395 horsepower and 400 pound-feet of torque. It could be had in either RWD or AWD. The Trailblazer SS competed most closely with the Jeep Grand Cherokee SRT-8, which debuted for the 2007 model year.
The Vortec 4200’s Racing Career
The Vortec 4200 actually 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 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 and supplied the drivers. The first race truck was 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 Vortec 4200’s second foray into racing came at Pike’s Peak. Again built by Herzog Motorsports and driven by Ragland, this GMC Envoy race truck took home the overall win for the 2000 Pike’s Peak Hill Climb. It was built on a Trans Am chassis with a modified front subframe, this 4WD Envoy rode on cut-and-grooved tires powered by the same hand-built race version of the Vortec 4200.
Herzog Motorsports also ran another Vortec Trailblazer in the Best In The Desert Nevada 2000. Four drivers tag-teamed the race and included Brian Stewart, Mark Miller, Bekki Freeman and Ryan Arciero. The 5.0-liter inline-six pushed the race truck to win the Trick Truck class, beating the next race team by more than 30 minutes and coming in second in the overall completion.
The End of an Era
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 cancellation 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.”
Technically, the inline-six would fit in GM’s full-size trucks like the Chevrolet Silverado and GMC Sierra. However, it was logistics that squashed 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. The V-6 was essentially a V-8 with two cylinders removed, much like the relationship among the Atlas engine family. That meant it could use the same engine mounts and accessories as the V-8, saving parts costs and engineering expenses.
GM’s decision to replace the body-on-frame Trailblazer with the unibody-based and FWD Equinox and Traverse meant the Vortec 4200 was no longer needed. Both the Equinox and Traverse utilized transversely mounted engines, and as Sutter mentioned, the straight-six was simply too long for mounting sideways. The Vortec 3700 five-cylinder was passed up in favor of a trio of V-6 engines in the car-like crossovers.
Nevertheless, the Vortec 4200 and its Atlas family soldier on in millions of GM vehicles the world over. The Atlas’ high-tech features and innovative construction seem lost to most enthusiasts who aren’t normally interested in early- and mid-2000s GM SUVs and compact pickups. Still, that doesn’t negate the unique nature of GM’s forgotten inline engine family.
|Vortec 4200 I6 (LL8)|
|Application||standard on Trailblazer and Trailblazer EXT|
|Type||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|
|Valvetrain||dual overhead camshafts, variable cam phasing – exhaust cams|
|Ignition system||coil-on-plug, dual platinum electronics|
|Fuel delivery||sequential fuel injection, electronic throttle control|
|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)||6300|
|Estimated fuel economy|
|(mpg city / hwy / combined)||TrailBlazer 2WD: 16/22/19|
|TrailBlazer 4WD: 15/21/18|
|TrailBlazer (Vortec 4200 I6)|
|GVWR, standard (lbs./kg)||2WD: 5500/2495; 4WD: 5750/2838|
|Payload, base (lbs./kg)||2WD: 918/416; 4WD: 1489/676|
|Trailer towing maximum (lbs./kg)||2WD: 6300/2858; 4WD: 6200/2767|
|Maximum tongue weight (lbs./kg)||400/181 (without sway control)|
|Fuel tank (gallons/liters)||18.7/71.5|