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2016 Cadillac CT6 Powertrain Detailed

Motor Trend logo Motor Trend 3/23/2015 Frank Markus
2016 Cadillac CT6 Powertrain Detailed

As part of a $12 billion investment that will bring eight new Cadillacs into five new segments by 2020, the company is working to broaden its powertrain portfolio, which we are assured will include V-8s for the foreseeable future. First up, and accounting for $540 million of that total, is a pair of brand new V-6 powertrains that will power the CT6. They share virtually nothing with predecessors of similar displacement, including cylinder bore spacing, which increases from 103 to 106 mm. Naturally, these new engines bristle with the very latest and greatest tricks for optimizing performance and efficiency, and the top-dog 3.0-liter twin-turbo is said to be the world’s only six-cylinder engine to combine turbocharging, cylinder deactivation, and auto stop-start. These engines are backed by a new eight-speed automatic (also designed in-house with a few new tricks), and a new ultra-compact all-wheel drive transfer case.

Related Story: 2016 Cadillac CT6 Chassis Uses Mix of Aluminum, Steel, Other Materials

Engines first

The base CT6 engine will be a new 3.6-liter naturally aspirated unit that will serve optional duty in the ATS and CTS. It marks the fourth generation of GM’s high-feature V-6 architecture, and its bore and stroke are both up slightly (1.0 and 0.2 mm respectively) from the outgoing 3.6, bringing displacement up 85cc to 3649. Compression is unchanged at 11.5:1, but the valves are 6 percent larger (39.4mm intake, 31.5mm exhaust) for 5 percent better breathing and 25 percent better in-cylinder mixing of the air and fuel thanks to increased tumble, all of which leads to improved flame-front propagation. Another key feature: the intake cams have a much wider “range of authority,” being able to vary the timing by as much as 70 degrees of crankshaft rotation (up from 50) for optimal efficiency. A novel parking mechanism allows the phasers to lock in an intermediate position that is ideal for cold-start emissions, which is a key enabler for this technology.

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GM also dreamed up a new actuator design for the Active Fuel Management (cylinder deactivation) system. Within the roller follower for each valve is a pin that connects the roller to the follower until oil pressure forces it out of the way, at which point the valve stays shut. The deactivated cylinders are the forward-most cylinder on the left bank, and the rear-most one on the right, for V-4 operation, which we’re assured is imperceptible. Other novel valvetrain features: cam sprockets that are not round and coated with rubber for noise control. The team chose a noisier but stronger roller-chain design, and to quiet it down the profile of the sprocket deviates from perfectly round just enough to reduce chain loads and prevent harmonic noises from arising. Then, on the cam sprocket that’s in tension (intake on the right, exhaust on the left bank), the area next to the sprocket teeth, where the chain links come to rest, is coated in a hard-compound rubber for noise isolation.

To optimize engine warm-up and reduce coolant flow demands (enabling a smaller pump that requires 50 percent less power to drive), coolant gets sent in parallel to the heads and block, instead of flowing from one to the other. Similarly, the oil pump has two output levels so as to meet the demands of high- and low-load operation with no wasted energy.

Add it all up and you get a 393-pound engine that produces an SAE-certified 335 hp and 284 lb-ft of torque (up from 321 and 275), with the AFM and auto stop-start helping deliver a claimed 9-percent boost in fuel economy. The new engine family will be manufactured in Romulus, Mich. GM also buried in their press materials the fact that a 2.0-liter turbocharged I-4 engine will be available on the CT6. However, no details on that engine have been revealed yet.

Related Link: Research 2015 Cadillac Models

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The 3.0-liter twin-turbo engine shares almost every feature mentioned above, but starts with an almost square 86 x 85.8mm bore x stroke. Its turbos are mounted outboard, directly to the cylinder heads, which incorporate the exhaust manifolds. A hot-vee arrangement was not explored, largely due to packaging constraints of a 60-degree engine. These turbos develop as much as 18 psi of boost, with vacuum-actuated blow-off valves regulating boost to limit torque. They also feature lightweight titanium-aluminide turbines, which reduce the rotating inertia by 51 percent, relative to the more typical Inconel turbines (the LF4 twin-turbo 3.6 introduced this technology at GM). Air exiting these turbos makes the super-brief trip up and into an air-to-water charge cooler capable of eliminating 30 kW of heat (bringing 284-degree air down to 104 degrees F) with only 1 psi of pressure drop. This setup results in about 60 percent less charge volume than systems that run the air through intercoolers up in the front fascia, which greatly reduces perceived turbo lag.

This engine has been designed from the outset to be turbocharged, and no naturally aspirated variant has been announced. It will power top-line all-wheel-drive CT6 models only, and output is currently estimated at 400 hp and 400 lb-ft of torque, shaming the primary German competition by at least 20 percent. Fuel economy is also expected to raise eyebrows, with the combination of cylinder deactivation and auto stop-start contributing a 6-percent boost in fuel economy. Total dressed weight is 443 pounds.

Read More: Most Improved Cars of the Last 10 Years

Transmissions

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The 8L90 eight-speed introduced in top CTS models will serve the twin-turbo, but its new baby brother 8L45 is joining the lineup this year in entry CT6, ATS, and CTS models. The team has earned two dozen patents on these transmissions, which package in the same envelop as the six-speeds they’ll replace. The ratio spread (first gear ratio divided by the eighth gear ratio) is a healthy 7.0. It features a new “off-axis balanced-vane oil pump,” which means that instead of being positioned behind the torque converter, the pump is now integrated into the valve body in the pan. This makes it smaller and more efficient, and it features two pressure levels to reduce losses during low-output operation. Also improving efficiency is a design that always keeps three of the five clutches engaged (open clutches introduce fluid-flow losses). A new synthetic transmission fluid further improves efficiency, for a total expected fuel-economy improvement of up to 5 percent relative to its six-speed predecessor. The new second-generation transmission controller features seven variable-force solenoids, and three Hall-Effect sensors which team up to help better monitor shifts for improved speed and smoothness.

Transfer Case

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Finally, thanks to the slightly skinnier transmission, the CT6’s AWD transfer case features a two-gear power takeoff, instead of the three-gear setup that used to be used to get the front prop-shaft out far enough to clear the trans. This design pulls some 5.5 inches out of the transfer-case width, all of which used to intrude on the front passenger’s leg environment. The system uses a wet multi-plate clutch to route torque forward, and the torque split is variable depending on the drive mode. Sport mode gets a 20/80 front/rear split, Tour gets 40/60, and Snow mode splits it right down the middle.

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