Caddy CT6's lightweight body = big car, small engine

The Cadillac CT6 at the Shanghai auto show in April 2015. Photo credit: BLOOMBERG

RAMONA, Calif. -- Of the three engine choices available on the Cadillac CT6 sedan, it was the smallest that seemed to generate the most buzz during a media drive here last week.

The 2.0-liter, four-cylinder turbo kicks out 265 hp, shy of the 272 hp that the same displacement engine produces in the compact Cadillac ATS. But the relative featherweight construction of the CT6 left it with plenty of pull for passing trucks on steep uphills and on undulating mountain roads northeast of San Diego.

The big, aluminum-intensive sedan is also propelled by a 3.6-liter V-6 and the range topper, a 3.0-liter, twin-turbo V-6 that makes 404 hp and is exclusive to Cadillac (read not shared with Corvettes and Camaros).

The four-banger was the favorite of the bunch for AutoPacific Inc. product analyst Dave Sullivan. He called it “dialed in.”

It’s the one that even Cadillac President Johan de Nysschen is most enamored with -- “it’s the powertrain that will surprise people,” he told me.

The Cadillac CT6 frame and body mix up aluminum and steel.

Four-cylinder engines don’t get talked about much in large luxury sedans because, well, there really aren’t any. Mercedes and Lexus don’t even bother with anything smaller than a V-8 under the hoods of their big sedans (Mercedes throws a 12-banger in for good measure on the S class).

That the CT6 offers a credible four-cylinder underscores just how different General Motors’ approach was to the car’s development.

Engineers melded aluminums and high-strength steels to produce a large sedan that weighs less than most luxury midsize cars (3,657 pounds vs. 3,814 for the smaller BMW 528i). The materials were mixed and matched in an effort to maintain the stiffness needed to keep it unusually quiet and planted for a big sedan.

Cadillac engineers say it would have been a lot easier to use aluminum for the whole car. But that wouldn’t have allowed for the vaultlike interior that drivers of big luxury cars demand. So it’s 62 percent aluminum -- mostly the front and rear body structures -- while the passenger compartment is mostly high-strength steel.

A modular design also saved mass, helping pave the way for the big-car/small-engine setup. An example: The front body hinge pillar -- think the section that runs from the A-pillar to the front wheel well -- is one large aluminum casting, consolidated from what would be about 35 parts on a typical car.

I don’t think of the CT6 as a game changer. But I think this mixed-materials approach marks a milepost on the path to building a smarter big sedan, one that carves out the spaciousness luxury owners expect while keeping it from getting too girthy to be a fun ride.

Expect to see the same approach on a next-generation Silverado pickup at your nearest Chevrolet dealership sometime in 2019.

You can reach Mike Colias at autonews@crain.com

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