DIG, AWD, 6-Speed Highlight New 304-HP CTS

The engine represents the most powerful naturally aspirated V-6 the auto maker has ever brought to market.

James M. Amend, Senior Editor

August 30, 2007

5 Min Read
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SAN JOSE, CA – General Motors Corp.’s ’08 Cadillac CTS for the first time will feature an optional 3.6L V-6 with direct-injection gasoline (DIG) that boosts output 15% and squares the sports sedan’s performance with that of its German and Japanese rivals.

The mill owes its bump in performance – from 258 hp in the ’07 version, optimized by variable valve timing, to 304 hp in the new model – to DIG technology that squirts the air/fuel mixture straight into the combustion chamber instead of using more traditional portfuel injection (PFI).

Mated to a Hydra-Matic 6L50 6-speed gearbox, another first for the CTS, the optional V-6 lays out 273 lb.-ft. (370 Nm) of torque, an 8% improvement over the ’07 model.

Compression ratio rises to 11.3:1 from 10.2:1, but fuel economy essentially stays put at 17/26 mpg city/highway (13.8/9.1 L/100 km).

Although the ’08 CTS also makes available all-wheel drive for the first time, the system’s nominal weight gain does not adversely affect fuel efficiency.

Additionally, cold-start hydrocarbon output is reduced by 25% with the new DIG engine.

The higher horsepower numbers make the new 3.6L the most powerful naturally aspirated V-6 the auto maker has ever brought to market.

The CTS’ former top-of-the-range conventional PFI 3.6L engine, which provides 263 hp and 253 lb.-ft. (343 Nm) of torque, becomes the base mill in the ’08 model. It phases out the 210-hp V-6, although GM will make the engine available on rear-wheel-drive Cadillacs sold overseas.

’08 Cadillac CTS direction injection boosts horsepower without sacrificing efficiency.

Direct injection, a process utilized by most diesel engines, results in more noise than traditional gasoline mills, particularly at startup, which has kept GM from bringing it to market sooner.

Specifically, pulses from the injectors run at about 1,750 psi (121 bar), far above traditional injectors. To help quiet the harsh ticking, GM engineers modified the engine structure. For example, they isolated the fuel rail, which prevents the transmission of injector noise into the engine.

The patented approach ranks as an industry first. GM also has switched to a structural oil pan cast from aluminum, rather than one that is stamped.

“It bolts directly to the transmission, stiffens the powertrain itself and helps isolate sound,” John Zinser, vehicle line director, Cadillac CTS, says during a media backgrounder here.

GM added acoustic treatments atop the engine and installed a front dash panel made of laminated steel between the driver and the engine compartment.

Additional acoustics have been added to the wheel wells. A 360-degree seal mitigates noise at hood-level, while a belly pan muffles unwanted sound that normally would bounce off the pavement at the vehicle’s underside.

DIG technology has progressed in recent years to the point where it’s now viable for a production application, Zinser says, noting injector durability has witnessed particular gains.

“It has to be able to take the combustion loads because the injector injects right on top of the piston,” he tells Ward’s.

That setup also leads to a more efficient burn, which improves fuel economy and cuts cold-start hydrocarbons without the addition of costly aftertreatment technology.

“When you’re trying to bump the horsepower up and you don’t have a clean burn, you need aftertreatments to meet (U.S. Environmental Protection Agency) requirements,” Zinser says.

The new DIG V-6 weighs roughly the same as its predecessor, although items such as a high-pressure fuel pump and the more rigid fuel rail slightly increase the entire system’s heft.

The new mill also takes advantage of a newly available 6-speed transmission with driver shift control. The closer gear ratios improve fuel economy and provide for smoother acceleration. GM engineers also went for a clutch-to-clutch setup, which makes for a lighter, more compact transmission than the previous generation’s 1-way clutch.

“It’s a much more complicated algorithm, but it also gives you more capabilities to control it,” Zinser says.

Drivers especially will notice a steeper first gear, which improves acceleration at launch, he says.

GM opted against paddle shifters in favor of a more conventional shift gate for “Sport” mode and manual “tap-up, tap-down” operation, although the auto maker promises the popular wheel-mounted controls in the future.

“It’s coming,” says Rob Kotarak-vehicle development manager. “You’d like to drive it into the product out of the gate, but the CTS-V is coming down the line in ’09 and (its features) could bleed into other cars in the lineup over time.”

Kotarak declines additional details on the high-performance variant, which in its four model years of production borrowed a 6-speed manual and 6.0L V-8 combination from the Chevrolet Corvette.

But for the ’08 CTS, GM did improve its performance-algorithm liftfoot technology, which adjusts shifts according to driving styles while in sport mode.

“Now it blips the throttle and matches the revs to the gear, so you get better acceleration out of a corner, for instance,” Kotarak says. “It keeps you in that optimum range.”

The more aggressive setup times out after “a couple hundred feet” of normal driving, he says.

For drivers who wish to use the tap-up, tap-down feature, GM engineers shaved the shift gap from 700 milliseconds down to 400 milliseconds.

As for the Aisin AY6 6-speed manual transmission that carries over on base models, GM reduced shift effort and shortened shift throws. It also claims quieter operation.

Cadillac expects a take-rate on the top-line DIG engine of between 60% and 70%, although it will cost buyers an extra $1,000. On ’07 models built through January, the most powerful engine option accounted for 60.4% of all installations, according to Ward’s data.

Related document: % Engine Installations on U.S. Domestic Cars and Lt. Trucks, ’07 Model Year

The ’08 CTS arrives at Cadillac dealerships in October and uses regular unleaded gasoline. A version of its new DIG V-6 also will appear on the larger STS sedan this fall.

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