Monday, October 12, 2015

The Knowledge: 12 Tech Tidbits on the First-Ever Turbocharged Porsche 911 Carreras

-The Porsche 911 Carrera and Carrera S are adopting turbocharging—previously the province of the eponymous Turbo model—for the first time for the 2017 model year. We’ve already covered these so-called 991.2 cars when they debuted, and we also went really deep into the tech-heavy new engine, a 3.0-liter flat-six strapped with two turbochargers and good for 370 horsepower in the Carrera and 420 horsepower in the Carrera S. Here we present some additional factoids we’ve learned since the car’s introduction, straight from the engineers to your eyeballs.-Active cooling flaps are hardly uncommon these days. They cut the airflow to the radiator to reduce aerodynamic drag when the engine doesn’t need much cooling. However, these shutterlike devices are usually fitted between a car’s grille and its radiator. On the new 911, the shutters are in full view because they essentially are the grilles on the two radiators at either corner of the front fascia.-Most cooling fans are designed to draw air through a radiator when a car is standing still or driving slowly. They usually shut off at higher speeds, when natural airflow takes over. But most cars have much larger openings than are possible on a 911, with its low-slung front end. As a result, when you’re hard on the accelerator, the Carrera’s radiators are starved for air, even at high speeds. The solution is to turn on some fans—powerful fans. On the new Carrera, each fan uses 400 watts. The S, with its greater cooling needs, uses 600-watt fans. The naturally aspirated outgoing models used 300-watt fans on both versions. To keep with these, and other, increased electrical loads, the new Carrera S gets a 2450-watt alternator, while the S has a 2940-watt unit—both up from 2100 watts on the previous versions.-The new Carrera’s air-to-air intercoolers are fitted behind the tires in each rear fender. Air to cool the intercoolers enters from the rear edge of the grille on top of the engine cover. It’s ducted through the intercoolers and exhausts via vents at the lower back of each fender. Although the inlet is not in a very high-pressure area, the exhausts are in a low-pressure zone, ensuring steady airflow through the system. However, if the intake air sensors detect too much temperature, the rear spoiler will deploy to create an air dam just behind the intercooler air intake to force more air through the system. This can happen even at low speed, so if you see a 911 with the spoiler deployed in town, there’s a good chance that the driver has been caning it.-Even with the most sophisticated turbocharger installations, turbo lag still exists at low and moderate rpm. But when you select the new Carrera’s Sport mode, among the changes are different engine parameters designed to maximize exhaust flow through the turbochargers. This means retarding the ignition timing and altering the camshaft phasing, which operates the engine in a less efficient mode. But less efficiency means more air and fuel being used to generate the same amount of power, and that means higher exhaust flow to spool those turbos more energetically.-The PDK automatic transmission has seven gears, but there are times—for example, when crawling through a city at moderate speed—that none of those gears are just right for best efficiency. This is particularly true in the lower three gears, which are spread further apart. Porsche’s solution is radical: engage two gears at once, partially. Each of the output shafts are coupled to the engine via their own multiplate wet clutch. By engaging both clutches very lightly to allow plenty of slippage, the transmission operates at a ratio that essentially splits the difference between the two gears engaged. Of course, as soon as you step into it, the transmission picks one gear or the other to avoid frying the slipping clutches with lots of power. The Turbo’s PDK already had this function, but it’s a first for the 911 Carrera.-Although the Carrera’s manual transmission and PDK automatic share many common elements, including having seven forward speeds, the ratios were different in the 991.1. First and second were the same, third was almost the same, fourth through sixth were the same, but seventh was shorter on the manual. Porsche felt that with kickdowns available, the automatic could employ taller ratios without compromising responsiveness. However, with the 991.2 and its considerably greater low-rpm torque, Porsche concluded that even the manual could tolerate the taller gears. The ratios now are identical between the two gearboxes.-The PDK transmission fitted to the new 911 has a number of upgrades to match the new engine. It has been reinforced to cope with the increased torque. It has a dual-mass flywheel with a centrifugal pendulum, which damps vibration and allows the engine to run smoothly at lower rpm. Porsche also has switched the shift pattern in manual mode so that you pull the shift lever back to upshift and push it forward to downshift, as you do in the GT3 and most racing cars. And in manual mode, the transmission neither downshifts nor upshifts of its own accord—selecting manual mode means you do all the shifting.-When you order the Sport Chrono package on the new Carrera, the mode selector moves to the steering wheel and takes the form of a small rotary knob mounted on its own pod. As you rotate the knob you can select among the usual Normal, Sport, Sport Plus, and Individual modes. There also is a "Sport Response" button in the middle of the knob. When you press this, all of the systems shift to their maximum performance mode—the optimum gear for the speed, the engine settings for best response, and the stiffest suspension. It stays in this mode for about 20 seconds, plenty long enough to complete a decisive pass, for example. There’s even a circular graphic on the dash that counts down how much longer this mode will be engaged.-Even though 911s aren’t as low-slung as Lamborghinis, the Porsches have plenty of front overhang and scrape their noses on steep driveways and ramps. Porsche has solved this problem on the new model with the optional nose-hiking Compact Lift System. The setup uses a pair of remarkably compact annular hydraulic cylinders that sit below the lower spring perches on the front struts. When activated, an electric hydraulic pump pressurizes these cylinders and takes about five seconds to lift the front suspension by 1.2 inches, which translates into 1.6 inches of additional clearance at the nose. The entire system weighs only nine pounds and automatically drops to normal ride height if you exceed about 20 mph.-In order to help offset the weight added by the turbos, intercoolers, and their associated plumbing, Porsche’s engineers looked to save weight elsewhere in the engine. One of the biggest chunks came from a new plastic oil sump that saved 4.4 pounds. The sump is comprehensively baffled to ensure a supply of oil near the pickup of the oil pump no matter how aggressively you’re driving, but the idea of an oil pan molded from even the most modern polymers doesn’t inspire confidence among those who expect robustness from their Porsches. To prove the pan’s durability, the engineers tested it by dropping an engine onto a concrete floor from a height of 39 inches. Thomas Brandl, one of those engineers, said the pan developed no leaks.-Porsche has long believed that a car’s brakes should be stronger than its engine. So when engine power rises, so does braking performance. The 2017 Carrera gets new front brake calipers—still with four pistons but with 17 percent more pad area. And the 13.0-inch brake rotor increases in thickness from 1.1 to 1.3 inches (28 to 34 millimeters). The Carrera S gets new six-piston calipers with 18 percent larger pads, squeezing 1.3-inch thick rotors that go up from 13.4 to 13.8 inches (340 to 350 mm) in diameter. Furthermore, these larger S rotors now get pin-mounted aluminum hat sections, which reduce their weight by about one pound each.-The rear-wheel-steering system already standard on the Turbo and GT3 is available as an option on the 991.2. The system can turn the rear wheels up to two degrees in each direction. Below 30 mph, the rear wheels turn opposite the front ones to increase agility. Above 50 mph, they turn in the same direction to enhance stability. In between those speeds, the direction they turn depends on the circumstances. The ratio of the front steering rack is also faster to increase agility even more—a change made possible only because of the extra stability imparted by the rear steering. But what we found fascinating was the mechanism that causes the rear wheels to steer. Each of the two toe-control links is replaced by a miniature electric power-steering rack that looks like a shrunken version of the big one steering the front wheels. Each side has a small electric motor that uses a tiny cog belt to spin a nut to move a screw that traverses the wheels.--
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