With the introduction of the 2017 911 Carrera and Carrera S, the word “turbo” as it relates to Porsche’s iconic sports car has been relegated to a mere badge rather than being singularly emblematic of one of the hottest versions. That’s because with this major midcycle update, Porsche has turbocharged the engines in even the lowest 911s and is heading toward a near-universal turbo future, following the successful trail blazed by Audi, BMW, and Mercedes-Benz.
-The motivation behind this change is improved fuel efficiency to meet both European and American requirements for reduced CO2 emissions. But Porsche also understood that while achieving the efficiency improvements demanded by the politicians of the world, it also needed to create a turbocharged engine that perpetuated the stirring sounds and high-rpm energy that 911 drivers expect.
-New—Yet Familiar
-The new engines, known internally as 9A2, are complete redesigns of the existing 9A1 powerplants, although they retain the traditional flat-six layout and general dimensions of their predecessors. The engine block still consists of two parts, each with a crankcase half and a block of three cylinders. However, these cylinder bores are now coated with iron that is plasma-sprayed onto the aluminum surface. The raw aluminum cylinders are machined with slightly undercut grooves, so that the 150-micron-thick (0.006 inch) iron coating interlocks mechanically with the aluminum. According to Thomas Brandl, one of the engineers working on the new engine, this RSW (Rotating Single Wire) process is more durable than either Nikasil or the silicon-reinforced Locasil process used previously.
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Moreover, this change allowed Porsche to employ a stronger aluminum alloy for the block. It’s a fairly common alloy used in cylinder heads called AlSi7MgCu0.5 and it’s cast with metal outer molds and sand-cast cores, using a rotary process that reduces porosity. The resulting block has a closed deck and slices 3.3 pounds from the engine’s weight. A simplified oil pump with variable displacement to reduce its power requirements saves another 2.5 pounds, while a new plastic sump saves 4.5 more.
-In the fashion of modern turbo engines, these powerplants for the 991.2—as these new 911s are called—are downsized, but not by too much. Thanks to a bore and stroke of 91.0 mm and 76.4 mm, the new engine displaces 2981 cc. This is down from 3436 cc and 3800 cc in the 991.1 Carrera and Carrera S models. The stroke is only reduced by 1.1 mm, while the bore dimensions came down by 6.0 and 11.0 mm for the two engines. The less oversquare dimensions make for a more compact combustion chamber for greater efficiency while the very similar stroke dimension accommodates high revs.
-The smaller bore has less room for valves, so the intake valves on both the new engines measure 34.3 mm in diameter, down from 39.5 mm on the previous Carrera and 41.8 mm on the Carrera S. The exhausts are similarly downsized, going from 43.2 and 36.1 mm to 29.7 mm on the new engine.
-Despite the narrower bore, the bore-center spacing remains at 118.0 mm—common to all of the water-cooled boxer engines. The limiting factor is the length of the crankshaft, which has six crank throws and seven main bearings. These are essentially the same size in the new engines as in the old, and they required the same amount of space.
-The cylinder heads also are similar to their predecessors, although with smaller intake and exhaust valves to fit the smaller bores. The valvetrain uses Porsche’s VarioCam Plus, with selectable concentric valve lifters to achieve variable valve timing and lift on the intake cams. Both intake and exhaust cams have phasing that is adjustable over a 50-degree range, and intake-valve timing is similar to the previous naturally aspirated engine’s while the exhaust timing has been adjusted to optimize turbocharger performance. The intake manifold, however, is simpler than in the previous engine with none of the variable plenum volumes and port lengths.
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Finer Details
-One key change is the relocation of the direct fuel injector from the side of the cylinder head, spraying across the combustion chamber, to the top, adjacent to the spark plug, where the fuel is sprayed straight down. Coupled with 3625 psi of injection pressure—more than twice the previous maximum—the new injector location results in finer atomization, better mixing of fuel and air, and less wall wetting.
-New intake ports promote more tumbling of the mixture to improve combustion efficiency and reduce raw emissions from the engine. This reduction includes particulates, which are sometimes a problem for gasoline engines with direct injection. As of the 2017 models, the EPA is phasing in a two-thirds reduction in particulates, so cutting them is now on everyone’s radar.
-The compression ratio in these combustion chambers falls from 12.5:1 to 10.0:1, to cope with the force-feeding delivered by a pair of small BorgWarner turbochargers. These units all have 45 mm diameter turbines, but the impellers on the compression side are 49 mm in the Carrera and 51 mm in the Carrera S. Boost pressure is also tailored for each engine, with 13.1 psi in the Carrera and 16.0 psi in the S. A pair of air-to-air intercoolers on each side of the engine keeps intake-charge temperatures under control.
-All told, the additional componentry adds some 77 pounds to the engine, but the various weight-savings measures—which also include a 15-pound-lighter exhaust system—have kept the total engine weight increase to 44 pounds.
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The Power Story
-In exchange for this poundage, the engines deliver usefully higher output. The Carrera’s peaks increase from 350 horsepower and 287 lb-ft to 370 and 332. The Carrera S jumps from 400 horses and 325 lb-ft to 420 and 369.
-Those increases, particularly on the torque side, will feel even greater than the figures suggest because, unlike the previous engines, which developed peak torque at 5600 rpm, the new engines deliver max torque starting at 1700 rpm. This makes for an enormous difference in the midrange. For example, at 3000 rpm, the previous Carrera engine could only muster 243 lb-ft, while the new one is delivering all 332—36 percent more.
-Even more important to 911 drivers, the new turbocharged engines still rev frenetically, achieving peak power at 6500 rpm and continuing to a 7500-rpm redline. Peak power only falls by about five percent in that last thousand revs.
-In a quick ride around the short course at the Hockenheim, the new engine’s energy at high power was in full evidence. The Carrera S rocketed toward its redline with the same energy as a 2015 Carrera GTS we had just driven. And it makes similar and gloriously unmistakable flat-six sounds. From what we could feel from the passenger seat, throttle response felt immediate at elevated revs and the driver could easily balance the handling with the throttle.
-What we still don’t know is how strong the throttle response will be at low revs. The dirty little secret of turbocharged engines is that, while they develop lots of low-rpm torque, they don’t necessarily produce it as soon as you squeeze the accelerator.
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Under intense questioning, one of the Porsche engineers admitted that at 1800 rpm, the engine needs three full seconds to produce full torque from a closed throttle, though he was quick to add that the turbo lag dropped to two seconds at 2000 rpm and only one second at 2300. If a bit of turbo lag at very low revs is the price for a 7500-rpm redline charge, I think most of us will happily pay it.
-One other key piece of missing information is the improvement in fuel economy with the new engines. On the European cycles, consumption drops by about 10 percent. EPA figures will become available closer to the on-sale date in March of 2016. But we would guess that most of the EPA ratings would increase by 2 mpg in both the city and highway measurements.
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Porsche makes the point that this new base Carrera, with its 370 horsepower, is 110 horses stronger than the landmark original 930-generation Turbo—which also had a 3.0-liter engine—while consuming barely one-third as much fuel on the European cycle. That’s great progress, but we’re eager to find out for ourselves if the engine really proves to deliver higher real-world efficiency while remaining as inspiring and satisfying as the naturally aspirated engine it replaces.
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