Porsche, Shelby show their high-performance approaches

  • 02-Apr-2013 10:14 EDT
14-911GT3-4.jpg

The Porsche GT-3 exterior includes lipped spoilers for increased downforce.

Porsche and Shelby approach ultra-high performance engineering from seemingly opposite directions, and at the 2013 New York International Auto Show each introduced 50th anniversary examples. For Porsche, it was the 50th for the iconic 911, and the showing of its fifth generation GT-3 variant. Shelby marked its 50th year at the NYIAS with an up-to-1200 hp (895 kW) edition of the Shelby GT500 Mustang-derivative called the Shelby 1000. Both are race cars that can be street-legal.

"Conventional wisdom" is that Porsche is all about sophistication and Shelby all about "American muscle." Perhaps to an extent, but the Porsche isn't a weakling and the new Shelby reflects considerably engineering expertise.

New boxer engine for GT-3

The GT-3, which goes on sale in November, has an all-new 3.8-L horizontally opposed "boxer" six-cylinder engine rated at "just" 475 hp (354 kW), but curb mass is only 3153 lb (1430 kg). The previous generation was rated at 435 hp (324 kW), although an available RS version delivered 450 hp (336 kW). The new GT-3 is based on the 911 Carrera, with an aluminum chassis and an aluminum-and-steel body but with specific front and rear parts not focused on "styling" cues. The GT-3 is built on a 96.7-in (2456-mm) wheelbase and is 178.9 in (4544 mm) long.

Its coefficient of drag is an unimpressive-sounding 0.33 and actually is higher than the previous generation GT-3's 0.32. That doesn't seem like progress, but a light car with all that power needs a lot of downforce, and Porsche engineering had to make choices for needed balance.

The new model, which incorporates front and rear spoilers, develops 260 lb (118 kg) of downforce at 300 km/h (186 mph), a 15% increase.  And the proof of the design is its performance. The car goes 0-60 mph (0-97 km/h) in 3.3 s vs. the previous 4.0 s. Top speed is 195 mph (314 km/h), achieved in this version in 7th gear (a 0.84:1 overdrive ratio) of the newly adopted PDK dual clutch transmission; that compares with 194 mph (312 kph) in 6th for the previous GT-3's manual.

The GT-3 is about more than acceleration and top speed; Porsche engineers focused on the driving experience. It has, for example, four-wheel steering and a unique algorithm for full-throttle burnouts. And it's certainly a high-revving experience: 9000 rpm redline vs. 8400 rpm on the previous GT-3.

The 3.8-L engine uses Porsche's new direct-injection design. The fuel system is fed by two high-pressure pumps with peak pressure of 2900 psi (20 MPa)—vs. one pump for the standard 911. Injectors are a six-hole design for increased flow and finer fuel atomization vs. the previous swirl type.

The new 3.8-L engine internal parts are lighter than in the previous, long-used "Metzger" design, saving 25 kg (55 lb) in overall engine weight; this is noteworthy because the Metzger already had forged titanium rods and forged aluminum pistons. The valvetrain uses rocker arms and hydraulic lifters—vs. direct-acting buckets—because rockers have lower moving masses for high engine rpm and provide large contact areas with the cam lobes. The dry-sump system's oil pump is an on-demand type and has seven oil pickup points, an increase of two. Compression ratio is 12.9:1 vs. 12.0:1 for the Metzger.

Electrorheological mounts

GT-3 engine mounts are filled with liquid and ferrous metallic powder, and they are electrorheologically regulated by an electronic module based on inputs from handling-related sensors, so they're soft in everyday driving. During performance driving, the module energizes electromagnetic fields in the mounts, causing the powder particles to agglomerate, "thickening" the liquid and stiffening the mounts.

The 911's PDK is specifically modified for the new GT-3. If the driver pulls back on both short-travel shift paddles, the gearbox shifts into neutral. New software reduces manual shift speeds to under 100 ms, so if the driver floors the gas pedal and releases the paddles, the car produces a smoky burnout. The rear axle is a fully variable locking type, but under extreme conditions the GT-3 can add selective use of wheel brakes for better stability control.

The rear suspension has an electromechanical actuator at each wheel, each with a range of up to 1.5 degrees. At speeds up to 31 mph (50 km/h), rear wheels steer opposite to the fronts. Over 50 mph (80 km/h), they move from straight ahead to steer in the same direction as the fronts, improving high speed stability.

Shelby 5.4-L V8 bored out to 5.8-L

The Shelby 1000, based on the Ford-built Shelby GT500, provides maximum-power bragging rights, with Bugatti Veyron rivaling numbers at a fraction of the $1.3 million price for that VW Group supercar. It has a version of the Ford supercharged modular V8, enlarged for 2013 from 5.4 to 5.8 L by a bore increase from 90.2 to 93.5 mm (3.55 to 3.68 in)—made possible because the engine has Ford-patented sprayed-in liners. As supplied by Ford, the Mustang variant is rated at 662 hp (494 kW) and has a claimed top speed of over 200 mph (322 km/h).

Ford's SVT group put considerable engineering time into the GT500, of course. However, Shelby American's Gary Patterson, Vice President of Operations, describes the GT500 as "the starting point" for the 1000, which can be considered the continuous improvement version of the 850-hp (634-kW) Shelby-built predecessor.

Engine output ranges from 950-1000 hp (708-746 kW) on premium pump gasoline and is 50-state emissions compliant. Remove the catalytic converters, raise supercharger boost pressure from 19 psi (131 kPa) to 23 psi (159 kPa), and use racing fuel, and the track version output climbs to 1200 hp (895 kW), Patterson said.

Aftermarket performance parts

The 1000 incorporates many parts produced to Shelby specs by leading American performance aftermarket companies.

The Shelby-spec supercharger is a 4.0-L unit—Kenne Bell and Whipple are Shelby supercharger suppliers—with a three-row intercooler. Most engine internal parts also are Shelby-spec, including the camshafts, rods, pistons and rings, bearings, oil pump, valve springs and titanium retainers, and balance shaft assembly. Shelby ports the cylinder heads and specs the main bearing studs, head bolts, water pump and heavy-duty cooling package, twin clutch, and flywheel. There is no grille as such, so the opening supplies increased cooling airflow.

The fuel system is a Shelby 1000 package including the injectors.

The Ford-supplied transmission is a heavy-duty Tremec 6060 six-speed linked to an MGW short-throw shifter. The driveshaft is a Shelby aluminum design.

The GT500 has been strengthened to handle the additional power. Suspension is a Shelby upgrade, including a billet aluminum Watts link in the rear. The unit body is welded to ladder-type longitudinal frame rails (front and rear sections on each side). Brakes include Wilwood forged aluminum calipers (six-piston fronts, four-piston rears). The exhaust system is from Borla, both the headers and cat-back exhaust.

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