So you think the Corvette Stingray and Jaguar F-Type play the wildest V8 soundtracks through their active-exhaust systems? Or you say nothing beats a Ferrari 458 for pure aural delight? Well, you haven’t heard Ford’s 2015 Shelby GT350 Mustang on rising throttle. From tip-in to WOT the new Shelby, powered by a unique naturally aspirated 5.2-L DOHC V8 that was coded “Voodoo” during development, plays a downright diabolical tune from its four exhaust outlets.
“When we would start up the first prototypes in the engineering garage, everybody’s heads turned: ‘What the heck is that?’” recalled Raj Nair, Ford’s Group Vice President of Global Product Development, in an interview with Automotive Engineering prior to the 2014 L.A. Auto Show. “When we’d transit the cars between facilities in Dearborn you’d hear their echo out on Oakwood Boulevard. Anybody at our proving ground who heard these cars knew there was something very different under the hood.”
There will be customers who will buy the latest Shelby Mustang for its 20% stiffer body structure (compared with the Mustang GT), its racetrack-focused chassis tuning, enormous Brembo brakes, responsive MagneRide damping, Recaro seats, and many other vehicle attributes. Others, however, will want the car for its howling exhaust note alone. The sound is created by the engine’s special flat-plane crankshaft and its related breathing and ignition characteristics.
So named because its four connecting-rod journals are spaced 180° apart, the shaft appears “flat” in a single plane when viewed head-on. The arrangement generally does away with the large counterweights used in the typical 90° "cross plane" layout which helps reduce crankshaft mass and enables the engine to rev more freely. The new Ford spins to a heady 8000 rpm before fuel cut-off, according to Nair, and has another 250 rpm of safe headroom. This from mildly oversquare 94 x 92.7-mm bore and stroke dimensions.
Long the province of European high-performance V8s from Ferrari, Lotus, Cosworth (the legendary DFV Formula 1 unit) and others, flat-plane cranks were also used in the first Cadillac V8 in 1914 . The configuration gives an alternating right-left-right-left firing order across the cylinder banks, allowing more efficient gas flow and exhaust-gas scavenging along with improved throttle response and exotic exhaust acoustics.
Compression ratio of the port-injected, Coyote-based 5.2-L is 12:1 and max cylinder pressure is 97 bar (1406 psi), an engineer said. Final SAE power and torque ratings are not yet finalized, but the V8 will deliver at least 500 hp (373 kW) and more than 400 lb·ft (542 N·m), according to Jamal Hameedi, Chief Engineer, Ford Global Performance Vehicles.
Hear the V8 sing in this video courtesy of Ford (turn up your volume setting first):
Flat-plane-crank engines have limitations. First, the lack of counterweight balancing typically limits cylinder displacement to about 4.5-4.6-L due to greater second-order vibration. Ford has solved that in the 5.2-L application with a new crankshaft-mounted damper system and extraordinary attention to NVH abatement during the design and prototype phases. According to Nair, the engine program (which was concurrent with GT350 vehicle development) nearly wasn’t approved for production.
“This [vibration] was our biggest engineering challenge even after we had the first prototype,” Nair noted. “Things were breaking and the technical guys were worried. Whether or not we continued down the flat-plane-crank path for GT350 came down to a critical prototype drive we had scheduled. After that drive, we all went into a meeting room for the debriefing. And we unanimously concluded that we simply had to have this motor! We were determined to solve the issues.”
What followed was, in Nair’s words, “a lot of stiffening of the cylinder block, exhaust system, and various brackets” achieved through an intense FEA analysis, plus “a lot of tuning.” Nair said the result yielded unique torsional-damping technology among other patent-pending actions aimed at taming the beast.
The 5.2-L is naturally aspirated by design; boosting was not part of the plan. “We really wanted to stay with naturally-aspirated technologies for instantaneous throttle response” along with a feeling of minimal flywheel effect, Nair explained. He’s familiar with both qualities as a longtime motorcyclist who rides a Yamaha R1 sportbike on the street and at track days.
Other challenges with flat-plane-crank engines include their tendency to deliver underwhelming torque. Nair admitted that the GT350 “doesn’t have that electric-motor feel off idle that the GT500 has” but the latter also uses a blown 5.8-L rated at 631 lb·ft/855 N·m. Increasing cylinder displacement (via larger bore) to 5.2-L, which is 700-cc larger than Ferrari's V8 used in the 458, helped overcome some of the torque deficit. Nair also noted that optimizing the intake and exhaust systems for power, efficiency, and meeting cold-start emissions was tricky.
“Even the unequal-length headers we selected, to some extent, to help with catalyst light-off,” he said.
According to Hameedi, the 5.2-L V8 has a linerless aluminum cylinder block that features bores finished using PTWA (Plasma Transfer Wire Arc), an advanced coating technology co-developed by Ford and Flame-Spray Industries that debuted on the 2011 GT500 5.4-L V8. For PTWA details see http://articles.sae.org/7624/.
Supplier sources involved with the 5.2L V8 program said Ford broadcast an initial volume plan for production of 5000 units.