2015 Ford Mustang GT: Coyote Voodoo Cross-Plane Flat-Plane
January 21, 2015
Fresh from the Detroit Auto Show, I am flush with thoughts of our long-term 2015 Ford Mustang's engine family. Ford's "modular" V8 in its current 32-valve, 5.0-liter "Coyote" form has potency and refinement in equal measure. It's a pleasure to drive. You might think this is as good as the Mod motor gets.
You'd be wrong.
The upcoming GT350's "Voodoo" engine is the flat-plane crank version of the Coyote. If you've been following the slow drip of GT350-related into, you know that specifics on the Voodoo's guts are sparse on the ground. I chatted with Ford engineering boss Jamal Hameedi and peered at a Voodoo cutaway in Ford's booth at the Detroit auto show. I'll spare you the foreplay of a basic technical crankshaft treatise and get right to the relevant engine-nerd bits.
Here comes the brain dump.
The single biggest challenge in creating a long-stroke flat-plane crank (FPC) V8 like the Voodoo is vibration (specifically a pronounced second-order lateral shake). So, given its huge-for-a-FPC 92.7 mm stroke, it's no surprise that quelling the Voodoo's vibes was one of Ford's primary development tasks. Addressing it entailed some 30 parts unique to the Voodoo including a stiffer block casting, revised accessory mounts and other approaches Ford's not yet willing to divulge. Incidentally, a nose-mounted crank damper is limited in its ability to snub this shake. Instead it damps torsional vibrations set up in the crank itself.
According to Hameedi, Ford benchmarked the vibration levels of the normally aspirated version of the Ferrari California. The California's front-engine 4.3-liter FPC V8 powertrain layout aligned closest to what Ford was creating, so the California became the natural bogey. Fast forward to today and Hameedi says the GT350's in-cabin vibration levels are lower even than the California's.
The GT350R that debuted at the show was fired up in Ford's booth and given a few very healthy prods of the throttle. To my distinct puzzlement, the Voodoo doesn't emit the characteristic flat-plane crank V8 sound I'd anticipated. It's missing the high-winding scream. The Voodoo is bassier than a Ferrari V8, yet more insistent than the lub-lub-lub of cross-plane crank V8s (like other American V8s including the Coyote). The Voodoo's aural signature is somewhere between the two.
The decision to craft the Voodoo's acoustics this way makes some sense. An exotic, Ferrari-style high-pitched exhaust note would probably alienate would-be GT350 buyers who expect a barrel-chested sound in their American performance cars. Doing so apparently led Ford's development team to take unorthodox measures. For starters, it has unequal-length 4-into-3-into-1 exhaust manifolds which contrast vividly with the equal-length 4-into-1 jobs of Ferrari V8s.
There's more. The Voodoo's crank doesn't adopt the crankpin layout of conventional FPCs like those of all Ferrari V8s, the McLaren 650S, Porsche 918 Spyder, Lotus Esprit V8, etc. When viewed as if lying on a table, these typical FPCs look like slightly stretched versions of inline-four cylinder cranks, with the four crankpins arranged in an "up-down-down-up" manner.
Conversely, the Voodoo's four crankpins alternate (they're arranged "up-down-up-down"). Weird! This layout results in a different firing order (it still alternates from bank-to-bank in true FPC fashion), too.
A side effect of the Voodoo's crankpin layout is an end-to-end vibration that traditional FPCs avoid. In geek-speak, the Voodoo has (in addition to the aforementioned second order side-to-side shake inherent to all FPCs) a first-order inertial unbalance. To counteract this end-to-end unbalance, the Voodoo's crank looks to have a big honkin' counterweight at each end that, say, a Ferrari V8 crank doesn't require.
It's hard to say at this point whether the Voodoo's crankpin configuration yields any performance benefit over a traditional FPC. Perhaps it just made packaging the unusual exhaust manifolds easier. In any case, the price the Voodoo pays for its atypical crank throw layout is somewhat higher crankshaft mass and inertia than a traditional FPC, but still less than a cross-plane crank. In fact, Hameedi says that even with its hefty, vibration-absorbing dual-mass flywheel added in, the Voodoo still manages to have less total rotational inertia than the cross-plane crank-havin' Coyote.
Hameedi also tells me the production Voodoo exceeded the development team's original performance targets. Ford is still mum on output, so I'll dip my toe in and speculate. Assuming it develops BMEP halfway between the Ferrari 458's 15.1 bar and the Coyote's 13.8 bar (a safe bet given the Voodoo's higher volumetric efficiency and elevated compression ratio), the Voodoo ought to generate a peak of 435 lb-ft of torque.
Jason Kavanagh, Engineering Editor