A few weeks ago we got an in-depth look the platform changes that the SVT/Ford Performance engineering crew made in the process of transforming the 2015 Mustang GT into the new Shelby GT350. However, a great chassis can only you get you so far. Every great sports car also needs a great powerplant and here the engineers appear to have come through again. Unlike the last couple of generations of SVT Mustangs, this time around the team have come up with something completely different, the 5.2-liter Voodoo V8.
While the Voodoo has its roots in the acclaimed 5.0-liter Coyote V8 that has powered Mustangs for the past five years, other than the basic dimensions and bore spacing, not much is carried over. The Voodoo is Ford’s first-ever production V8 with a flat-plane crankshaft, something you typically only find in dedicated racing engines and Italian exotics. Pretty much the only significant parts shared with the Coyote are the phasers for the variable cam timing system.
“We came into this project with the 2012 Boss 302 engine as our performance baseline,” said Adam Christian, Ford Performance intake, combustion and exhaust technical specialist. “The Boss 5.0-liter generated 444-horsepower and 88-hp/liter. The target for this car was at least 500-horsepower, 100-hp/liter and a redline of more than 8,000 rpm.”
The raw crankshaft forging on the right and the finished machined part on the left
The GT350 was always intended to have a very different character from the last few generations of SVT Mustangs. Just as the GT350s of the 1960s were the track-oriented cars meant for road racing while the GT500s were the muscle cars, the same holds true today. Thus a free-revving, normally aspirated V8 was the order of the day. With the basic parameters of 500-hp and 8,000 rpm defined, Christian set about determining how to get there. It soon became clear that a new flat-plane crank V8 would be needed.
air * fuel = power
Since fuel has to be delivered to an engine in proportion to the air, the only way to get more power was to increase the airflow. The GT500 achieved this by force-feeding the engine with a big supercharger. Christian chose a different path for what would become the Voodoo. Improving the breathing without a blower requires removing flow restrictions and adding speed. The intake and exhaust valves would need to be larger and that meant increasing bore size from 92.2-millimeters to 94-mm. New camshaft profiles also increased the lift to 14-mm and opened them faster than before with steeper ramps. Back upstream, the GT350 engine gets an open-element air filter and a massive 87-mm throttle body to minimize restriction.
The 87-mm throttle body is the largest ever on Ford production engine
Getting all this hardware up to 8,000 rpm and beyond would also require a significant reduction in rotational inertia and internal friction. That’s where the flat plane crankshaft comes into play. Unlike a traditional American V8 where the cross-plane crankshaft throws are spaced at 90-degree intervals, a flat-plane crank spreads the throws at 180-degree intervals. This layout ends up looking more like two inline four-cylinder engines mated at the crankshaft. It’s a setup that provides both advantages and new problems.
A cross-plane crank inevitably ends up with at least two adjacent cylinders on the same bank firing in succession. This causes problems with vibrations and exhaust scavenging. The vibrations are addressed with heavy counterweights on the crankshaft.
With a flat-plane crankshaft, the spacing of the pistons allows the cylinder firing order to alternate between the engine banks L-R-L-R-L-R-L-R. This maximizes the time between cylinders on the same bank firing and provides for better scavenging of exhaust gases and improved overall breathing, which is particularly important at high speeds.
Flat-plane crankshafts also have much smaller and lighter counterweights, significantly reducing the rotational inertia. Ford further reduced the weight of the forged-steel crankshaft by gun-drilling through the center of the shaft, taking out material wherever possible without sacrificing durability. Less inertia means the engine revs faster and in this case it redlines at 8,250 rpm That’s nearly 2,000 rpm higher than the Coyote which should contribute to making this a great track day machine.
The exceptionally slim piston rings enabled by the deck-plate honing process reduce friction
Friction is addressed in multiple ways but one of the most significant came about through the use of deck-plate honing. Like GT500 blocks since 2011, Ford is using plasma transfer wire arc technology it developed (and subsequently licensed to Nissan for use in the GT-R) to apply a steel coating to the aluminum bores of the redesigned block. When cylinder heads are bolted on to a block, they inherently cause some distortion of the bores making them slightly out of round. The springy piston rings have to fill the gap and more tension is required when the bore isn’t round. When race engines are built, a massive steel plate is bolted to the block to predistort the bores before the final honing process. After machining the bores become out of round again, but the final assembled engine has much truer cylinders. As a result, they are able to use slimmer piston rings with less tension while maintaining the same seal.
The same process, known as deck plate honing is being applied to the aluminum blocks when they are machined at Ford’s Windsor engine plant. The plasma deposition process is now done in-house by Ford rather than an outside contractor and then the deck plates are bolted on, complete with a cylinder head gasket. Once the machining is complete, the plates are removed and the used bolts are discarded for recycling.
Careful shaping of the lightweight forged pistons and the combustion chambers in the new heads helps to virtually eliminate knock. Even while retaining port fuel injection, the GT350 has a 12.0:1 compression ratio and runs on 93-octane fuel.
You might expect an engine of this type for a track-oriented car to have a dry-sump lubrication system, but again the Ford Performance team have been contrarian. For packaging, weight and cost reasons, they developed a new 10-quart composite oil pan for the Voodoo. In the Camaro Z28, the remote sump is mounted up high in the engine compartment and a similar setup would have been needed for Mustang. Instead, the composite pan has integrated baffles and venting that keep the oil where it needs to be without getting all whipped up at high speeds. Did I mention it’s also lighter?
Composite 10-quart oil pan
Like the 5.0-liter, the 5.2-liter gets fabricated tubular steel exhaust headers. Thanks to the new firing order, these now feature a 4-into-3-into-1 layout and they generate an amazing sound at the tailpipe.
Christian and the team ran millions of simulations using computer aided engineering before fabricating any parts. Most major parameters were settled before building an engine including the 94-mm bore and 93-mm stroke for a slightly oversquare 5.2-liter displacement. The result was the first prototype engine they built was generating more than 500-hp on the dyno within the first week of testing. Over the past three years, these engines have done thousands of hours of durability testing in the Dearborn dyno cells and countless miles on the road and track.
When the first GT350s are delivered to customers this summer, they will deliver 526-horsepower at 7,500 rpm and 429 lb.-ft. of torque at 4,750 rpm. More importantly the torque builds quickly and reaches nearly its peak at just over 3,000 and then stays there through most of the rest of the operating range.
While flat-plane crank V8s eliminate most of the vibrations inherent in a cross-plane design, they do have some vibrations of their own. The Ford Performance team acquired a Ferrari California, the most powerful front engine flat-plane V8 available to use a benchmark for noise, vibration and harshness (NVH). Everything about the GT350 NVH had to be equal or better than the Ferrari.
Like the GT500 engines before, the Voodoo V8 will be hand-assembled on a special line at Ford’s Romeo, Mich. engine plant
“We tweaked many elements of the car in order to avoid nodal alignments where the resonant frequencies of different components are equal,” said Jamal Hameedi. “Thoughtful changes everywhere allowed to us balance everything out.”
Another change from the last GT500 is the return to a steel 2-piece drive-shaft in place of the carbon fiber drive shaft used before. With the new characteristics of the Voodoo engine and the independent rear suspension, the carbon shaft wasn’t stiff enough.
Ford is being surprisingly aggressive in pricing the new GT350 with the base model going for $49,990 with delivery and the GT350R at $63,495. That’s a significant savings over the current Camaro Z28, although it’s safe to assume no dealer will actually let one off their lot for anywhere close to MSRP. Having seen what has gone into the new GT350, I can’t wait to drive this beast.
