2015 Ford Mustang GT: More Insights On the GT350 Voodoo V8
June 22, 2015
Ford recently (finally?) dropped details on the 5.2-liter "Voodoo" V8 that's going into the 2016 Ford Shelby GT350, GT350R, and a handful of other 2015 models. As a follow-up to my previous pontifications on the subject — the gist being that this is one of the most fascinating modern performance car engines to come out of Detroit — let's take a more informed look at how Ford arrived at this engine's unusual anatomy.
Yeah, this has little to do with our long-term 2015 Ford Mustang GT, specifically. But if you're a Mustang geek or an engine nerd, read on.
As I noted in my previous piece, the Voodoo has an unconventional crankpin arrangement by flat-plane crank (FPC) standards. Ford's crank throws are arranged up-down-up-down (UDUD), rather than the expected up-down-down-up (UDDU). At the time I could only speculate on how this evolved because the company wasn't ready to talk about it. But now, they are. A little.
Ford's engine guys tell me the Voodoo's crankpin arrangement was the product of the engine's manifolding. The team modeled approximations of the intake and exhaust systems and assessed the engine's breathing characteristics in each of the three possible FPC configurations. The UDUD crank ultimately got the nod from this exercise.
It follows that the geometries of said manifolds are also unusual in the context of FPC V8s. First, have a look at the Voodoo's weird, unequal-length 4-into-3-into-1 exhaust manifolds (the top one is the driver side manifold, mirrored for easy comparison):
The layout of the manifolds was driven largely by the cramped confines of the Mustang's engine bay and the desire to facilitate catalyst light-off by minimizing their internal "wetted" area. Performance considerations alone would have resulted in paired-length 4-2-1 or the equal-length 4-1 manifolds that FPC V8s often exploit by virtue of their bank-to-bank alternating firing order.
On the fresh air side, the Voodoo's intake manifold has a single plenum feeding all eight cylinders at all times — another uncommon sight in FPC V8-land. Typical intake manifolds on FPC V8s carry dual bank-specific plenums or, as found in the Ferrari F430 and 458 Italia, a fancypants two-stage manifold that can couple and decouple its two plenums as desired.
A common-plenum manifold like the Voodoo's has the benefit of being far more packaging- and cost-friendly than a dual-plenum unit. The latter necessitates two of everything else: throttle bodies, ducts, mass flow sensors, filter boxes, intakes, et al, plus the attendant calibration overhead.
One downside to a UDUD crankshaft, relative to UDDU cranks, is mass and inertia. To recap, a UDUD crank imparts a first-order, end-to-end imbalance that UDDU cranks do not exhibit. To mitigate this imbalance, Ford had to seriously bulk up the Voodoo's two outboard counterweights. They're approximately double the width axially and larger circumferentially than the crank's remaining counterweights:
Compare that to the typical UDDU crank (Porsche 918 Spyder crank shown) below, in which all counterweights share one pared-back geometry:
Ford engineers say that crankshaft inertia "was not a prime consideration in the selection of the crank configuration." Nevertheless, the resulting additional mass of the UDUD crank had to have made a Dearborn engineer or two wince. After all, they went to the trouble of milling little pockets out of the Voodoo's heads to shed mass. Heads are aluminum and stationary. Cranks are steel and rotate.
Incidentally, it turns out the GT350's "Voodoo" V8 crank has a 93.0mm stroke, not 92.7mm as I'd expected. This small difference is enough to hand the Voodoo the crown of the highest mean piston speed (25.6 meters per second) of any production engine. Previously this title was carried by Audi's 4.2-liter V8 (25.5 m/s) and not the Honda S2000's F20C (25.2 m/s) you were probably expecting.
Ford's first crack at a flat-plane crank V8 is a whopper. At a much more attainable price, the Voodoo generates BMEP within a stone's throw of those produced by manufacturers that have a lot more seat time in developing them. And it does it without the benefit of direct injection, ideal exhaust manifolds, a dry sump scavenging system or exotic materials. There's not a titanium rod or valve to be found.
All this and it can be serviced at your local Ford dealer.
Jason Kavanagh, Engineering Editor