Flat Out: An In-Depth Look at the Ford Shelby GT350 Mustang Engine

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It's all-new. Its 5,163 cubic centimeters produce 526 hp at 7,500 rpm and 429 lb-ft of torque at 4,750 rpm, and it revs to 8,250 rpm -- making this Ford's highest-revving V-8 and its highest-output naturally aspirated engine of any type. That's less power but more torque than Ferrari coaxes out of its 4.5-liter V-8 (562 hp at 9,000 rpm and 398 lb-ft at 6,000 rpm, though that includes ram-air effects at high speed). Of course, Ford wasn't trying to build a 458-fighter. It's building the ultimate track star Mustang, so it designed and engineered this engine from the ground up in Allen Park, Michigan, paying little attention to that prancing horse V-8.

Even the crankshaft is different. Yes, it's flat, but did you know there are three different flat-plane crank designs? Looking at the crank lying on a table with the front on the left, the connecting-rod throws can be up-down-down-up (like Ferrari's, and like most four-cylinder engine cranks), up-up-down-down, or up-down-up-down. Ford modeled all three designs extensively in the computer and prototyped at least two of them before determining that the UDUD concept was the one that breathed the best with its layout. Maximizing airflow through the engine is a primary driver of the flat-plane crank design, which inherently delivers improved exhaust airflow, because you never have two cylinders on the same bank firing within 90 degrees of crank rotation. That causes a traffic jam in the exhaust header unless you connect the header pipes a long way away from the engine, which makes it impossible to light off a catalyst. (Tell your friends: The flat-plane crank is a green development!) Anyway, Ferrari treats each bank like a separate I-4 engine with its own intake plenum, so UDDU works better; Ford has one big intake plenum for the whole engine, which the UDUD crank serves better in terms of airflow.

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Maximizing airflow was a prime directive for the team developing the Ford Shelby GT350 Mustang's engine, so let's follow an air molecule. It enters through a giant open air cleaner element and flows through a gaping 87-mm throttle body (Ford's largest). It then enters the scroll-type intake manifold, which features runners as long as those in the 5.0-liter V-8 but much bigger in cross-section. (They also incorporate flaps to increase swirl for higher efficiency at light-load cruising speeds.) Next, it's into the combustion chamber through valves that are considerably larger in diameter and that lift an impressive 14 mm. That's super high for an engine designed to safely over-rev to 9,000 rpm using hydraulic lifters. "Soft-landing" cam profiles prevent excessive noise, harshness, or wear, and the slight "loft" (where the cam throws the valve farther open at high speed) actually improves breathing a bit more at redline. These cylinder heads, by the way, feature fully machined combustion chambers and ports, with additional machining performed just to reduce weight. (They're lighter than the 5.0's by 6 percent.)

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Let's pause here for a word about the block. To accommodate those big, free-breathing valves while preserving the 100-mm cylinder-bore spacing that allows this all-new block to be machined alongside the 5.0-liter in Windsor, Ontario, Canada (the deck height is also the same), the bore was increased to 94 mm by eschewing iron liners in favor of a plasma-transferred wire-arc cylinder liner technology. The crankshaft is also "gun drilled," with big holes in the counterweights to allow air to flow more freely between the front two and back two chambers to reduce pumping forces. This also further lightens the crank so that its rotational inertia is about 15 percent less than the 5.0-liter's cross-plane crank, making it freer to rev. The cylinders are deck-plane honed to further reduce friction. That involves torqueing down a plate that simulates the cylinder head to distort the cylinders the way they will be when fully assembled before honing. This results in a much more perfectly cylindrical shape that in turn permits lower-tension piston rings, reducing their friction by an undisclosed amount. (It typically accounts for 20 percent of overall engine friction.) A novel composite oil pan includes a composite insert that incorporates the oil baffle, pickup tube, and windage tray while increasing oil capacity by 2.0 quarts and slashing weight by 20 percent relative to the 5.0L.

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Back to our air molecule. By now it's been mixed with port-injected fuel, squished at 12:1 compression (up from the 5.0-liter's 11:1), and ignited by a fairly advanced spark for a slowish burn rate. (Knock resistance is exceptional with this chamber design.) Then it exits via similarly huge exhaust valves (also opening fully 14 mm). It flows into a header manifold that is TIG-welded by hand with a four-into-three-into-one design. (Four-two-one is ideal, but packaging around power steering forced this design.) Flowing aft, the molecules from each bank are mixed well in an X-shaped crossover pipe (this sounds better than the 5.0-liter's H-shaped crossover) before exiting through a two-stage muffler that automatically opens under high loads and can be opened at will via the user interface. This is Ford's lowest back pressure on a road-legal Mustang exhaust system to date.

It's worth noting that computer-aided engineering tools allowed this engine to be developed in just three years, the very first physical prototype producing 500 hp inside a week of its assembly. Naturally, the engine enjoys Ford's normal five-year/60,000-mile warranty.

During the Q&A we asked why it doesn't include direct injection. Intake-combustion-exhaust specialist Adam Christian explained that although DI is vital to turbocharged engines, it does add cost, weight, noise, and complexity. Increasing the compression ratio further to take full advantage of the technology's cylinder-cooling effects would force too many other compromises, and with the current setup delivering 14-bar peak cylinder pressures and 526 hp and 429 lb-ft, it just wasn't deemed worth the cost and effort. Having been sold on the merits of the flat-plane crank, we asked if it might find its way into pickup trucks, to which Adam replied that it's not as strong down in the 1,000-2,000-rpm range where those workhorses spend much of their lives. And when asked what the magic bullet was that enabled this engine to surpass the historical max size for flat-plane-crank V-8s, Christian explained that it wasn't any one thing but a holistic approach to the entire engine design. He also speculated that Ferrari just isn't into big V-8s the way Ford is. Maybe if Enzo had sold out to Hank the Deuce back in 1963 …

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