Tom Wilson
February 3, 2012
Photos By: Courtesy of Ford Motor Company

Liner Notes

A spray bore is special because it eliminates the iron cylinder liners. This technology saves weight since the mass of iron lost is significant and it comes off where every Mustang could stand to lose a few pounds, right over the front axle. But, most importantly in the 5.8, eliminating the liner's thickness allowed the absolute maximum bore the aluminum block architecture would allow.

The trick is, pistons and piston rings can't live without the durable, non-galling iron cylinder surface. Bare aluminum is far too soft to coexist with the piston and rings, and so some other, harder material must be added to the raw aluminum bore. Many have worked on this problem for decades, including Reynolds Aluminum 40 years ago in big-block Chevy Can Am engines. Porsche is another notable experimenter. These earlier methods involved both adding material into the as-cast aluminum, along with wet sprays such as Nikasil, which are applied after casting.

Ford's approach, first used in the 2011 GT500 block, differs in that it is a post-casting, dry application of molten iron, resulting in a thin, durable, lightweight, oil-holding metal lattice joined to the aluminum bore's surface. The process is called Plasma Transfer Wire Arc, and was developed jointly by Ford and Flame-Spray Industries in Long Island, New York. Besides the GT500 blocks, it's used in gas turbines and Caterpillar remanufacturers its diesel cylinders with it, so it's tough stuff. Ford says the iron liners removed from the 2011 GT500 block weighed 7 or 8.5 pounds (depending on who you ask), and the PTWA coating is only 150 microns (0.006-inch) thick, so little weight is replaced.

For the GT500 block, the application begins in Germany, where casting specialist Honsel gravity-pours the GT500 block as a sand-casting with chill plates. Honsel then machines a spiral groove into the cylinder to act as a ledge for the sprayed iron to grip onto. Next, what is essentially a rotating arc welder is inserted into the cylinder. It uses a wire feed, an electric arc, and compressed air to blast a stream of 35,000-degree iron plasma onto the cylinder walls. The molten iron droplets are tiny, just 20 to 30 microns (0.0008 to 0.0011 inch) in diameter, and they dry in 10-6 seconds. The wire-fed plasma jet is maneuvered to form the lattice pattern; later the cylinder is diamond-honed for final crosshatching.

Ford is certainly invested in spray-bore technology, holding 25 patents in this area, and it's paying off. Nissan licenses the technology from Ford for its GT-R V-6 bores, and empirically, we've heard of no issues with '11-'12 GT500 blocks. The SVT engineers say the iron has a strong purchase on the aluminum, so liners may be a thing of the past with Ford performance engines. One consideration is that the thin iron coating can't be overbored, and we're not sure a GT500 block could be sleeved should things go awry after a night of heavy passion by an aftermarket tuner. SVT says the bores can be re-sprayed, but obviously this is not a field fix, so it sounds like a new block is required should damage occur.

In any case, the new 5.8 uses the identical aluminum block casting as the 5.4 with a few extra machining steps. And thanks mainly to spray-bore technology, SVT was able to increase the bore from 90.2 to 93.5 mm and thereby reach 5.8 liters with the stock 5.4 stroke.

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Get Shorty

Because the thermal activity in the 5.8 at full chat is probably adequate to give the devil sweats, attention to the cooling and oil systems was required. Specifically, SVT needed to verify coolant flow completely around the cylinders just below the block deck, as well as between the exhaust valve seats in the cylinder heads. In the block's bottom end the pistons are subjected to diesel-level heat and pressure and so they needed oil cooling jets.

For the cooling system upgrades, SVT turned to computational fluid dynamics modeling. Perhaps the greatest need was to get coolant flow between the exhaust valve seats, a journey that begins in the block as the coolant flow is from the water pump through the block, up through the heads, and out to the radiator. Existing 5.4 coolant flow directed water up through the block's deck and nicely around the exhaust valves, but not between them. That's where the CFD came in especially useful, letting the SVT engineers establish the additional paths required to route coolant between the exhaust seats at the correct flow velocities and volumes. These additional coolant paths in the heads needed to be fed by new passages in the block's deck.

Furthermore, flowing water around the top of the cylinders meant another new set of passages was required between the cylinders. Luckily, the CFD showed the solution to both new cooling paths turned out to be small passages drilled in the block and heads. Because simple drilling operations gave the needed extra coolant flow, there was no need to make a casting change in either the block or heads, which is one reason why both the head and block castings are carryover 5.4 parts.

In the bottom end of the block, the big 5.8 attraction are the piston squirters. These are actually tiny spring-loaded valves positioned in newly drilled passages in the main bearing bulkheads. Access to the squirters is found under the main bearing inserts, and their shooting path is through another carefully placed diagonal hole drilled into the side of the main bearing bulkhead. Considerations on how to aim the squirters include both aiming at the desired portion of the piston--that would be the underside of the dome and the pin boss, and not the skirts or especially the connecting rod's beam--and the length of time the oil stream is on target. The engineers say they're hosing down the piston about 60 percent of its stroke, an on-target average that took considerable effort, they report.

The oil squirter valves are tensioned to open at 50 psi. This is necessary to assure adequate oil flow at the critical main bearing-to-crank journal clearance. It also keeps the squirters from filling the crankcase volume like a bunch of drunk frat boys playing with fire hoses. Only under higher loads and crankshaft speeds do the oil squirters provide maximum flow, and that's exactly when the piston needs the extra cooling. Otherwise, having the squirters continuously flow at full volume would only cause excessive windage, which drags on the crankshaft costing horsepower and fuel economy.

What the oil squirters do for piston temperatures is "awesome," say the engineers. That's a good thing, as the 5.8 piston is subjected to incredible heat and pressure. To put numbers to it, the cylinder pressure in the Condor 5.4 is already an impressive 1,650 psi, while the 5.8 endures a crushing 2,000 psi or so at maximum load. That literally flattens the dome on a 5.4 piston--not from detonation, but simply from combustion pressure.

As you might recall from physics class, such piston-collapsing pressure is due to tremendous heat. And it's never far from the engine designer's mind that the piston dome is not only subjected to searing heat, but it also has one of the longest heat rejection paths in the engine. So besides needing a new, larger-diameter piston, the 5.8 designers needed a much stronger piston.

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