Coyote engines are widest...
Coyote engines are widest at their exhaust manifolds, and so the manifolds are designed to lay tightly against the engine to clear the chassis shock towers. It will take long-tube headers to best these manifolds, and then you'll probably want to open up the exhaust port, too.
"So basically the benefits of the tubular headers in a nutshell is about 15 lb-ft and 6 hp," Adam added. "The thing that you'll notice is, you know what a set of Tri-Y headers are supposed to look like-a simple side and a complex side. On all previous Ford engines, the complex side is always on the driver side. This engine is swapped because the firing order is changed. The complex side should be on the passenger side, which is nice for the steering-shaft packaging and everything. What you'll see on these headers though is that we look like we don't know what we're doing, and they are actually simple connectivity on both sides-front pairs, rear pairs, both banks."
"The reason is that you have to have the catalysts very close to the engine-they have to light off-and when you have that kind of length and you try to separate the 90-degree cylinders, which is what you pick for connectivity, you don't have enough length. What ends up happening is you take the blow-down pulse that occurs in the second cylinder, and you push its pulse into the overlap period of that first cylinder, and you actually destroy the volumetric efficiency," Adam continued. "You've helped the pumping because you've moved that pulse out of the pumping portion of that cylinder, but you've hurt its Vol-F [volumetric efficiency] and the net result is zero; you don't get anything for it. And if you look at [Brand T], they're made that way. [Brand C] tends to do just straight-up manifolds. They're nice manifolds, but just straight up."
"This literally was a morning-shower epiphany thing ... you don't know the amount of work [it was] to push that exhaust flange down as far as it is. The catalysts are short. They're actually stacked on top of each other. The bricks have no separation between them at all, they're just crammed together. They touch; there's no cat monitor in-between. Usually there is a HEGO in-between and we don't have it," Adam said. "So we had pushed the package as far as we could and there just wasn't enough length to get it to work, and then I thought, 'What if we just don't try to pair the 90-degree cylinders? What if we just try to bring them together as much as possible?' And that's what you see, particularly the right bank; right-bank cylinders 1 and 2 come right together, and those two fire right on top of each other. You see the secondary pipe is actually bigger than the rest-that's to take the larger blow-down of those two."
Like all modulars, the Coyote...
Like all modulars, the Coyote uses exhaust manifold studs, not bolts. Enlarged to 10mm from 8mm on the 4.6, the Coyote studs are larger to resist shear loads from shrinking headers. After thousands of heat cycles at high mileages, the manifolds can shrink and pull on the studs, something the 8mm stud apparently can't quite handle. Also, the nuts are prevailing torque pieces and carefully torqued to less than maximum clamping load. This allows the manifold flange to float slightly when glowing hot, retaining a gas-tight seal and not shredding the gasket or overly stressing the manifold. The prevailing torque nuts also resist turning by the ballooning hot manifold. Now you know why your 4.6 header bolts are always coming loose.
"So we've separated the 180-degree cylinders because we have enough length that we have fixed the Vol-F on all those cylinders so they scream. And the 90-degree pairs are also happy in terms of volumetric efficiency-but they have a pumping hit. So that's the best trade-off; basically, if you have to be that short, this is the type you want to have," he said.
"I have to hurry up and apply for a patent on these, 'cause no one else builds them this way," Adam confessed. "Our peak Vol-F, which is at peak torque, is 110 [percent]. It depends on the dyno cell, right, but we've hit as high as 110, 108, so it's pretty impressive. And at peak power we're pretty close to 100. I don't know, typically 98, 99 [percent]."
Certainly the end result is impressive. "Torque is almost 400 lb-ft out of 5.0 liters; no one else comes close. And it's these type of things that help-the intake runner lengths, the port volumes-because we could have gone with a super-short intake and sold out all the torque to go for peak power. It's those small details, the TiVCT, those are the things that let us get that kind of torque," Adam elaborated.
Or, in the words of Gary Liimatta, "This is a really, good engine, but it is the culmination of a many, many, many small details all pointing in the right direction. The successes we've had are by very hard work."
Some of the hard work in places far from the exhaust paid off in delivering these headers as production pieces. Asked how a stainless steel tubular header compares to a cast-iron exhaust manifold in cost, Mike Harrison spoke right up. "To run a fabricated tubular header on a production engine is a decision that's not taken lightly, and we revisited it on a number of occasions."
Politically, headers are highly visible targets to the cost-cutters. "They are more than double the cost of a cast manifold," explained Mike. "At 6 hp it's hard to justify, but we wanted to build the best 5.0-liter engine out there. And sitting with the [management] team and educating them on the details, defending them ... but we set up cost targets early in the program and we hit the cost targets, so there really was no leg for the more senior management team to stand on. In fact, if I had been overrunning my costs, I would have had to give something up and [the headers] would have been it. [But] we were able to contain this within the overall cost target of the engine, so we were able to deliver our metrics, and, you know, that helped."
Durability is another tubular-header concern. "The problem is the manifolds grow with heat and this pipe tries to pull the short one right out of the collector," explained Adam. "Those rear, short primaries need to twist, so it grows to the rear." These durability concerns led to some of the otherwise non-optimal intersections around the Coyote headers collectors.
Another issue is getting a tube header to work at the vehicle assembly plant, where the tools are huge and time is precious. The header, "has to be durable, work, and can be assembled [with workable] decking zones and tool paths," said Adam. Coyote engines are fitted to Mustangs from the bottom at the plant and are the widest part of the engine, so tucking them close to the engine was important.