"Bob put me in touch with Bill Barr, who was one of the powertrain engineers. He next became a manager at Ford and now he's retired," Mike said. "He lives down in Florida, comes up in summer, and... we just kind of hit it off. We went to the bar and talked on how the original Boss 302 came about ... [how] that engine was tooled, with the production one element, but then the racing models with the big ports had a lot more potential..."
Looking much like a Coyote...
Looking much like a Coyote connecting rod, the Boss 302 rod is pressed from upgraded powdered metal and is considered absolutely bulletproof. The rod cap continues with a fractured parting line, and the rod bolts are carryover as well. The latter is possible because Ford wanted the good Boss 302 bolts in the Coyote rods from the beginning.
Coyotes, like the 4.6 and...
Coyotes, like the 4.6 and 5.4 modulars, use a relatively hard aluminum bearing. They are durable, but for the increased loads in the Boss 302, scuff resistance is key, so more compliant copper-lead bearings were used. In fact, these are the main and rod bearings from the 5.4-liter in the Shelby GT500, so no engineering was required to reinvent this wheel.
Strength through forging is...
Strength through forging is the big deal with the Boss 302 piston at left. Compared to the hypereu-tectic Coyote piston at right, the Boss piston is mechanically much stronger so it can withstand the Boss’s higher pressures and rpm stresses without the Coyote piston squirters. The other Boss piston detail difference is the valve reliefs—they accommodate the RoadRunner’s larger exhaust valve. Also upgraded is the top piston ring, which has a spray plasma inlay unique to the Boss’s increased scuff and durability needs.
"What we decided... was to have the street car get a legitimate race car engine as well so that the typical customers didn't really have to do a lot of additional work to it--tear it down, port it, do all the other stuff you have to do to a hot-rod engine in order to deliver, you know, credible track performance," Mike revealed. "So, the basis for the performance development was, certainly, to have a high-revving engine. We wanted it to be around 7,500 rpm, maybe a little higher. We wanted that, but we didn't want to trade off that low-speed torque because we know that only five percent of Boss 302 customers are ever going to take them to the track day or whatever. A lot of them are only going to be driven on Sundays, gently, and preserved, and we wanted that driving experience to be pretty good, too."
As he was talking, Mike had a few Boss parts and documentation on hand. Picking up a dyno sheet, he continued: "This is the power curve for the base 5.0 liter, and as you can see, the power curve turns over pretty hard at 6,500. And then the Boss manifold with an approximately 2 inches shorter runner, it really keeps going. We rev limit it at 7,500, that's just about where it's making its peak power, but much above this, the computer system has a hard time... So, the engine's got a lot of potential, and it's that short-runner manifold that sets up the characteristic of the curve, and instead of tuning for 6,500 as peak power [in the Coyote] it tunes for 7,500. ...we have some trade-offs in the midrange torque, but it's only 10 lb-ft and it really isn't that noticeable. It feels linear. ...it's actually got more torque at 1,000 rpm--it's just the nature of the tuning. And so you kind of lose a little in the middle, and pick up some at the bottom and you pick up some at the top."
As the highest-revving production Ford V-8, the RoadRunner posed some issues. One of the first was to the development dynos. As Mike explained it, "We'd never spun anything this fast in the dyno cells and... the jackshafts and all of the system wasn't really set up to take it. I think the Ford Racing cell was--it was set up with a lightweight driveshaft and a clutch system." That Ford Racing cell used to run Formula One engines and is now used for NASCAR powerplants. It could easily handle the Boss 302 prototypes, but it was only one dyno cell, and several were needed to run the multiple-durability-test RoadRunners.
Furthermore, the RoadRunner had to pass all standard Ford durability tests. "This isn't like something we'll run for 50 hours and if it passes that it's good to go. We ran the regular fatigue tests, thermal tests, piston-scuff tests, everything like that," noted Mike. So the durability test dynos had to be upgraded. "We kind of had to do a better re-engineering of our durability cells because... Ford Racing hasn't run any long-term durability and we were just shaking dynos apart every 20 hours."
Eliminating the Coyote’s piston...
Eliminating the Coyote’s piston squirters was as easy as fitting these four plugs to the carryover Coyote aluminum block. Each squirter features two arm-like nozzles and a single feed hole; the bolt-in plug closes off the feed hole and is held in place by the bolt, which uses the same threaded mounting hole as the squirters.
Inspecting the bottom side...
Inspecting the bottom side of the Boss 302 piston uncovers piston-maker Mahle’s logo and a generally fine-yet-strong structure. Careful attention to the pin boss and associated bulkheads assures this critical area can handle 7,500 rpm as long as you can dish it out. While it definitely looks lithe and slippery here, the Boss piston actually has more skirt area than the Coyote’s slug.
Forces increase dramatically...
Forces increase dramatically with rpm—for proof, consider the Ford engineers when the Coyote clutch pressure plate and flywheel kept vibrating apart at high rpm. They had to safety-wire the clutch bolts to keep the pressure plate in place during testing. The production cure was to go from six to nine pressure-plate retention bolts in an oddly staggered pattern. Otherwise, the flywheel follows Coyote practice, with eight bolts at the crankshaft and neutral balance. At the other end of the crankshaft, the harmonic damper is uniquely balanced for the Boss 302’s high-energy torsional vibrations.
"But if the engineers in the trenches could identify the high-revving engine as a Boss 302 must-have and then design the necessary intake manifold to achieve that rpm, that didn't mean management was going to approve the intake manifold. "When all the planners saw the torque curve, they all said, 'No way. Standard manifold! There is absolutely no way we're putting that in the program. There is no way we're going to lose any low-speed torque--the driveability would be awful.'"