As RoadRunner development continued, it met an unexpected challenge: The Coyote, which was finishing up its development, was making more and more power. This put more pressure on the RoadRunner crew--most of whom worked on Coyote as well--to better themselves after just putting a maximum effort into the base 5.0-liter engine.
Mike recalled the progression of his two new V-8 engines by saying recent engineering advances mean everyone expects his teams to spool up ever-increasing power ratings, but it isn't easy.
"When we went through the product approval gateway, [the RoadRunner] was approved at 430 hp. And at that time, the base 5.0-liter was going to be 400. That 30hp delta was really where we felt [we needed to be] for a limited-edition car, and then, unfortunately we kept getting better and better on the base 5.0-liter and closing that gap. So, we just had to really... you know, the development work that went on in the end to find the last 1 or 2 hp on the dyno was pretty intensive... I think within a couple of months of the first media launch at Laguna for the historic races last year we committed to 440, and they were like, 'Well, we knew you were going to deliver that.' So we managed to scrape another 4 hp over and above my final offer--which was hard to find."
"The cool thing about doing this was that everybody was into it. This wasn't a burden to anyone. Everyone on the program--the complete Mustang Team, the complete engine team--were all fired up about doing the Boss 302 again, and that makes it really easy to get whatever you want done because people want to work on it."
Was there anything the enthusiastic RoadRunner team wishes they could have done to the Boss 302's prime mover? "People ask, 'What else would you have done?'...and there's really not a lot left."
Finishing the Boss 302 combustion...
Finishing the Boss 302 combustion chambers via CNC machine helps in several ways. The locating pad between the outer edges of the exhaust seats is eliminated, several areas around the chamber are unshrouded, and mainly, the chambers are consistent one to the other and accurately placed. In fact, half the intake flow gains are due to the chamber modifications, plus the more consistent chamber volumes mean more accurate and less invasive spark retard from the knock sensors. The compression ratio remains 11:1 because the basic Coyote/Boss combustion chamber is already at maximum brake torque. That means given pump gasoline it doesn’t respond to an increase in ignition timing or compression, and increasing either means decreasing the other to avoid detonation.
Sectioning a Boss 302 cylinder...
Sectioning a Boss 302 cylinder head means trained eyes can likely see the floor of the exhaust port looks a little low on the short-side radius—the surprising path to power as it turned out. Ford’s CNC machines finish the port after the valve seats are installed, so all ports and seats are matched and accurately placed relative to the chamber and valve guides. It’s not so important here, but CNC finishing the septum—the nose of the splitter dividing the ports—allows getting just the right shape where it counts in the intake port. All said, the Boss intake port gained about 4 percent flow, while the exhaust picked up “huge gains at the low lift,” lost a little at mid-lift, and then picked up again at high lift. On average, the Boss exhaust port outflows the Coyote by an amazing 10 percent. As Adam Christian put it, “The last 4 in the 444 hp is from the exhaust [port].”
Cast from a different aluminum...
Cast from a different aluminum alloy and changed in almost every respect, the Boss 302 cylinder head is definitely a Coyote descendent, but still its own animal. While it still passes as a bolt-on replacement for a Coyote head, the finished Boss casting is stronger and worth about 10 hp more than a Coyote, thanks in part to CNC porting and a larger exhaust valve.
What sets the Boss 302 engine apart from its Coyote starting point is its high rpm. No other production Ford engine has revved this high--7,500 rpm--much less lived through Ford's rugged durability tests at such speeds. The key piece of hardware enabling the rpm is the short-runner intake manifold, but almost every aspect of the 5.0-liter Coyote was addressed to meet the RoadRunner's frenetic gait.
Another Boss 302 fundamental is higher cylinder pressures--it's simply the physics of making more power. As a consequence, in many places inside the Boss 302 not only are things happening faster due to the higher rpm, greater pressures are being generated, such as in the cylinders or at the bearings. This, too, translates into strengthened parts.
For this review, we'll use the 5.0 Mustang GT engine, the Coyote, as the starting point. If a part isn't mentioned here, such as the block, you can reasonably assume it was carried over unchanged from the 5.0. Curiously, the Coyote's piston squirters were one of the first things to go because at high rpm they resulted in massive windage. There was just so much weight of oil whipping around in the crankcase that it slowed the crank and rods, costing horsepower.
But without the piston squirters the Coyote's hypereutectic pistons were on the durability edge. Cylinder pressures were higher, plus piston temperatures climbed enough to invite a forged piston with its fine grain structure and lack of porosity. Interestingly, given Ford's immense engineering resources, short-block engineers Jay Bolyard and Claudio Battistini didn't waste time testing just how far into forged-piston territory the Boss was reaching. They knew the Coyote's hypereutectic piston was out of its league, and a forged piston could handle the Boss's pressures and stresses, so they specified a forging right away.
With a heavier piston and higher rpm, it was also obvious a stronger piston pin was required. This turned out to be no more difficult than reaching into the GT500 parts bin for the 5.4's bulldozer-spec unit. It is slightly shorter in length but slightly thicker, built from a stronger material, and nitrited for surface hardness.
Taking another look at the...
Taking another look at the cutaway Boss 302 head, this time on the intake-port side, reveals the general streamlining and gorgeous blend between port, valve seat, and combustion chamber. It’s tough to remember this is a production Ford cylinder head. Another CNC advantage is it allows a multi-angle valve seat. Ford calls it a five-angle valve job, but there’s actually a sixth angle hiding in there. It’s just another step toward the holy grail of fully radiused valve seats. Intake port volume remains at 193cc, same as the Coyote, to avoid velocity losses at low rpm. It’s going to be interesting to see how racers and hot rodders can improve these heads.
Comparing cutaway Coyote valves...
Comparing cutaway Coyote valves at top to their RoadRunner counterparts at bottom easily shows the Boss part’s hollow stems. It’s also just possible to see the Boss exhaust valve’s slightly flatter head design at bottom compared to the Coyote’s more tulip-shaped valve at top. After lightening the Boss exhaust valve, it was necessary to use a sodium filling to better transfer heat from the head to the area of the stem surrounded by the valve guide; obviously the sodium filling is not shown here.
Boss 302s use this 37x31.8mm...
Boss 302s use this 37x31.8mm valve package and upgraded beehive valvesprings. Necking the valve stems down to improve airflow through the port was tried, but it didn’t prove worth the effort. Sharp eyes might be able to spot how the valve heads are cut back to improve low-lift flow. This tactic was especially productive with the exhaust valve, which uses a 40-degree seat instead of the more typical 45-degree seat to promote low-lift flow.