Along the same lines, cutting material from the backside of the exhaust valve head helped because the team could see the airflow was more sideways, across the valve head, rather then the textbook example of bending around the head in a classic tulip shape.
Of course, valves are part of the valvetrain, which was handled by a separate team of engineers, principally John Carter, Mark Nowak, and Kevin Shinners. Their story is similar to the rest of the RoadRunner team. Having just finalized the Coyote valvetrain, they turned right around and re-engineered it for the Boss 302's higher rpm and loads, plus accommodate its more voluminous breathing.
Computer modeling is the engineering...
Computer modeling is the engineering turbocharger that has enabled the tremendous advances in automotive technology we take for granted today. This is a screenshot from one of Beth Ann Dalrymple’s intake manifold “analyticals” or simulations. It models the air velocity and direction in a single thin slice through the intake’s plenum. The simulation takes several seconds to run through 720 degrees of crankshaft rotation to bring every cylinder into play, resulting in a surging wash of multi-colored arrows. This simulation easily identifies dead areas of slow air movement, or confused eddies of seemingly directionless air that can be energized by reshaping the plenum.
Intake manifold guru Beth...
Intake manifold guru Beth Anne Dalrymple is a died-in-the-wool, three-car-owner Mustang enthusiast who didn’t bother auditioning for the RoadRunner team but just started working on it. She generated many hundreds of analytical models such as the ones seen in this article when designing the already iconic Boss tunnel-ram intake.
In this simulation of the...
In this simulation of the Boss intake, the velocity data is displayed in smoothed color gradients. Directional information is not so clear, but the surging interplay of the intake air among the eight intake runners is more defined. Spotting lazy areas, such as to the outer plenum walls just in from the throttle body opening, is thus quite simple. Again, this is a screenshot from a short video representing 720 degrees of crankshaft rotation; red denotes the highest velocity, blue the slowest.
It really didn't take much new hardware to meet those goals. Of course, the Coyote valvetrain architecture had just been well laid out for performance to begin with. Or, as Kevin put it, "Once you have a valvetrain architecture, you just can't change it to get another 5 hp."
One of the more important points is the Coyote was designed to accept up to 13 mm of valve lift but used only 12 mm, so there was a spare 1 mm of valve lift available. Kevin laughed, recalling the extra capacity the Coyote team had built-in. "Someday we'll use this," he recalled thinking. "And someday turned out to be much sooner than we expected!" It's a good thing the RoadRunner team delivered a solidly stable valvetrain because it could easily have been a mechanical rpm limit. Without a stable, high-rpm valvetrain all of the 7,500-rpm intake tuning would have been for naught. Because the induction benefitted so much from the new intake manifold, the cylinder head and valvetrain didn't require much. The porting was optimized, as we just heard, but the intake valves were simply hollowed to reduce weight.
To keep pace, the exhaust valve diameter was increased 0.8 mm and it's face angle lowered from 45 to 40 degrees, both to improve breathing. The exhaust valve was also hollowed to help the valve-springs at high rpm, but a little weight was regained with sodium filling for cooling.
Titanium was briefly considered as a valve material, but it proved too much for multiple reasons. "We looked at titanium," said John Carter, "but there was a packaging issue and timing of the program; trying to keep some of the other carryover components, we couldn't package titanium. It was around getting a lash cap on the [valve] tip. The keeper grooves were pretty close to the top; I didn't have any room to make the rocker arm any wider." Tim added, "There are not a ton of people who are going to do titanium in our volume," which is a good illustration of just how much high-end technology Ford is delivering in the Coyote and Boss 302 for the money.
No changes were necessary to the roller-finger followers (rocker arms) or lash adjusters. "We didn't do anything unique for Boss," Kevin said. "It was already downsized [for Coyote], so low mass and good stiffness [were already available]. It would have to be really exotic to make any improvements here."
Definitely unique to the Boss, however, are the valvesprings. As lightweight as the hollow stem valves, the extra energy from the 7,500-rpm redline plus the extra 1mm of exhaust valve lift demanded more control from the valvesprings.
We should specify that for a valvetrain specialist such as John, Boss engine speeds do not end at the production car's 7,500 rpm. "We have a design guide so that if the fuel shut off is 7,500, we want the valvetrain to be stable to 400-rpm over that in case of over-speed events, missed shift on a downshift, or whatever. [This is to] make sure that if there is a momentary over-speed, the valvetrain doesn't come apart. If you throw a rocker arm or bend a valve--and it doesn't take long to do that under the wrong conditions--then you can lose your whole engine." Told that one of the Grand Am racers had managed to wing a RoadRunner to 9,000 rpm on a missed downshift with no apparent damage, Mark's commented, "I can only warranty it to 7,900, but I'm glad it worked!"