So, when Ford says redline is 7,500 rpm, their valvetrain engineers are really aiming closer to 8,000 rpm. John detailed the spring design process. "Because of higher lift, the lobe profile on the exhaust side, and the higher speed, we went to a stronger spring. To handle the stress from the higher speeds and the higher lift, we improved the grade of steel used in the spring, going to one with a higher tensile strength so we wouldn't have fatigue failure problems."
"The spring is typically designed so that at max lift there is not a lot of room for the coils to surge up and down. The base spring (Coyote) was designed for 12mm lift, and there is only another millimeter before it goes completely solid, which wouldn't have worked, so we lowered the solid height on the spring so we could get to the 13mm lift and still have a half a millimeter of clearance before it went fully solid. So in addition to the higher load, we had to take out some coils."
Like the intake valve, the intake camshaft needed no changes and was carried over from the Coyote. Only the exhaust cam needed that extra 1mm of lift, a bit of a job for cam-lobe specialist Kevin. He cited the short time available and the need to maintain all of Ford's durability requirements when taking an already aggressive cam profile and increasing its lift, while not changing its duration.
"It was a challenging assignment, but exciting to push the envelope and do something you haven't done before. ...It still meets all the durability, NVH concerns, seating velocities, roller contact forces. All the dynamic constraints we use for a normal production engine in a Taurus, Focus, or whatever, we're still meeting all of those metrics with RoadRunner."
Not because it's the first thing you see when you open a Boss 302 hood, but the Boss 302 intake manifold is the absolute key player in setting the RoadRunner engine's rev-happy personality. That's because the tuned intake runner lengths of this carefully designed, shall we say, tunnel-ram intake, set the tuning peaks and valleys the rest of the engine runs to.
Like just about anything that is involved in making things go bang in the combustion chamber, we found Adam Christian at the core of setting the all-new intake's performance goals. He identified and shuffled the available power peaks, playing a crafty game of corporate poker in the process.
"Coyote cannot make power any higher than 6,500--the intake manifold shuts it down. This one (the Boss 302 intake) won't shut it down until 7,750. And the reason that I picked 7,750 is that there are actually... you've seen the torque curves on Coyote, there are three very distinct peaks, wiggles in the curve, and I call those 'teeth on a sprocket' So they are tuning modes of the runner; if you change the length of the runner enough, you can actually move one whole tooth."
Any sort of fluid flow can...
Any sort of fluid flow can be modeled in Ford’s computers. This analytical of the Boss 302’s righthand exhaust header and catalytic converter is color-coded by cylinder. The direction and relative velocity of the header’s internal gas flow patterns are easy to see. When running this simulation, we were surprised at the amount of reversion, or backward flowing gas, and that the gas often breaks into separate segments.
Ford was really thinking of...
Ford was really thinking of you with this 13mm thick spacer plate at the H-pipe flange. It exactly mimics aftermarket dump valves, so if you want to add the valve, it’s a simple bolt-on swap for the spacer plate. The thin blocking plate with the small hole keeps the sound level to within 0.1 db of the legal limit. You can keep it as is, drill the hole larger for more sound, or remove it for the most sound—and it sounds really, uh, boss with it out. Horsepower isn’t affected in any case as there is no meaningful gas flow through the sidepipes, which are purposefully mounted 90 degrees to the main gas flow at the H-pipe. Think of the sidepipes as the exhaust equivalent of the induction sound tube—fun, but nothing you’ll see on a stopwatch.
One look under a Boss 302...
One look under a Boss 302 explains how the quad exhaust is laid out. The regular exhaust system remains unchanged, but with the sidepipes extending outward from heavy mating flanges, through flexible couplings, and finally to down-turned tips. The flanges and a single bolt out at the rocker panel are the only attachments, adding more just introduced buzzes and moans into the cockpit. The same goes for the expensive flex coupling—without it, the system drives way too much buzzy, loud exhaust energy into the car.
"So I took the tuning peak on Coyote that was at 5,250, moved it to 6,500, took the one that was at 6,500 and moved it to 7,750. And what that did for us was... at the time we didn't know if the hardware could support 7,500 rpm. At the time we started, we thought it might be a 6,500-rpm engine. So we said, 'OK, we'll get about 4 hp from going to this style--we'll give up a lot of torque--but we'll get about 4 hp and we'll protect for 6,500 by putting the tuning peak there. But then we had this other one that's way out there. And we all kinda... I mean it was the plan," Adam said. "Basically we gave the company enough rope to hang itself. Because once they start dyno-testing and they start seeing the power out there, then there's this relentless push to get all the way to 7,800 basically. And ultimately it was the PCM that was the one thing left that couldn't take it anymore as far as engine speed.