5.0 Mustang & Super Fords
2013 Ford Shelby GT500 Trinity 5.8L V8 - Power Of Three
How Ford's Special Vehicle Team stuffed 100 extra non-gas-guzzling, emissions-legal, Warrantied horsepower into the 2013 Shelby GT500 engine
The new piston is supplied by Mahle. While a conventional design, it does feature thick bulkheads to better support the piston dome. The angled geometry of these bulkheads is such that the small end of the connecting rod would no longer fit, so the small end of the rod was ground off at an angle, with the top of the small end narrower than the bottom of the piston-pin bore in the rod.
"Of course, if you change one thing you change another, and narrowing the small end of the rod closed the edge distance from the piston pin oiling hole in the top of the rod so much that Brendan Vido's computer modeling showed the rod became weak. Luckily the oil squirters put so much oil onto the bottom of the piston that there's no need for the oil hole in the 5.8, so the hole was deleted and rod strength preserved. Otherwise, the 5.8 connecting rod is unchanged from its 5.4 starting point.
Likewise, the main and rod bearings are unchanged. The 5.4 was the last Ford engine using a high-quality, tri-metal bearing insert, and it runs just fine in the 5.8. The tri-metal bearings are more expensive, but "really forgiving of any debris, contamination, high load, all that kind of stuff." This is a good example of the premium nature of the 5.8--even the new Coyote and its hot-rodded RoadRunner offshoot use the standard aluminum-backed modular bearing.
Because the larger, stronger 5.8 pistons are heavier than the 5.4 variety, the crankshaft balance falls out of tolerance on most 5.8 engines. This is handled by inserting a heavy tungsten weight into the front counterweight on 99 out of every 100 5.8 crankshafts, according to SVT.
Such balance concerns take on more, ahem, weight, when considering the mandate for a 7,000-rpm over-rev capability. The Achilles' heel of the modular engine family is its small cylinder-bore capability. We've beat this topic to death in our Coyote and RoadRunner stories, but as a quick refresher, the modular's 100mm bore spacing limit puts an emphasis on long piston strokes, and that in turn means high piston speeds. The 5.4/5.8 modular stroke is 13mm (0.510-inch) longer than the 5.0 Coyote/RoadRunner, for example, which is why the GT500 redline has traditionally been 6,250 rpm. Even the vaunted '00 Cobra R 5.4--a raging, naturally aspirated beast if there ever was one--saw fit to quit at 6,250 rpm, and all because that's an F1 or NASCAR-like piston speed with the 5.4/5.8 stroke.
So here comes management with a need for an occasional 7,000 rpm over-rev capability. That's 24.7 meters per second, or 4,861 feet per minute for us old guys, and let's just say that's way, way out there for a production engine. The result is a need to keep everything as light as possible in the piston and ring packaging. It also underscores SVT's labeling the 7,000 rpm as a trick up your Nomex sleeve and not a daily habit (anyone who can rev a 2013 Shelby to 7,000 rpm on a daily basis is living large on any account...). We asked Jeff Albers if the Copperhead PCM counts time over 6,250 rpm or anything like that, and he said no. "You can go there as often as you want," but if coolant temperatures exceed 251, the computer will progressively limit throttle opening and rpm.
The idea of the over-rev is track-oriented. “We maintained a continuous rev limit of 6,250 rpm,” Albers explained, “which is the same as the 5.4, but we allow a temporary over-rev up to 7,000 rpm to facilitate things like quarter-mile runs where you want to stretch the motor a little bit more, or say on a race track, a road course, where you might want to hang onto a gear through a corner and not shift, that sort of thing. So, we give time at the higher rpm, enough for any reasonable use of the car and the engine, but limit the amount of time up there just to limit exposure to those high rpm.”
Relatively little work was required to turn the 5.4 cylinder head into the 5.8 head. In fact, from a power standpoint, the ports handled the increased airflow delivered by the increased boost and more aggressive camming, so no porting work was required. What was left for the SVT engineers was making the exhaust valves live through the resulting higher temperatures. This took development of the valves, valve seats, and cooling system.
We’ve already touched on the coolant passage improvements and that after extensive computer analysis the only mechanical changes were drilled passages to promote coolant flow between the exhaust valve seats. This sounds simple, but the engineers were set on evening coolant flow as much as possible throughout the cylinder head, and that entailed many computer simulations.
In the end, they were pleased with the 5.8’s top-end cooling as they got water flowing evenly between the exhaust valve seats, plus the coolant flow balance between the engine’s two banks is an even 49 percent on the right and 51 percent on the left (this is affected by water pump direction of rotation, which might also partially explain why cylinder No. 3 is always the hottest). Furthermore, this was accomplished with the first physical modifications as the development work was done completely in the computer. This saved tremendous time and money.
Getting the exhaust valve to live was done with improved materials and a little extra mass. Incredibly, the extra mass found on the combustion chamber side of the valve head is there simply to withstand the hellacious cylinder pressure. The original concave profile of the 5.4 valve deformed at 5.8 temperatures and pressures, so the head of the valve was made slightly thicker.
The harder valve material is Nimonic, trade name for a super steel alloy, along with a Stellite ring inlaid into the valve’s seating face. The Stellite is fitted to a groove machined into the valve face, and welded and machined in place. In the cylinder head, the exhaust valve seats were upgraded from W236 to W100 Stellite. That’s truly hard stuff normally found in “dry-gas” applications such as propane-burning engines. In the 5.8, the W100 is there strictly for its increased surface hardness as lubricity was not an issue.
To support the extra airflow coming off the 5.8’s larger supercharger, SVT employed the age-old expedient of reaching for an already developed and proven camshaft, or in this case, all four cams from the 5.4-liter Ford GT engine. These are more aggressive grinds than the 5.4 GT500 sticks, as seen by the gain of 1.1 mm of intake and 1.4 mm of exhaust valve lift, and the engineers say they really helped. No other valvetrain modifications were necessary, so the intake valves, all valvesprings, retainers, roller-finger rockers, lash adjusters, timing sprockets, timing chains, tensioners, and pulse wheels are carryover 5.4 parts. The front timing cover is carryover as well, as are the valve covers and even the ignition system. Well, to help combat gap growth, the 5.4 platinum-tip spark plug has been replaced by an iridium-tipped plug.