Cylinder heads are where the power's at. It's a principle so fundamental to producing horsepower that even not-so-smart engine builders have figured it out. Using this logic, where cylinder head airflow reigns supreme, the short-block merely serves as a container for cylinder pressure whose primary purpose is harnessing the power of the combustion process without blowing up. Substantially increasing airflow through the cylinder heads by throwing a couple of 88mm turbochargers into the induction package takes this concept to the extreme. Although the short-block itself adds very little to the horsepower tally, blowing it up means all that yummy airflow goes to waste. Pressurizing a 547ci big-block Ford to 15-20 psi of boost and gunning for 2,000-plus horsepower requires a seriously stout container to hold it all together. That's why we shipped stacks of premium aftermarket parts to the School of Automotive Machinists (www.SAMRacing.com) to make it happen.
Five years ago, the School of Automotive Machinists (SAM) built us a 532ci big-block Ford good for 786 hp and 9.81-at-141 mph timeslips in our street/strip '93 Mustang project car. While we had grown quite fond of that pump-gas motor that logged over 100 dragstrip passes without a hitch, changing from a strict diet of barometric pressure to a gluttonous diet of positive manifold pressure mandates revamping the entire combination from top to bottom. This goes far beyond lowering the compression ratio and beefing up the internals, as the bearings, clearances, ring gaps, quench, and camshaft design must all be changed to play nice with the additional heat and cylinder pressure associated with turbocharging.
Since this isn't an engine combo that will compete in any specific class—but rather something intended to run 7s in a street-friendly package—we weren't bound to a rulebook when scheming up its specs. So while we could have stroked our Ford Racing A460 block up to 598 ci, we settled on a smaller 547ci total instead. Evidently, bigger isn't always better. According to several Outlaw 10.5 racers we consulted with, turbo big-blocks in the 520-540ci range tend to work quite well on small-tire applications. Likewise, a seemingly marginal increase in displacement (between 565 and 572 ci) can make a car running 10.5-inch-wide tires much more difficult to hook up coming out of the hole. As such, we retained the 4.300-inch Eagle crankshaft from our prior 532, and matching it up with a 4.500-inch bore, netting a total of 547 cubes. We considered an even smaller bore, but the 2.450-inch intake valves in the Trick Flow A460 cylinder heads we will be running require a 4.500-inch minimum bore diameter.
Complementing the Ford Racing A460 block and Eagle steel crank are Oliver 6.700-inch E4340 billet connecting rods, and custom 8.5:1 MAHLE forged pistons. Thanks to the 10.3200-inch deck height of the big-block Ford architecture, the pistons boast a generous 1.405-inch compression height, which allows for thick ring lands. It also enables pushing the top ring down and away from the piston crown, further isolating it from combustion heat.
Sharp mathematicians have probably noticed that a 4.300-inch stroke, 6.700-inch connecting rods, and pistons with a 1.405-inch yields .065-inch of deck clearance on a 10.320-inch deck height A460 block. When combined with a .045-inch-thick head gasket, this nets .110-inch of quench clearance, which is almost three times as much as what's typically seen in a naturally aspirated combination. Both in theory and in practice, tight quench clearance creates turbulence within the combustion chamber that improves air/fuel homogenization, decreases the potential for detonation, promotes a quicker burn rate, and increases power. Notwithstanding, SAM Instructor Chris Bennett says, tight quench isn't always desirable in a forced-induction combination. "In an engine with a lot of swept volume, you have to run a very big dish to lower the compression ratio. MAHLE put a 36cc dish in these custom pistons, which is about as big of a dish as you want to put in a piston," he explains. "To get the compression down to 8.5:1 in this 547, in addition to having a big piston dish you have to run relatively loose quench. In any nitrous or boosted application, you want to slow the burn rate down as much as you can. Having a lot of quench and softening the transition in the piston crown and combustion chamber helps slow the burn rate down, and broadens the tuning window as well."
Providing a solid foundation for the twin-turbo 547 is a Ford Racing A460 block (see Part 1 in the August issue for a complete walk-around of this solid piece.). To prep it for prime time, SAM students and instructors bored and honed the cylinders, deburred all sharp edges, align honed the mains, the installed the cam bearings.
In the unlikely event of catastrophic parts failure, circulating shrapnel through the oil system only increases engine damage. To prevent this from happening, SAM epoxied custom screens over the oil drainbacks in the lifter valley.
Since off-the-shelf pistons capable of handling 2,000 hp are hard to find, MAHLE designed a custom set for our 547. From top to bottom, these impressive 2618-alloy forgings are built to endure intense cylinder pressure and heat. They feature an extremely thick crown, and a hard-anodized top ring land that prevents micro-welding and allows running a tighter vertical ring clearance. The skirts use MAHLE's proprietary Grafal coating for reduced friction and enhanced cylinder seal.
On the bottom side of the pistons, the support struts extend from the piston skits to the pin boss for additional reinforcement and strength. The heavy wall tool-steel wristpins provide serious support for the connecting rods. MAHLE's phosphate coating serves as a dry lubricant and gives the pistons their signature gray appearance. It provides a lubricant film in the pin bores and ring grooves to improve lubrication at start-up.
To help endure the extreme heat and cylinder pressure generated by turbocharging, MAHLE matched the pistons with a nitrided stainless steel top ring and a ductile iron second ring. The additional forced-induction heat calls for larger ring gaps as well. "In a naturally aspirated motor, the top ring gap is usually set at .0045 inch for every inch of bore diameter. In a forced-induction motor, we set the gap at .0065 inch for every inch of bore, so on a 4.500-inch bore motor the top ring gap comes out to roughly .029," Chris Bennett explains.
At 1,500-plus horsepower, billet steel or aluminum rods are the only viable option. Since aluminum rods are iffy in street-driven combos, we opted for Oliver 6.700-inch billet units. Built from aircraft-grade E4340 alloy, Oliver's Big-Block Max rods have proven reliable in countless 3,000hp turbo big-block applications. To achieve this level of strength, they employ a parabolic beam design, and are subjected to a multistep heat treat process that produces a very consistent grain structure.
Clevite H-series main and rod bearings are particularly well-suited for high-load, high-rpm operation. They have enlarged chamfers for additional crankfllet clearance, steel backings, a high crush factor, and medium level eccentricity. As with the ring gaps, the rigors of forced induction require larger-than-usual bearing clearances. "Generally, you want to run about one thousandth of an inch of clearance for every inch of journal shaft diameter in a naturally aspirated engine. With forced induction, you open that up .0005- to .0007-inch more," Chris Bennett says.