“There is no replacement for displacement,” as the saying goes. While power adders are fun, sometimes, the application calls for brute strength. Such is the case with this engine build brought to us by our friend Jeff Huneycutt.
Put together by Prestige Motorsports for a customer who will eventually install it into an airboat (yes, an airboat), the engine has several special considerations and interesting touches which make it a very unique project.
“The point of this engine is to create an astounding amount of power out of a naturally aspirated pump-gas engine,” explains Huneycutt. “The Prestige team decided to do it with displacement. It’s an all-aluminum engine, so even with its large size, it’s still lighter than a smaller iron-block combination.”
10.7 Liters of Brute Strength
Building an engine with such a large amount of displacement can really only be done in one of two ways — with a block with altered bore centers allowing for a much larger bore, or with a tall-deck block allowing more stroke length from the crankshaft. For this engine, tons of RPM is not in the performance envelope of the engine, so stroke is the obvious source of the additional displacement.
In order to use a crankshaft with a lot of extra stroke, you need a block with additional deck height. For that, the team turned to Brodix and the 11.100-inch deck height version of its aluminum big-block engine block. Made in the big-block Chevrolet style, this well-engineered chunk of metal has extra-thick sleeves to accept a 4.600-inch bore, a .400-inch raised cam tunnel, and, of course, the enlarged 11.100-inch deck height.
In addition to the out-of-the-box features, Prestige also machined the block for oil squirters. Spraying a jet of oil on the underside of each piston will help keep everything cool since this engine will be run hard for extended periods.
To get the massive stroke demanded, a Callies Magnum crankshaft was chosen, both for its ability to provide a 5.00-inch stroke (and is available in even larger stroke-lengths) as well as its immense strength. The 4340 steel undergoes multiple heat treatments, has eight counterweights which are fully profiled and lightened rod journals.
That crank is held into the block by the billet-steel four-bolt main caps included with the Brodix block. Between the cap material, the splayed outboard bolt configuration, and beefed up webbing, the combination should hold up to a lot of use and abuse.
Attached to the crankshaft is a set of 7.00-inch billet-steel I-beam connecting rods from Oliver. The rods use an upgraded 7/16-inch rod bolt that is specifically designed to handle stupid amounts of power in tall-deck applications thanks to revised design geometry.
Containing all the cylinder pressure in this combo is a set of custom 4.600-inch JE pistons. In order to keep the compression ratio at a pump-gas-friendly 10.4:1, massive 44cc dishes are incorporated into the slugs. The pistons feature a .043-inch ring package also from JE. The standard top ring is gapped to .023 inch and the Napier second ring is gapped to .025 inch.
All Of The Airflow
To top off the monster short-block, the Prestige realized that the engine would need every last possible cubic-foot of airflow. For that, they chose a set of AFR’s 18-degree Magnum cylinder heads. Starting with massive 457cc intake ports that have been raised .500-inch, and 143cc exhaust ports, AFR then added 2.400-inch intake and 1.800-inch exhaust valves.
Between the port’s shape and volume, the flowbench shows almost 500 cfm of airflow at 1.00-inch of lift and stout flow curve up to that point, making the Magnum 18-degree heads the highest flowing heads in AFR’s lineup.
The heads are secured to the block with a set of ARP head studs, which will have their hands full in such a robust combination. Handling the valves is a set of dual 240 lb/in valvesprings with 650 pounds of open pressure, coupled with titanium retainers and 10-degree valve locks.
Controlling the valves is a custom COMP Cams solid-roller camshaft, driven by a Jesel belt drive. With .717-inch gross lift on the intake side, and .715 inch on the exhaust, coupled with .050-inch duration numbers of 274 degrees, intake, and 282 degrees, exhaust, and a 110-degree lobe-separation angle, the camshaft moves a lot of air.
Since those cam specs were actually a revision to add lift, late in the build process, the team had to go in and manually enlarge the valve reliefs on the pistons so the intake valve is sure to clear everything.
Bam solid-roller tiebar lifters ride on the cam lobes, with 7/16-inch diameter, .120-inch wall-thickness Manton pushrods — 10.0-inch intake, and 10.7-inch exhaust — engaging the rocker arms. A set of T&D Machine aluminum 1.7:1 ratio shaft rockers physically actuate the valves, and are set to a .004-inch cold lash, which will open up to a .022-inch (intake) and .024-inch (exhaust) hot lash.
Fitting A Cast Intake To A Tall-Deck Block
As if the build wasn’t interesting enough up to this point, the team realized that in its intended environment (an airboat), the vibrations would more than likely crack the welds on a sheetmetal intake manifold, which are the norm for an 11.1-inch deck application.
Instead, Prestige decided to adapt one of AFR’s cast aluminum Magnum single-plane intake manifolds (designed for a 10.2-inch deck height) to the engine. First, they machined the flanges to accept O-rings, eliminating the need for traditional intake gaskets.
Then, Prestige machines a set of intake spacers, along with a set of custom china wall spacers for the front and rear of the block to properly support the intake manifold. Not finished with the custom work, the customer also wanted to run a traditional distributor, which most places would call impossible in this application. However, with some careful measuring, cutting, and welding, they were able to pull it off.
To fuel the fire, A 1475cfm Holley Gen-3 Ultra Dominator race carburetor was bolted to the intake manifold, along with a one-inch thick four-hole carb spacer. A Moroso vacuum pump system was also fitted to ensure a more-than-adequate vacuum supply for the engine.
Putting this bad boy on the engine dyno, Prestige knew it was going to make a ton of power, but wanted to do a little testing as well. In addition to the normal BBC dyno headers, the team also wanted to dyno test several variants of the actual custom airboat headers that will be run on the engine, as well.
The first thing Prestige did was set a relatively conservative RPM limit on the tests, due to the long stroke. If you read the Bore Vs. Stroke article we recently published, the method for calculating piston speed (the main concern when limiting RPM in a long-stroke application) tells us that for this combo, 6,200 rpm is pushing the piston speed pretty hard. However, in the airboat application, the most the engine should see is mid-5,000 rpm.
In the first pull with the dyno headers, the engine made 730 lb-ft of torque at the start of the test, at 3,500 rpm. Peak torque was 910.7 lb-ft at 5,000 rpm. Peak horsepower was 984.5 at 6,200 rpm, and still climbing.
Switching to the first iteration of the custom 2.25-inch primary, 2.375-inch-step airboat headers saw an increase in power and torque to 5,700 rpm. Peak torque increased almost 16.5 ft-lb and occurred slightly earlier, while peak horsepower dropped to 961.6 horsepower.
Adding 12-inch extensions to each primary tube saw almost no difference across the curves, with a slight reduction in peak torque and horsepower values, proving that the initial airboat header design was the best for this engine, especially considering it will probably never see above 5,700 rpm in the real world
Regardless of what it’s in, this engine is an absolute brute and definitely shows what a long-stroke and a lot of displacement is capable of.