One horsepower per cubic inch. Back in the 1950s, that was the target. Those newfangled overhead pushrod engines that revolutionized how passenger car engines were created only served to heat up the power-per-cube race. When the new Chevrolet big-block engine was introduced in 1965, it lit the fuse for bigger power and renewed the focus on the power-per-cubic-inch wars.
Today, making big horsepower is still the ultimate goal; the one-horsepower-per-one-cube of displacement target is but a mere dot in the rearview mirror of old aspirations. But complications to the engine-building process have surfaced recently. For many builders, the engines they create must make excellent power and still be compatible with today's low-quality pump gas (91-octane). In addition, they must be able to generate adequate vacuum and excellent daily-use drivability. Just as today's performance cars are compatible with formerly antiperformance style options, such as A/C and power steering, most current engines don't need 114-octane race fuel to avoid detonation while hitting the big power figures. The fact is that no engine builder wants to tell his clients that their freshly built engines require such coddling. Why should they when proper component selection will help them avoid such behavior?
A Little Big-Block History
The big-block Chevy engines of today come in a variety of shapes and styles. While the Mark IV, as introduced in 1965, went through some initial changes, it was not until 1991 that the Mark V engine came to be. Today, the Mark V and its brother, the Mark VI, which debuted in 1996, are great platforms for engine builders, offering great versatility and availability over the seemingly harder-to-find Mark IV engines. Featuring a strong internal webbing, integrated oil pan gasket, and one-piece round rear main seal to avoid oil pan drips, the Mark V was perfect for our needs. Paired with the hot performing RHS Pro Action aluminum cylinder heads, we were excited about the potential for what we believed would be an excellent combination for our street-based project.
The selection of the Mark V platform was the work of engine builder Jim Shewbert, who was hired to create the potent big-cube engine for Ted Yurek's '70 Chevelle. The car, while outfitted with a number of advanced suspension upgrades, was a true dual-purpose machine; both track and cruise time were planned for the beautiful blue-and-SS-stripe-clad heavy Chevy. While the engine was assembled in Southern California, the Chevelle was expected to run on pump gas once dropped between the inner fenderwells of the Coeur d'Alene, Idaho-based Chevy. Owing to the huge amount of distance between L.A. and Idaho, it was imperative that the engine be powerful and reliable.
The Build Up
The building of the Mark V engine was not unique, for the most part-a fairly straightforward assembly using Scat 6.385-inch 4340 H-beam connecting rods, Akerly & Childs extreme rings, and 4.5-inch bore Keith Black Hypereutectic aluminum pistons. The pistons feature a 1/16-inch top moly piston ring and similar-sized cast-iron second ring. The pistons have a flat-top, making for excellent flame propagation, and when used with a 9.800-inch deck block, generate 10.25:1 compression. An Eagle forged steel crankshaft was used with a 4.250-inch stroke, making for 532 cid total displacement (540 cid for you government workers).
The cylinder heads were state-of-the-art RHS Pro Action 360cc aluminum heads. The heads started plenty clean from the factory with excellent airflow and internal water-jacketing for consistent temperature control, and were ported to the next level by Toby Allison. Because of the high horsepower that Jim wished to generate, he felt a Toby porting job would help to eek out the engine's top horsepower numbers.
In the valvetrain department, Shewbert clearly wanted a big camshaft to take advantage of the high-flow cylinder heads and horsepower and torque-handling attributes of the super-strong Mark V short-block (see CamQuest 6 sidebar). To that end, he selected a camshaft that featured large lift and duration numbers requiring a mechanical roller valvetrain, which included COMP's new Endure-X mechanical roller lifters. In combination with the big camshaft, the COMP Cams tech line folks (1-800-999-0853) helped him select appropriate valvesprings, retainers, lifters, spring cups, lash caps, and even the Pro Magnum roller rockers that were to be used with this combination.
Topping the engine was a Dart intake ported by Allison and a Demon reworked 850-cfm carburetor. Big-tube exhaust headers and low-restriction Flowmaster tailpipes were slated for use, along with a five-speed manual transmission and high-performance clutch.
On To the Dyno
The folks at Westech Performance were hired to test the limits of our engine. Engine builder Jim Shewbert installed the camshaft with 4 degrees of advance, along with the Dart intake, Demon 850-cfm carburetor, and MSD ignition. Using 91-octane fuel, Westech's Steve Brule fired the engine and spent a considerable amount of time warming the engine, then resetting the valves and warming the engine again with some short low-rpm pulls. After an hour of break-in time, they proceeded to make some test runs to see if we had done the proper homework.
The Mark V required a few jetting changes, and we adjusted the timing slightly during our four test pulls. In the end, our 540-cid Mark V big-block turned out an impressive 649.2 hp at 6,200 rpm and 613 lb-ft of torque at 4,900 rpm. The most impressive part of the engine's power and torque output was the consistent power delivery. From 5,700 through 6,700 rpm, the engine maintained a minimum of 640 hp-less than a 10hp variance for over 1,000 rpm. In addition, the torque level reached 590 lb-ft of torque starting at 4,100 rpm, and stayed above that level through 5,700 rpm. This is usable street horsepower that would be more than up to any race challenge encountered by the classic Chevelle.
In the end, we were quite happy with the performance level of the 540-cid engine. Making 650 hp on pump gas is no small accomplishment, and one that proved-street or race-big-block Chevrolets have deservedly earned their reputation for relatable power, even the often-overlooked Mark V.