How to Upgrade a 421 Small Block

Concealed Weapon

Steve Dulcich Oct 28, 2013 0 Comment(s)
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We all know what to look for when scoping out the high-powered competition, whether on the street or the track. The visual clues to big power couldn't be more obvious. The well-trained eye will quickly tune into hardware like a trick set of spread-port 18-degree heads, or a Dominator carb. Your ears will pick up the sharp crackle of a high-compression ratio, and the distinctive clatter of a mechanical valvetrain. The air will be filled with the distinctive scent of burnt race fuel. Yeah, you might think the telltales always give away the game. In the case of this deceptive street small-block Chevy from Dr. J's Performance, you'd better not let down your guard. While it sports ordinary-looking 23-degree cylinder heads, a single 4150 carb, a hydraulic valvetrain, and burns pump unleaded, this isn't your ordinary 400hp street small-block.

Building It
The crew at Dr. J's approached the build with the idea of taking what seems like ordinary street hardware to the next level. In a normally aspirated engine combo, this necessarily means bumping up the cubes, and turning up the wick on rpm capacity. To do this reliably requires parts that can take the punishment. The foundation for the build is one of Dart's stout SHP blocks, a unit that has the capacity for a large bore size, and comes with a revised oiling and enough metal in the right places to go the distance. The block was taken to a bore size of 4.155 inches, and mated with a 3.875-inch Callies Compstar crankshaft for a displacement of 421 ci. While more stroke can be stuffed into the Dart block, the moderate arm and large bore are right on target for reliability and longevity as the rpm fly.

Dart Shp 2/17

Filling the bores are a set of CP Bullet flat-top pistons wrapped with their 1.5, 1.5, 3mm ring package. Bryce Mulvey of Dr. J's tells us, "I've tried a lot of pistons and the CP slugs always seem to deliver just a little more power. The Bullet line gives me high-end features at a price comparable to ordinary pistons, so it is a good performance value." The pistons swing from a set of Callies 6.00-inch Compstar H-beam rods, completing the block's internals.

Of course the bottom end can be built like an anvil, but without the right combination of power parts, all that hardware is just along for the ride. To take advantage of the solid bottom end, the cam, valvetrain, heads, and induction have to deliver the goods. Mulvey tapped Isky for a custom hydraulic roller measuring 258/262 degrees at 0.050 inch, cut on a 108-degree separation, and good for 0.640-inch lift with a set of COMP Cams 1.6:1 rockers. Providing the valvetrain control is a set of Manley springs delivering 170/500-pound spring load, while the balance of the valvetrain consists of Isky hydraulic-roller lifters, and Manley pushrods, retainers, and locks.

Topping the short-block are the key parts that bring the whole package together, starting with a set of Dr. J's AirWolf 220 cylinder heads. These conventional 23-degree castings deliver flow capacity typically reserved for more exotic stuff. The fully CNC-ported heads unleash 325 cfm at 0.700-inch lift from the intake ports, while the exhaust moves 244 cfm at the same lift, breathing through 2.125/1.600-inch valves. That's the kind of airflow capacity that can produce serious power. Feeding the cylinder heads is a Dr. J's fully ported "Hurricane" intake manifold topped with a Holley 950 Street HP carb.

Callies Comptar Forged 3/17

On the Dyno
With the fire provided by an ignition system from MSD, and exhaling though a set of Schoenfeld dyno headers, the 421ci mouse was put to the test on Dr. J's DTS dyno. Getting into the upper 600hp range from 421 ci of small-block means turning some revs. From the numbers, the top end parts definitely have the capacity to feed the engine's high-rpm appetite, but where many builders fall short is in the cam and valvetrain when running a hydraulic roller. Right from the jump we could see this thing just worked upstairs. Peak torque was coming in at about 5,200 rpm, with plenty of twist on tap, closing in on the 600 lb-ft mark. With torque coming up that high in the rpm range and in such massive quantities, the only thing that could prevent big horsepower at the top is a cam and valvetrain that can't keep up. The Isky hydraulic roller setup proved its worth by pulling clean and sharp up to our test redline of 7,000 rpm. The dyno sheets showed 682 hp coming in at 6,600 rpm, and a usable powerband right up to redline. That kind of firepower makes this innocent-looking street small-block armed and dangerous.

Cp Bullet Series Pistons 4/17

The CP Bullet Series pistons are rugged and yet lightweight, and come with the advance piston shape and design CP is famous for. The flat-top pistons deliver 11.5:1 compression with 67cc heads. Rods are 6.00-inch Callies Compstar, while the 1.5, 1.5, 3mm rings come matched from CP.

Isky Hydraulic Roller 5/17

Bryce Mulvey of Dr. J’s tells us the Isky hydraulic-roller lifters in conjunction with their cam prove very stable at high rpm, a critical aspect to making high-rpm power with a hydraulic.

Isky Hydraulic Roller 6/17

A custom Isky hydraulic-roller profile operates the valvetrain, featuring specs of 258/262 degrees duration at 0.050, and 0.640-inch lift. The lobe separation angle is 108 degrees.

Cloyes Adjustable Timing 7/17

Turning the camshaft is a Cloyes adjustable timing set, dialed in for an installed intake centerline of 105 degrees.




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