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How to Upgrade a 421 Small Block

Concealed Weapon

Steve Dulcich Oct 28, 2013
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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.

Vital Specs: Dr. J’s 421 Chevy Small-Block
Block: Dart SHP
Bore: 4.155 inches
Stroke: 3.875 inches
CID: 421 cid
Compression Ratio: 11.5:1
Crankshaft: Callies Compstar
Camshaft: Isky Hyd Roller
Timing Chain: Cloyes
Lifters: Isky Hydraulic
Pushrods: Manley
Springs: Manley 22441-16
Installed Ht: 1.900
Spring Load, Closed: 170-pound seat
Spring Load, Open: 500 pounds at 1.100 inches
Retainer: Manley 10-degree Titanium
Valve Locks: Manley 10 degrees
Duration Intake: 258 degrees at 0.050
Intake Lift: 0.640 inch
Duration Exhaust: 262 degrees at 0.050
Exhaust Lift: 0.640 inch
Rockers: COMP Cams
Rocker Ratio: 1.6:1
Lobe Separation: 108 degrees
Installed Centerline: 105 degrees
Ring Pack: CP 1.5/1.5/3 mm
Ring Endgap: 0.020 Top/0.025 Second
Piston: CP bullet Pistons
Piston Clearance: 0.005
Main Fasteners: ARP/Dart
Rods: Callies Compstar 6.00 H-Beam
Main Bearings: Clevite H
Rod Bearings: Clevite HN
Main Journal Diameter: 2.49 inches
Rod Journal Diameter: 2.100 inches
Main Clearance: 0.003 inch
Rod Clearance: 0.003 inch
Cylinder Heads: AirWolf 220
Chamber Volume 67 cc
Head Fasteners: ARP Custom Studs
Intake Manifold: Dr. J’s Performance Hurricane
Intake Port Flow: 325 cfm at 0.700
Exhaust Port Flow: 244 cfm at 0.700
Intake Valve: Manley 2.125
Exhaust Valve: Manley 1.600
Cylinder Head Gaskets: Fel-Pro 1144 (0.041 inch)
Piston/Head Clearance: 0.040 inch
Engine Gaskets: Fel-Pro 1206
Carburetor 950-cfm Holley Street HP
Carb Spacer HVH
Header: Schoenfeld
Coil MSD Ignition
Ignition MSD Ignition
Spark Plug Wires: MSD Ignition
Damper: Professional Products
Water Pump: Meziere Electric
Oil Pan: Moroso Oil Pan
Oil Pump: Melling M55a
Oil: Dello 400 30w

Melling Oil 8/17

Adequate lubrication is ensured by the improved oiling system of the Dart block, while a conventional Melling oil pump pressurizes the system.

Airwolf Cylinder Heads 9/17

Taking 23-degree cylinder head flow to extremes are the AirWolf heads from Dr. J’s. Peak flow measures 333/260 cfm, and at the lift used in this build the engine is still tapping into the 320/240 range. Very impressive for a conventional head that does not require shaft rockers.

Moroso 10/17

The large kick-out–style Moroso pan reduces power-robbing windage losses while substantially increasing the oil capacity.

Valvetrain Isky Manley Comp 11/17

The completed valvetrain with components from Isky, Manley, and COMP Cams deliver the high-rpm stability needed for serious power numbers. The Manley springs are installed to deliver 170/500 pounds of spring load.

Intake 12/17

The wide-open intake ports provide a generous minimum cross-sectional area of 2.65- inches—the kind of size needed to make power upstairs.

Fel Pro Multilayered Steel Head 13/17

Fel-Pro’s multilayered steel head gaskets seal the combustion gases, aided by the clamping of a custom set of ARP studs. With the piston at 0.001-inch positive deck, the quench clearance works out to 0.040 inch.

Intake Manifold 14/17

A Dr. J’s ported "Hurricane" intake manifold helps the engine breathe deep, thanks to its fully ported and flow-balance interior. The intake accepts a 4150 carb configuration.

Dyno Results
Chevrolet 421 Small-Block
DTS Engine Dyno Tested At Dr. J’s

3,000 451 257
3,100 466 275
3,200 482 294
3,300 493 309
3,400 506 327
3,500 520 346
3,600 532 364
3,700 541 381
3,800 547 396
3,900 554 412
4,000 560 426
4,100 566 442
4,200 571 456
4,300 574 470
4,400 576 482
4,500 577 495
4,600 581 509
4,700 584 522
4,800 589 538
4,900 593 553
5,000 596 568
5,100 596 579
5,200 596 591
5,300 595 600
5,400 592 609
5,500 590 618
5,600 587 626
5,700 583 632
5800 583 644
5,900 580 651
6,000 579 661
6,100 573 665
6,200 569 672
6,300 563 675
6,400 557 679
6,500 549 679
6,600 543 682
6,700 531 677
6,800 519 672
6,900 506 665
7,000 494 658

Cylinder Head Airflow
SuperFlow 600 Flow Bench
Dr. J’s AirWolf 220
2.125 intake; 1.600 exhaust; cfm at 28-inch water depression

Lift Intake Exhaust
0.100 78 65
0.200 155 116
0.300 217 162
0.400 266 196
0.500 302 219
0.600 320 233
0.700 325 244
0.800 328 252
0.900 331 257
1.000 333 260


Inside the plenum, the detailed port work is revealed. In addition to high flow, the manifold is designed to optimize flow balance and fuel distribution.

Holley 950 Street Hp 16/17

For high-rpm power, a carb of sufficient capacity is a must. This Holley 950 Street HP offers substantial airflow and strong throttle response.

Chevy Small 17/17

When it was all said and done, this deceptively simple looking Chevy small-block delivered 682 peak horsepower at 6,600 rpm, and a phenomenal 596 lb-ft of torque at 5,200 rpm on 91-octane California pump gas. CHP


Automotive Racing Products
Ventura , CA 93003
Comp Cams
Memphis, TN 38118
CP Pistons
Irvine, CA 92614
Cloyes Gear And Products Inc.
Ft. Smith, AR
Manley Performance Products
Lakewood, NJ 08701
Mahle Motorsports
Fletcher, NC
Professional Products
Hawthorne, CA 90250
Callies Performance Products
Fostoria, OH 44830
Melling Engine Parts
Jackson, MI 49204
Meziere Enterprises
Escondido, CA 92029
Isky Racing Cams
Gardena, CA 90248
Moroso Performance Products
Guilford, CT 06437
Dr. J's Performance
Orange, CA 92868
Holley Performance Products
Bowling Green, KY 42101
Dart Heads
Troy, MI 48084
Southfield, MI 48034
(248) 354-7700
HVH (High Velocity Heads)
Knoxville, TN
MSD Performance
El Paso, TX 79936



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