Bolting it all together — we were getting Stoked On Stroke!

Words and Photos By: Richard Holdener

How exactly do you define a hot street small-block? The most obvious answer is the one that manages to destroy some unsuspecting Mustang in the other lane, but for our needs, it might be helpful to get a tad more specific. Back in the heyday of the Muscle-Car Era, the magic number often tossed around was one horsepower per cubic inch. Credited as the first small-block to achieve this goal, the 283-hp 283 was a mouse-motor milestone, but it was soon eclipsed by larger and even more powerful versions.

Eclipsing the one-hp-per-inch mark were the 350-hp 327, the 360-hp 350, and the top-rated 375-hp 327. Never mind the power numbers were often generated as much by the marketing department as the dyno, these factory power plants were just the starting point. With help from the aftermarket, Chevy owners soon found ways to improve upon the factory power. Stepping up from the 350-375 hp factory ratings to the 400-hp level quickly became commonplace. This was the start of the power escalation, as 400 hp soon became 450 hp, then 500 hp, and so on.

The problem with this type of power escalation is that power and problems often go hand in hand. The most common sacrifices when power levels climb include idle quality and overall drivability. Big peak power numbers are almost always possible, but combining them with reasonable street manners is a completely different ball game. Let’s use some simple math to help illustrate the point.

Using a typical 350-inch small-block as an example, suppose we tune it to produce 350 hp (one horsepower per cubic inch). Such a combination would combine an abundance of low-speed torque with a smooth idle and excellent drivability. Stepping up in power to a 400-hp 350 (1.14 hp per inch) using ported heads or the proper camshaft might decrease idle and drivability slightly, but such trade-offs would be minimal. The components required to eclipse 450 hp (1.285 hp per inch) or 500 hp (1.428 hp per inch) would certainly tax the levels of acceptable drivability. Shifting torque production higher in the rev range with ported heads, wilder cam timing, and single-plane intakes has a negative effect on both low-speed power production and drivability.

The problem with enthusiasts is we want the big power numbers of the wild combo AND the drivability of the milder one. The question now, is it possible to get race-motor performance with street-motor drivability? For our needs, we defined race-motor performance as 600 hp, which we hoped to combine with street manners acceptable enough for a daily driver. Making things more interesting was the fact we wanted this power to come without resorting to boost or nitrous. The key to the success of any performance motor is the proper combination of components, but the real key to the success of this build was adding inches. Our math examples told us trade-offs increased in relation to the specific output. That is to say a motor that produced one horsepower per cubic inch would offer improved idle quality and overall drivability compared to one that produced 1.40 hp per cubic inch (or more).

If we know drivability is inversely proportional to specific output, what happens when we ask a small-block sporting just 350 cubes to produce 600 horsepower? The resulting specific output would be an incredible 1.709 hp per inch? That level of elevated specific output would almost certainly produce a wild, cantankerous race machine.

Rather than attempt to coax 600 hp from a 350, we decided to actually reduce the specific output by increasing the displacement. Rather than ask the 350 to produce 1.709 hp per inch, (or 1.566 hp/in with a 383), we dropped the specific output down to 1.405 hp per inch by building a small-block sporting no less than 427c.i. This is roughly equivalent to a 491-hp 350.

The Speedmaster block allowed for even larger displacements, but we purposely left room (bore diameter) for future rebuilds. Knowing increased stroke beyond the proposed 4.0-inch mark required significant clearancing, we decided the combo worked best at 427c.i. Besides, how cool is it to have a big-block like, 7.0L 427 tucked under your small-block valve covers?

Naturally, the build started with a four-bolt, race block supplied by Speedmaster. Production Chevy blocks cannot be bored .125-over to produce the desired 4.125-inch bore. Working with the race block was a 4.0-inch, forged steel, stroker crank, 6.0-inch, forged steel rods, and an SFI damper from Speedmaster. Though we chose an iron block for this build up, Speedmaster also offers an aluminum version for those looking to save some weight. Completing the rotating assembly was a set of flat-top pistons from JE and a Total Seal ring package.

For machine work, balancing, and assembly of the short block, we turned the 427 over to the guys at L & R Automotive. Additional components employed on the 427 included a new oil pan, pick up and windage tray from Moroso, a 292XFI HR13 cam, and 1.6-ratio roller rockers, both from COMP Cams. The hydraulic roller cam offered a .584/.579 lift split, a 242/248-degree duration split, and a 113-degree lobe separation angle. COMP Cams also supplied additional valve train components, including hydraulic roller lifters, double roller timing chain, and hardened pushrods.

Naturally, a small-block sporting big-block dimensions required plenty of head flow. This is especially true when running milder cam timing to maintain drivability.  To feed the 427, we installed a set of 220 Race Ready heads from Airflow Research. Peak flow from the AFR heads checked in at 320 cfm for the intake and an equally impressive 230 cfm for the exhaust. For even more power, AFR offered a Competition version that promised an extra 10-15 cfm from a revised valve job and more detailed porting. The 220cc AFR heads offer airflow number that match many off-set rocker, race heads, but we liked the fact these AFRs required no special hardware. Basically, these AFR 220s were the ultimate bolt-on performance head with flow numbers capable of easily supporting our goal of 600 hp.

In addition to the elevated flow rates, the AFRs also featured 75cc combustion chambers (allowed us to keep the static compression below 11.0:1 with a flat-top piston), a 2.10/1.60 stainless steel valve pack with lightweight 8mm stems), and a dual valve spring package with titanium retainers. So equipped, our hydraulic roller 427 had no trouble running cleanly to 7,000 rpm.

For street use (even street/strip), we would normally recommend a dual-plane intake. To reach our power goal, we stepped up to the more powerful single-plane design. Supplied by Speedmaster, the single-plane Shootout intake was designed to maximize power production higher in the rev range than a typical dual-plane intake. Given the massive displacement, the Shootout intake seemed like the ideal choice for our big-block sized small-block. A dual-plane might make more low-speed power, but there would be no shortage of low-speed torque from our big motor. Besides, we hated the thought of missing our power goal by just a few horsepower, even if it meant having extra torque down at 3,000 rpm.

Hedging our bets even further, we shipped the Speedmaster intake out for porting to further improve the flow rate. Why go to all the trouble of running heads that flow 320 cfm if you are going to choke them down with an intake that flows less, right?

The 427 was completed using nothing but quality components that included a Holley 950 HP carburetor, a full MSD ignition system (including a billet distributor), and Meziere electric water pump. In addition to the distributor, MSD also supplied a 6A ignition amplifier and complete set of 8mm plug wires. To cover their powerful cam and timing chain, COMP Cams supplied a two-piece aluminum front cover, cam button, and cast aluminum valve covers.

Stoked on Stroke 427 Small Block
Did we reach our goal of 600 hp with the Stoked on Stroke 427? You bet. The 7.0L produced peak numbers of 605 hp and 577 lb-ft of torque, but just check out that torque curve. Street use is more a function of the torque curve, which (in this case) exceeded 550 lb-ft from below 4,000 rpm to 5,700 rpm. Note also how the power curve carried out past 6,500; though you won’t need to, you can rev this baby safely all the way to 7,000 rpm.

Before running in anger, the 7.0L was treated to a few break-in cycles after pre-lubing the oiling system using an electric drill (cheap insurance against abnormal wear). After two computer-controlled, break-in cycles lasting 30 minutes, we drained the conventional (break-in) oil and replaced it with Lucas 5W-30 synthetic.  After minor adjustments to timing and jetting, the 427 demonstrated the worth of additional displacement by producing peak numbers of 605 hp at 6,500 rpm and 577 lb-ft at 5,200 rpm. Torque production exceeded 550 lb-ft from 4,000 rpm to 5,600 rpm. With more than 600 hp and a massive torque curve, is it any wonder we were so stoked on stroke?

Sources: Airflow Research, airflowresearch.com; COMP Cams, compcams.com; Holley/Hooker, holley.com; JE Pistons, jepistons.com; L&R Automotive, lnrengine.com; Moroso, moroso.com; MSD, msdignition.com; Speedmaster, Speedmaster79.com.