What exactly is the definition of a hot, street small-block? From a production standpoint, the magical performance number seemed to be any engine that offered at least 1 horsepower per cubic inch. The 283hp 283 is credited with being the first to achieve this goal, but Chevy followed up with even more powerful combinations. During the musclecar era, Chevy offered a 350hp 327, 360- and 370hp 350s, and capped it off with a fuel-injected 327 rated at an amazing 375 horsepower.
Regardless of the fact that the marketing department was every bit as responsible for the power rating as the engineers, factory outputs were just the starting point. Thanks to the aftermarket, enthusiasts quickly found ways to improve upon the production powerplants. Acceptable power output for streetable small-blocks began to escalate. If the factory offered 375 hp, then shouldn't enthusiasts be able to tune for 400 hp? If 400 hp is good, then isn't 450 hp even better? Or how about 500? Much like the Cold War, escalation was both inevitable and dangerous.
As always, power and problems go hand in hand. Serious enthusiasts always want more of the former, but there is often a price to pay for it. Some simple math works well to illustrate the trade-offs. A typical 350 tuned to produce 350 horsepower (1 horsepower per cubic inch) would offer a smooth idle, excellent driveability, and an abundance of low-speed torque. Depending on the chosen components, a 400hp 350 (1.14 hp per inch) might decrease idle and driveability slightly in exchange for the additional power, but the tradeoffs would minimal. Jumping up past 450 hp (1.285 hp per inch) to 500 hp (1.428 hp per inch) requires some serious components, not the least of which would be high-flow cylinder heads, and an aggressive cam designed to produce peak power much higher in the rev range. These combinations also shift the torque curve higher in the rev range. The downside is the shift reduces low-speed torque production. The wilder cam lobe profile required for these elevated power numbers also has a negative effect on idle, driveability, and fuel mileage.
The trade-offs inherent in high-horsepower small-blocks clearly illustrate the difference between stock and race motors. Obviously, many performance street/strip motors fall somewhere in between. The question now is, how do we get race-motor performance with street car driveability? How do we get our normally aspirated small-block to produce an honest 600 hp, and still allow us to drive it on the street? The answer obviously includes the proper combination of performance components, but the real key to success comes from displacement. You will remember from our simple math problems that the trade-offs in driveability increased with the specific output. That is to say, a motor that produces 1 hp per cubic inch would offer improved idle quality and overall driveability compared to one that produces 1.40 horsepower per cubic inch (nerd speak for the simple statement that mild powerplants drive better than wild ones).
The key to producing 600 hp from a streetable small-block is to reduce the specific output. This is accomplished by increasing the displacement. Using our 600 hp as an example, it is possible to reduce the specific output of the 600hp 350 (1.709 hp/in) down to 1.566 hp/in with a 383 stroker or even down to 1.477 hp/in with a 406 small-block. Looking at this trend, we chose a big-block-like displacement of 427 cubic inches for our streetable small-block. This results in a specific output of 1.405 hp per inch (the equivalent of a 491hp 350). It is possible to go even bigger in displacement, but we wanted to be sure to leave sufficient bore diameter for future engine work. Additional stroke required significant clearancing, so we decided that our 600hp street motor would work best at 427 cubic inches.
Think about having a big-block tucked under your small-block valve covers. How cool is that? It might even be worth running a set of factory valve covers that read 283! How's that for the ultimate sleeper?
For this buildup, we were less interested in the sleeper mode than producing 600 hp, while maintaining the proper idle and driveability that constitutes a real driver. When it comes to idle and driveability, the single biggest concern is the cam lobe profile. The problem is the cam lobe profile is also a major player when it comes to power production. Cam lobe profile and power/driveability are inversely proportional. Power production requires increased lift and duration, while idle quality and driveability require just the opposite. The key to the success of our build was choosing a cam lobe profile that provided enough lift and duration to achieve 600 hp, without the penalties associated with a lumpy race cam. In the end, it was our choice of the other components that allowed us to choose a relatively mild cam profile. High-flow cylinder heads, a fully ported intake manifold from Dr. J's, and even the displacement itself all combined to provide a powerful combination that did not require an aggressive cam profile to achieve our goal.
Starting with the basics, the short block consisted of a four-bolt iron race block from Procomp Electronics. In addition to the four-bolt mains, the beefy block also provided the required 4.125 bore. Procomp Electronics also supplied the 4.0-inch forged-steel stroker crank, which combined with the 4.125-inch bore produced the desired 427 cubic inches. The 6.0-inch forged-steel rods (also from Procomp) were attached to a set of flat-top pistons from Probe Racing. L&R Automotive was responsible for the machine work, balancing, and assembly of the short block. Additional components included a set of performance piston rings from Total Seal, a new oil pan/pick-up/windage tray from Moroso, and SFI damper from Procomp Electronics.
Being a dedicated street motor, we chose a hydraulic roller cam from the Comp Cams catalog. The 292XFI HR13 offered 0.584/0.579 lift split, a 242/248 duration split, and a 113-degree lobe separation angle. The lift figures were generated with 1.6-ratio rockers and took full advantage of the airflow offered by the 220 Race Ready heads from Airflow Research. Comp Cams also supplied the hydraulic roller lifters, double roller timing chain, and one-piece hardened pushrods for our test motor. Combined with the flat-top pistons, we achieved a net compression ratio of 10.8:1, a little high but totally liveable for 93-octane pump gas.
Cylinder heads were another critical element in the success of our healthy 7.0L small-block. The key to producing the desired amount of power with the relatively mild cam timing was lots of head flow. Insufficient head flow requires excessive cam duration to achieve a given power output. Our aluminum 220 Race Ready heads from Airflow Research offered nearly 320 cfm on the intake and 230 cfm on the exhaust. For serious racers looking for those extra 10-15 cfm, the 220 heads are also available in a Competition version that includes a revised valve job, and more detailed porting.
The 220 heads offer airflow numbers that match many offset rocker race heads but don't require offset rockers and retain all the standard valve and stud locations. These are the ultimate bolt-on performance heads with flow numbers that will support well over our intended goal of 600 hp. Additional features include 75cc combustion chambers (allowing 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. The spring package allowed our motor to rev cleanly to 7,000 rpm.
Normally we would recommend a dual-plane intake for most street applications, but the additional displacement combined with our desire to reach 600 horsepower steered us toward a single plane. Procomp Electronics provided one of its single-plane Shootout intakes. Designed to maximize power production higher in the rev range than a typical dual-plane intake, the Shootout intake seemed the ideal choice for our big-block-sized mouse. A dual-plane might make more low-speed power, but since these intakes were designed for your typical 350-inch small-block, the typical single-plane/dual-plane trade-offs are less pronounced on larger motors. Besides, we hated the thought of having the motor make 580 hp, even if it did offer more torque at 3,500 rpm. So concerned were we with the 600hp target that we further improved the flow rate and power potential of the intake by shipping it off to Dr. J's for porting. According to Dr. J's, the porting can be worth 20-25 hp on the right application. Who can't use an extra 20-25 hp?
Knowing we couldn't skimp on even the small components, we finished things off with a set of 1.6-ratio Ultra Pro Magnum roller rockers from Comp Cams, a Holley 950 HP carburetor, and a full MSD ignition system, including a billet distributor (with centrifugal advance), an MSD 6A ignition box, and complete set of 8mm plug wires. Also present was a two-piece aluminum front cover, cam button, and cast aluminum valve covers from Comp Cams, a Meziere electric water pump, and a set of 1-3/4-inch long-tube headers. As is our practice, the new motor was given a proper break-in procedure after prelubing the oiling system using an electric drill. This is cheap insurance against abnormal wear and tear during the initial start-up. After 25 minutes of alternating load and engine speed, we performed an oil change from the conventional stuff to 5W30 synthetic that included a new K&N oil filter. Initial runs were promising, and after running through minor jetting and timing loops, we were eventually rewarded with peak numbers of 605 hp at 6,500 rpm and 577 lb-ft at 5,200 rpm. Torque production exceeded 550 lb-ft from below 4,000 rpm to 5,600 rpm. Plans are to stick this stroker in an early Chevy II chassis. That should make one heck of a street car!