When it comes to forced induction, perhaps the best thing about owning a supercharged Chevy is the ability to literally dial in the desired amount of horsepower. If the motor is assembled properly using the right performance components, it is possible to adjust the boost pressure to achieve the desired output.
To illustrate the power gains offered by supercharging in general and then by pulley changes in particular, we decided to build a supercharged stroker and subject it to the rigors of the dyno. What follows is the build up of the dedicated big-block Chevy street stroker, along with the effects of adding a supercharger. After installing the Holley 420 Mega Blower, we took it upon ourselves to adjust the boost pressure by altering the blower drive ratio (via pulley changes).
After it was all said and done, we increased the power output of our already impressive normally aspirated stroker motor by a whopping 223 hp. What enthusiast wouldn't want an extra 223 hp?
The Holley/Weiand Roots blowers offer more than visual horsepower, as instantaneous boost provides immediate response and a wave of torque. The immediate boost response and impressive low-speed torque is one reason the Roots blowers continue to be so popular. When it comes to getting that big Chevelle, Camaro or even Impala moving in a big hurry, there is nothing like the feeling of a blown big-block. Taking the more-is-better route one step further, we decided that the only thing better than a big-block is an even bigger big-block. Combining additional cubic inches and forced induction can elevate your powerplant right to the top of the proverbial performance food chain.
Our test subject started out life as a 461 big-block. The 461 was originally equipped with a set of ported 049 oval-port heads, an emissions-legal Crane cam (203/212 duration) and an Edelbrock Performer 2-O intake. So equipped, the 461 produced 432 hp and 542 lb-ft of torque. For our needs, the 461 had two strikes against it in terms of performance as an optimum blower plant, namely the displacement and static compression. The displacement was too low while the compression was too high. Though the 461 offered decent low-speed torque production, our plan of attack was to build an even more impressive combination capable of bettering both the normally aspirated horsepower and torque production of the 461 while simultaneously allowing the combination to run effectively and effortlessly with our Holley 420 Mega Blower.
Given its mild state of tune, bettering the power output of the original 461 was not particularly difficult, as all we had to do was improve the efficiency of a few key components, namely the cylinder heads, cam and intake manifold. While adding free-flowing heads, a wilder cam and a high-rise intake manifold would surely improve the peak horsepower number, we also wanted to improve the torque peak-without sacrificing the impressive low-speed power already present.
Making life even more difficult was the fact that we had to achieve all this while dropping the compression down near 8.25:1 in preparation for boost. The game plan chosen was to combine the required low compression with an increase in displacement and efficiency. The additional cubic inches came from a stroker crank offered by Coast High Performance. The stock 4.0-inch (454) crank was ditched in favor of a cast 4.25-inch crank. Completing the stroker assembly was a set of forged I-beam rods and Probe Racing pistons. The forged pistons featured a single intake valve relief, as there is generally plenty of piston to (exhaust) valve clearance. The 4.25-inch crank was combined with the .030-over pistons to create a final displacement of 489 cubic inches.
We made full use of the valve relief in the Probe Racing forged pistons by replacing the wimpy hydraulic flat-tappet cam run previously in the 461 with a custom hydraulic roller cam. Comp Cams supplied the Xtreme Energy hydraulic roller profile, which offered a 236/242 duration split at .050 along with over .600 lift. The intake featured a tad more lift, checking in at .646, while the exhaust was down only slightly to .623. The cam featured a wide 114 lobe separation angle, helping produce a broad power band and minimizing overlap. The custom Comp hydraulic roller cam was run with a set of Comp hydraulic roller lifters, hardened pushrods and a double roller timing chain.
The use of the hydraulic roller cam required a cam button to control the cam movement. Unlike hydraulic flat-tappet cams, roller profiles do not have angled lobes to keep the cam from walking forward. It was necessary to set the cam button lash using the shims provided in the two-piece timing cover kit. The two-piece aluminum timing cover made the necessary adjustments to the cam button much easier, not to mention kicking the cool factor up a couple of notches.
With more cam and cubes, we turned our attention to the cylinder heads. Though we had good luck with the factory 049 heads (ported by Joel Marsh of Competition Heads in Fullerton, California), we knew the aftermarket had much to offer in the way of performance headgear. We contacted Canfield cylinder heads, and they shipped out a set of their 310cc aluminum heads. Right away the Canfield heads were a step up from the 049 heads, as the aluminum heads offered a dramatic weight savings (ever had to lug a set of big-block Chevy iron heads?) along with a reduction in detonation potential thanks to the superior heat dissipation of aluminum.
The final improvement involved the flow rate of the Canfield heads. Though we installed the heads out-of-the-box with no porting, the Canfield 310s easily out-flowed the ported 049 heads, by as much as 40-50 cfm per runner. The Canfield heads were equipped with a 2.25/1.88 stainless steel valve combination and treated to a performance three-angle valve job. The headwork was performed by Dougan's Machine shop in Mira Loma, California. The Canfield heads were completed with dual valve springs, retainers and spring seats, all from the Comp Cams catalog.
The only thing missing from the 489 stroker long-block was an intake, carb and ignition. We installed 2-inch Hooker Super Comp headers (for a Chevelle chassis) used previously for dyno testing, but the Canfield heads were topped with an Edelbrock Performer RPM Air Gap intake. We chose the dual-plane intake for its superior torque production. Sure, the racy single-plane Victor Jr. or Super Victor will make more peak power, but we were interested in having a broad torque curve capable of offering impressive acceleration and even improved fuel consumption over a single plane. For street use, nothing beats a dual-plane intake for maximizing torque production.
The Air Gap intake was topped off with a Holley 950 HP carburetor to ensure plenty of air and fuel for our thirsty stroker. Additional mods included a TCI Rattler balancer (external 454 balance), an MSD billet distributor (and wires) and a set of 1.7-ratio roller rockers. The motor was run on the dyno with the Hooker headers feeding a 3.5-inch exhaust equipped with mufflers.
After filling the pan with Lucas oil and a proper break in procedure, we were initially rewarded with 545 hp and 571 lb-ft of torque. After playing with the ignition timing, the carb jetting and finally installing a 1-inch carb spacer, we managed to coax 577 hp and 592 lb-ft of torque out of the normally aspirated 489 stroker. Even more impressive was the fact that the power came on Union 76 unleaded pump gas, thanks to a compression ratio of just 8.25:1.
Satisfied with the results of our normally aspirated 489, we quickly got to work and yanked the Performer RPM Air Gap intake and 950 HP carb and replaced them with the 420 Mega Blower and dual 750 (blower specific) Holley carbs. We reduced the ignition timing from 35 to 31 and proceeded to make some serious power-after all, the low-compression stroker was built with supercharging in mind.
Not surprisingly, adding the Holley 420 Mega Blower made a dramatic difference in the power output of the 489. Running a 56-tooth blower pulley and a 50-tooth crank pulley produced a blower drive ratio of roughly 11 percent under-driven (meaning the blower was spinning only 89 percent of the engine speed). The result of the 11 percent under-driven Mega Blower was an increase in peak power from 577 hp to 728 hp. The drive ratio produced a peak boost pressure of just 4.3 psi, with most of the run registering under 4 psi.
It is important to note that adding the supercharger changed where the stroker made peak power. We decided to limit the engine speed to just 6100 rpm for our testing, but we noticed that the power curve was still climbing. In normally aspirated form, the 489 made peak power between 5600 and 5800 rpm, and then began to fall off thereafter. Installing the 420 Mega Blower system included replacing the dual-plane Performer RPM Air Gap intake with a dedicated blower intake.
The Mega Blower lower intake offered much shorter runners than the Edelbrock piece, along with a common plenum similar to a single-plane intake, something that changed the effective operating rpm. The short runners allowed the engine to continue to produce power all the way to 6500 rpm, where the 489 eventually experienced valve float. While we continued to produce more power with engine speeds beyond 6000 rpm, we decided to purposely limit the tests to just 6100 rpm (with the exception of the one experimental run).
With the "more is better" principle ringing loud in our heads, we proceeded to pull the 50-tooth crank pulley and replace it with a slightly larger 52-tooth pulley. Changing the pulley was easy, taking all of two to three minutes.
The 52-tooth pulley increased the drive ratio to 7 percent under (blower spinning 93 percent of engine), and the boost climbed to a peak of 4.9 psi. As with the previous 50-tooth run, the boost pressure was lower than the peak (roughly 4.3 psi) for most of the run. Increasing the peak boost pressure by .6 psi resulted in a gain of 30 hp, bumping the peak from 728 hp to 759 hp. Peak torque was up as well, from 690 lb-ft to 712 lb-ft.
After our success, we continued by installing a 54-tooth crank pulley, which upped the peak boost pressure to 5.7 psi. The .8 psi gain in boost pressure added 13 hp, bringing the total to 772 hp, while the torque jumped to 731 lb-ft.
The final run involved the installation of a 57-tooth crank pulley, pushing the drive ratio over 100 percent for the first time. The 102 percent overdriven Mega Blower pumped out a maximum of 7.0 psi, and with it produced 797 hp and 745 lb-ft of torque. Running 7 psi, our 489 stroker was now pumping out nearly 800 hp and 750 lb-ft of torque.
In normally aspirated trim, the 489 produced 577 hp and 592 lb-ft of torque. Adding the Holley 420 Mega Blower (roughly equivalent to a traditional 6-71) and a pair of 750 blower carbs, we upped the power output to 728 hp, while torque jumped to 690 lb-ft. The impressive part is that all of this was accomplished at a hair over 4 psi.
One of the best things about owning a supercharged motor is the ability to literally adjust the power output at will. Of course there are limits to just how far you can go with increasing the boost pressure, but as is evident by the four distinct power (and attending torque) curves, reasonable boost levels can result in dramatic power gains. The gains in boost pressure were accomplished by altering the drive ratio of the supercharger relative to engine speed. Basically we changed the size of the crank pulley to increase the boost pressure.
Starting with the smallest 50-tooth pulley, the 489 produced 728 hp and 690 lb-ft of torque at a peak boost reading of 4.3 psi. Installing a slightly larger 52-tooth crank pulley increased the peak boost pressure to 4.9 psi and upped the power peak to 759 hp, while the torque jumped to 712 lb-ft. The next crank pulley to be installed was a larger 54-tooth, bringing the drive ratio to 4 percent under driven. The result was an increase in peak power to 772 hp, while the torque was up to 731 lb-ft. The peak boost registered with the 54-tooth crank pulley was 5.7 psi.
The final 57-tooth pulley tested (102 percent overdriven) produced 7.0 psi, 797 hp and 745 lb-ft of torque. Note that the third pulley combination (56/54) was run beyond 6100 rpm and the power kept climbing right to 6400 rpm. Had we elected to run the 57-tooth crank pulley to the same engine speed, we would have easily exceeded 800 hp, but that was not the intent of the pulley test. This 420 Mega Blower was possibly capable of as much as 1,000 hp, but our 2-bolt block and cast stroker crank were probably not, at least not for long.