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.