One trick the Magnuson offers that no other Roots-type blower does is a bypass valve. This simple vacuum diaphragm is connected to the manifold vacuum. At cruise speeds with light load, manifold vacuum is high, which actuates an internal bypass valve.
It's also important to note that a Roots-style supercharger, including a Magna Charger, is not a true air compressor. Roots blowers should more accurately be termed air movers, pushing air fast enough through the blowers that it stacks up inside the intake manifold creating pressure, or boost. This is pressure above atmospheric, which improves engine efficiency and power by pushing more air into the cylinders than would normally occur with atmospheric pressure.
Given the above description, this means the discharge temperature from a more efficient supercharger will be lower than a blower that is less efficient for a given outlet pressure, or boost. The discharge temperature we measured out of this blower on our test was a mere 118 degrees F, which is exceptional, but the temperature is also affected by the heat removed from the air with the introduction of fuel into the air stream by the carburetor. Overall, generating 7.2 psi of boost with only a 118-degree air temperature going into the engine is the main reason this engine makes so much power from such a small supercharger.
Here is the bypass valve that feeds inlet air directly to the engine, bypassing the twin three-lobe rotors. This reduces pumping losses and improves fuel economy at part-throttle. At wide-open throttle, low manifold vacuum closes the diaphragm and valve, allowing the blower to make boost.
The Test
The dyno testing was almost too easy. The intake manifold and blower were carefully bolted and torqued in place on the stock iron Vortec heads. The rest of the engine was already assembled, so all we had to do was drop in the Performance Distributors HEI, attach the plug wires, and plumb the 750-cfm mechanical secondary Holley supercharger carb. Almost as easy was installing the blower-drive assembly. We had to finesse the alternator position to ensure it would clear the blower, then we just bolted everything together. The Magnuson kit comes with a large-diameter crank pulley that allows the use of a larger blower pulley to ensure adequate belt engagement to minimize belt slippage-often problematic on serpentine blower beltdrives.
Because this could easily be a daily-driver street package, we limited the timing down low to keep the engine from rattling because our motor is equipped with cast pistons that don't like detonation. This turned out to be a challenge because the Magna Charger wants to make boost right out of the gate, cranking out nearly 5 psi as far down as 2,000 rpm. At 2,400 rpm, the Magna Charger was worth a solid 114 lb-ft of torque over the normally aspirated combination. As you can see by the graph on page 68, the torque curve is amazingly flat, which means the engine will be a whole bunch of fun to drive.
This NACA duct shape is the blower discharge area, which is shaped to maximize air movement efficiency through the supercharger and even out the pressure distribution in the intake plenum. If the opening were just a rectangle, air exiting the rotors (the top in this photo) would tend to "stack" on that end of the intake manifold. | 
Bolting on the blower was no more difficult than bolting on a regular intake manifold. |
The Magna Charger beltdrive system is simplistic in its approach, and it worked flawlessly. We tried a smaller, 3-inch pulley compared with the stock 3.25-inch pulley, but the extra boost caused detonation difficulties. | |