"The cool thing about centrifugal superchargers," declares Bob Endress, Vortech Engineering's product manager, "is that all you have to do to increase your boost level is change the pulley." That, in a nutshell, was what we were thinking as we embarked on this experiment. We'd start with a proven supercharged combo, make a couple of pulley changes to increase boost, and document the results on the dyno. But is anything ever really that simple? Of course not, and as we learned during a brief test and tune session at Vortech, several factors must be taken into account when contemplating a boost increase: fuel supply, air/fuel ratio, and the actual level of boost among them. We won't claim that we got our test subject perfectly dialed in-that quest goes on-but we made more power and learned loads about tuning a boosted application.
Pounds, Not Inches
Our mule for this experiment was Shane Ganzel's '67 RS/SS Camaro. The 9.1:1-compression 383 small-block under the hood was built with force-feeding in mind, and the heart of its supercharger system is a Vortech S-Trim compressor. Spinning a 3.33-inch pulley, this blower setup boosted the stroker's output by 103 hp and 71 lb-ft of torque. During our baseline runs, we measured slightly less than 7 pounds of boost at peak horsepower. The operative term here is that we measured it. "Pulleys are measured in inches, not pounds of boost," says Vortech's Chris Whalley. The reason for this is that the same pulley will produce different levels of boost on different applications. Anything that affects airflow through the engine-carb, intake, heads, compression, headers-will influence the amount of boost being made. And without a boost gauge, there's no way to know what that amount is.
The best place to measure boost, according to Endress, is at the intake manifold, right below the plenum. This will give the most accurate reading of how much boost the engine is actually seeing. The pressure can also be read from above the carburetor, which is what we did, using a port located in the discharge tube. Readings taken here will be higher than those measured below the carb; while not as precise, it does provide a margin of safety. If you're looking to work right at the edge of permissible boost-911/42-10 pounds in a carbureted street setup-you'll need to measure boost at the plenum. "A small change in inches makes a big change in boost," says Whalley.
It's All In The Timing
We mention boost first, since the gist of this story is the quest for more boost, and therefore more horsepower. But the first thing that must be addressed when working with a supercharged application is timing. "The important thing is to retard the timing first, then iron out the air/fuel ratio, and then add timing," explains Whalley. "I'd take out a degree per pound of boost, preferably with a boost retard." Indeed, a boost retard is the best way to go, since timing is only pulled out when the engine is under boost. On the other hand, our experts say it's pretty common to see setups in which total timing is retarded.
That was the case here; the 383's timing was retarded from 36 degrees in naturally aspirated trim to 30 degrees with the supercharger in place, and that setting didn't change for this test. One downside to employing this method is that the engine also sees less timing when it's not under boost, so low-rpm performance suffers. "You can play with the mechanical advance curve to compensate," Endress tells us. In other words, a spring or weight change will bring in more timing at low rpm, improving performance at the lower end. Endress sums up this aspect of supercharger tuning pretty well, we think: "We didn't get into timing here, but conservative is better."
Installing a centrifugal supercharger...
Installing a centrifugal supercharger system is the hard part; with that done, it's really a simple proposition to bolt on a smaller pulley, thereby spinning the compressor faster, making more boost, and producing more power. Remember, any given pulley will produce different results on different engines. In our case, a 3.12-inch pulley created 10 pounds of boost so we didn't run the even smaller 2.95-incher shown. The only way to know for sure is to measure the results with a boost gauge. Thinking about adding some boost? Vortech can create a performance estimate for your combo and recommend a pulley.