Supercharger Wastegate - What A Waste!

A wastegate is an item typically associated with turbo systems, but Detroit-area Stenod Performance has found great success by weaving them into supercharged engine systems to boost torque. In fact, on a pair of '10 Camaros equipped with identical bolt-on ProCharger centrifugal blowers, Stenod coaxed 60 maximum rear-wheel pound-feet more out of the wastegate-equipped car over the comparable, non-wastegate Camaro. At some points on the dyno chart, there was more than a 70 lb-ft advantage.

No matter how you slice it, that's a significant increase, and the path to delivering it is relatively simple--if you're handy at welding and tuning. And it was a solution that Stenod's founder, Joe Borschke, says fits the bill for optimizing blower performance across a broader rpm range. "We've been really impressed with the fit, finish and general performance of the ProCharger systems for new Camaros. But they don't seem to make as much torque as they do horsepower," says Borschke. "That's primarily a function of the boost curve of the centrifugal design."

When it comes to supercharged power, positive displacement superchargers (Roots and Screw-type) and centrifugal blowers produce it differently. In simple terms, a centrifugal supercharger's boost increases exponentially with engine speed, while a positive displacement supercharger's airflow is linear--with maximum boost occurring very low in the rpm band. That means a Roots or twin-screw blower that delivers, for example, 500 cubic feet per minute (cfm) of air at 2,500 rpm will push 1,000 cfm at 5,000 rpm. A Roots or twin-screw blower makes a small amount of boost whenever the engine is running, while a centrifugal supercharger's boost builds in a non-linear way, much like a turbocharger. As rpm increases, the airflow from the compressor will increase at a faster rate. Because of that, maximum boost is not achieved until the engine's red line, or maximum rpm level. So, the centrifugal "rolls" into its boost, and is generally easier to launch, with a stronger feel through the mid-range and upper rpm levels.

The non-linear airflow delivery also makes the centrifugal supercharger better-suited to drag racing, because the graduated boost application enables an easier launch, with greater power coming on as the rpm increases. Of course, with peak boost not occurring until redline, the blower's effectiveness is not fully realized at lower rpm. In other words, unless you're performing full-throttle launches at every stop light, you're not going to feel the maximum punch of a centrifugal blower. That's not to say centrifugals are weak on the street. We've driven a number of ProCharger-blown '10 Camaros, and they're admirably powerful. But there's no denying that more torque delivered lower in the rpm band would help overcome the Fifth Gens' considerable mass.

"We've installed, tuned, and dyno tested a bunch of ProCharger systems upgraded to the D-1SC compressor. You can pretty much bank on them making 550 hp and 500 lb-ft of torque at the wheels, with about 8 pounds of boost," says Borschke. "We wanted to get more torque out of them to make the cars feel stronger and more fun on the street. We turned to the wastegate, then started experimenting with smaller-diameter blower pulleys to push more torque higher in the rpm band. "In a turbo engine, a wastegate bleeds off excessive exhaust gas pressure to regulate the speed of the turbine. It's the same idea with a centrifugal supercharger, the compressor of which performs similarly to the turbine of a turbocharger.

"With the wastegate, we can spin the blower faster to produce greater boost, and bleed off the extra pressure that's not necessary," says Borschke. "This allows the boost to come on much sooner, gaining midrange power--especially torque--that you can really feel on the street. But more importantly, it helps maintain the right boost for to prevent detonation in a stock engine. "Of course, there's a fine line to balance with the size of the wastegate. Stenod uses a 44mm unit on LS3 Camaros, which Borschke reports is plenty large enough. "If you go too big, you bleed off too much pressure and go backwards on power," he says.