Parasitic Load Vs. Backpressure
Although hot rodders are well aware of the parasitic power loss that comes with driving a supercharger off the crankshaft, the detriments of the exhaust backpressure created by a turbo system are often overlooked. Although it's difficult to put exact figures on how much power loss is associated with both a supercharger's parasitic load and a turbocharger's backpressure, the numbers could be much closer than most people think. "The theory that turbos give you free horsepower is ridiculous. Turbos don't have parasitic loss, but the exhaust backpressure they have to deal with means that superchargers make more power on pump gas," he says. "In race gas applications, superchargers and turbos run similar e.t.'s at similar boost levels, so the power loss from parasitic load and backpressure may be similar. Plumbing a turbo in the exhaust system is like putting a banana in your tailpipe. Any exhaust that isn't scavenged out of the cylinders adds heat to the combustion chambers, which lowers the detonation threshold and lowers how much boost you can safely run on pump gas. In a high-compression late-model engine like an LS3 small-block, a supercharger can run 15 to 20 percent more boost, and that more than offsets any parasitic power loss."
Air naturally heats up as it is compressed, and the more efficient the compressor, the less heat it adds to the compressed air charge. According to Ken, although standardized tests can effectively measure compressor efficiency in a lab, there are a multitude of factors that lab testing can't simulate. "We think that SAE J1723, a testing standard intended to measure compressor efficiency, is highly problematic. It only measures the efficiency of the supercharger itself, and does not account for the effects of engine compartment heat transfer, intercooling, oil temperature, and supercharger mounting location," Ken explains. "Most people don't just buy a supercharger, they buy a supercharger system. By mounting a centrifugal superchager off to the side of the engine, you effectively isolate it from engine heat. Since heat rises and positive-displacement superchargers are mounted on top of the engine, they are very susceptible to heat soak. Also, limited hood clearance often results in an undersized intercooler, and any ornamental engine shrouding a manufacturer bolts on top of the engine traps in even more heat. In the case of a turbocharger, exhaust gas temperature can exceed 1,600 degrees. That heat transfers over to the compressor side of the turbo and decreases efficiency. Likewise, a self-contained oiling system runs much cooler than a turbo running off engine oil, resulting in less heat transfer into the compressor. Pump gas is the real litmus test for compressor efficiency, and centrifugal superchargers are proven to make the most horsepower on pump gas."
Minimizing inlet air temperature is imperative to maximizing power and durability in any forced induction application. Although intercooling is arguably the most effective method of lowering IAT, the plumbing on the induction side of the compressor is very important as well. "To properly engineer a supercharger system, you have to monitor IAT from the air filter, to the compressor outlet, to the intercooler, to the discharge piping, and to the throttle -body throughout a quarter-mile run. If you have restrictions in flow path anywhere between the air filter and air inlet on the supercharger, it increases parasitic power consumption," Ken explains. "In the case of a turbo, it will actually increase backpressure. If locating the air filter away from heat requires making a long inlet tube with tons of bends, you're better off bolting a rear-facing filter directly on the blower. Heat off the headers will rise into the filter at idle, which is obviously very bad, but last I checked you're not racing at idle. As a car moves down track, by the time it reaches 20 mph IAT will be within 3 to 5 degrees of ambient temperature. Sure you can pulley the blower down to overcome the restriction on the inlet side, but all that does is increase parasitic power loss and generate more heat. If you try to hot lap a forced induction motor at the track, the shortcomings of the induction system become very obvious. With a properly designed intercooler, recovery time from heat in the induction system is very quick."