A closer look inside reveals...
A closer look inside reveals the similarities between centrifugal blowers and a turbocharger's compressor side.
But its biggest disadvantage from a performance standpoint is that it has the lowest adiabatic efficiency among any commonly available supercharger in the order of 50 to 60 percent. When air is compressed, it heats up as expressed by the ideal gas law, PV=NrT. Adiabatic efficiency is the difference between how much the actual discharge temperature of air compressed by the supercharger rises versus the theoretical calculated temperature gain dictated by the ideal gas law. Good adiabatic efficiency is important to tuners because the higher it is, the cooler and denser the intake charge will be. A higher efficiency blower will also steal less power from the crank to make an equal amount of boost pressure.
The fact that roots blowers are external compression devices by design makes them less efficient. As a roots blower's lobes move air from the intake chamber to the intake manifold, the backed-up high-pressure air in the manifold remains at a higher pressure than the air being moved by the blower. When spinning lobes discharge the air into the manifold, the higher-pressure air momentarily tries to rush back through the lobes where it's pumped back into the manifold again. A portion of the air moving into the engine gets thrashed by the lobes, picking up heat with each trip. Not good!
Although many aftermarket companies make roots-type blowers, Eaton makes the one with the greatest number of applications on the market. The Eaton blower is a well-engineered, proven piece. Since it's built as an OEM part for several different manufactures including Nissan, Pontiac and Saturn, Eatons have undergone extensive testing for reliability and durability. Such credentials perhaps make Eatons the most reliable superchargers on the market. Eaton superchargers also have adiabatic efficiencies of more than 60 percent compared to the typical 50 percent or less when considering older roots blowers.
HKS superchargers feature...
HKS superchargers feature superb OEM-like quality.
The improvement in efficiency comes from several things. Eatons have closer tolerances than older roots blowers, which allows for less internal leakage. Additionally, with their precisely timed side-port intake configuration, which is much like a rotary engine or twisted rotor geometry, Eatons actually allow some compression to take place internally. Roots blowers, with their pulsating flow, are typically very noisy. The pulses sound like a sort of loud whine. The Eaton's intake porting and exit port geometry eliminate most of this noise. Parasitic losses at part throttle are also a big problem with roots blowers. Eaton has a good solution for this issue as well. At light loads and partial throttle Eaton blowers feature internal bypass valves, which keep the supercharger from boosting when vacuum is present in the intake manifold. This reduces strain on the engine and helps fuel economy at part throttle by partially decoupling the supercharger under light-load conditions, reducing parasitic drag. Once throttle is applied and vacuum drops the valve closes allowing the supercharger to produce boost.
The bypass valve doesn't perform perfectly though. When it's open, air is circulated around the supercharger, often several times before it's ingested into the engine. Each time air passes through the supercharger it picks up heat due to partial compression. Because of this recirculation, intake temperatures can soar under light-throttle loads. Such heating is not ideal for an engine to ingest and can severely heatsoak the intake tract thus affecting full-throttle power.