Even the supercharger:Supercharging has a direct benefit in pressurizing the air intake side,forcing more air into the combustion chambers above atmospheric pressure.NA engines however use large amounts of valve overlap to get the whole system to work properly at higher RPM.However,neither valve overlap amounts nor perfect exhaust system designs can keep air flowing in the right direction.No matter how long the exhaust is exposed to the intake system through valve overlap , air can flow backwards in an NA set up since they operate at similar pressures.This is one reason why cam shaft choice is important as it can determine the area in the RPM band where power is best served.
Suoercharger compressors utilize vanes , screws or rotors that are operationally termed positive displacement.They compress the intake air into a smaller higher pressure volume that send the denser charged to the cylinder heads.That means they guarantee a certain amount of airflow (measurable working volumetric efficiency) regardless of RPM.Because a supercharger operate almost all the time (belt driven from the crank) power is available right off idle and rises with engine speed.However,they need a large heavy duty drive train , such as a wide toothed belt driven off the crank shaft by means of a custom made pulley.Some hard earned torque power we finally get must go back into powering the supercharger,an unfortunate condition known as parasitic loss.It is said that between 15 to 20 hp is taken directly from the engine in order to generate just 5 to 6 psi on a small engine and even more from larger engines.
Roots compressors-This was the first blower design which comprises of two belt driven motors , each with a minimum of two lobes per rotor. Modern designs (screw) added a third lobe to each rotor , and then twisted them axially for greater efficiency and less noise when compared to the basic roots type blowers.The two rotors interact differently as they spin:the lobes interlock to form nearly airtight sections within the housing that offers better thermal efficiency and much improved high pressure boost performance.Advance with modern designs pulls air in from the rear of the unit,and then pumping it through carefully designed outlets for increased performance.The roots compressor makes matching engine airflow demands easy and boost possible at low engine RPM.Moreover , long term reliability and easy installation makes the roots compressor desirable.On the downside,its thermal efficiency is inherently lower than the centrifugal design and the large compressor makes placement harder in cramped engine bays let alone adding an intercooler.Heavy internal parts also mean high parasitic losses when boosting.
Centrifugal compressors:These utilize a vane wheel (gear) which spins faster than the engines speed.At high RPM,air from the center inlet is captured by the scroll of the compressor housing and channeled to its outlet.This initial outward motion is like a large air centrifuge relying on high air speed and RPM to work.As the air slows down in the scroll and beyond,it gains in pressure and temperature,thus creating compression.It takes little power to drive and the pressure builds up gradually from 1000 rpm at a linear rate,providing drivability and useable power.
Crank driven centrifugal compressors generally operate around 60,000 RPM maximum , while turbocharger compressors can exceed 120,000 RPM.Although turbo charger compressors are more efficient at high RPM,they are largely ineffective at creating meaningful boost at lower RPM because of the surge limit.Centrifugal compressors therefore provide higher efficiency levels over large flow rates and low levels of parasitic drag versus boost.They are also more popular because of its light weight and small size for easy installation,hence also enabling the use of an intercooling unit .However,they need a heavy duty comprsssor,therefore affecting long term reliability because of its internal tolerances must be precise,its inability to be driven at high RPM during during low speed engine operation make boost available only at moderate RPM.Although there are ways around this,suffice to say that there is no simple to control boost levels on crank driven designs.A centrifugal unit is best suited for cars that need to be at high RPMs anyway,such as DOHC VTEC Honda's and also cars taht naturally have lower end power to compensate,such as the Nissan Z's.
Like turbo charging,supercharging increases the volumetric efficiency of an engine thereby increasing compression pressure.Therefore , you need to increase the anti knock characteristics of the fuel by upgrading to a higher octane fuel or add an octane booster everytime you fill up with regular fuel in order to avoid pre ignition . The other alternative is to reduce the compression ratio so that the compression pressure will be the same as before supercharging.This is achieved by increasing the volume in the cylinder head(overbore) resulting in a slightly lower thermal efficiency,but the power output is increased,since more fuel is burned.Consider also synthetic oils as increasing power level can stress engine internals and generate heat,requiring the extra protection and resistance to heat break down.
Supercharger kits are normally sold complete,with ECU control and Intercooling.The Intercooler is basically a radiator that cools the compressed intake air being force fed into the cylinders.A byproduct of compress air is heat.Colder air is better.(denser) for both power and reliability.An intercooler especially the more efficient front mount intercooler which uses airflow through the fvront bumper is highly recommended for any forced induction set up especially for boost numbers that go beyond 8 psi, a forced induction system can produce air inlet temps over 200 degrees Fahrenheit making the engine susceptible to detronation.
All the things the turbo charging is bad at the surpercharger is good at.However,there arent any dump valves to make a nice sound.The supercharger can be found as standard on some higher end continental cars manufactured today. An after market supercharging kit is a lot cheaper and less of a hassle maintained if compared to a turbo kit.Though it is less powerful,it will usually fit better if all things being equal.
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