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Features of the four main types of charging systems at a glance.

Supercharged 1999 Subi Project :

412hp/332tq, SOHC, 20psi, 93 octane, started at 165hp, Dyno Sheet

UPGRADED, NOW AT 550hp/450tq, 25psi, 93 octane

Twin-Screw Supercharger
Centrifugal Supercharger Turbocharger Roots Type Supercharger
Response No Lag. Instant boost - full boost to redline. Lag. Poor low-rpm response. Turbo lag. Poor low-rom response. No lag. Good low to mid range power.
Driveability Increased lugging ability in higher gears. Constant boost through powerband. Must downshift to maintain boost level. Must downshift to maintain boost level. Increased lugging ability in higher gears.
Engine Torque Curve Virtually flat curve - ideal characteristic. Often worse than uncharged engine at low speeds. Often worse than uncharged engine at low speeds. Wide range - power can fall off at high rpm.
Heat Buildup Low - intercoolers are usually not needed for low, yes for high boost Low - intercoolers are usually not needed. High - may need intercooler High - may need intercooler.
Noise Virtually silent - extremely low noise levels. Typically very noisy. Very low noise levels. Typically very noisy.
Adiabatic Efficiency 75-80+% 60-78% 60-80+% Peak 40-60%

Supercharging for More Power

Turbos and superchargers both perform the same basic function - they’re pumps that force-feed an engine its fuel air mixture at a greater than atmospheric pressure. But the difference between them lies in how they go about getting the job done.

A supercharger is driven mechanically from the engine’s crankshaft to provide the necessary huff, while a turbocharger utilizes exhaust waste gases to spin an impeller at tremendously high speed and generate the puff. The supercharger looses out to the turbo in some ways because it drains power from the crank, while the turbocharger, driven by waste exhaust gases, gets it power for free. But because the supercharger is driven mechanically with no slippage, it’s already ready to deliver buckets of extra grunt at virtually any revs, from idle upwards, while the turbo is at a disadvantage at slow engine speeds, when the exhaust gases lack the energy to get the impeller really spinning.

A supercharger is connected directly to the crankshaft by a belt, unlike a turbocharger, which is driven by exhaust gases. A supercharger provides improved horsepower and torque, at lower rpm's, by pumping extra air into the engine in direct relationship to crankshaft speed. The positive connection yields instant response, in contrast to turbochargers, which must overcome inertia and spin up to speed as the flow of exhaust gases increases. The supercharger is a way to get around "turbo lag". The lubrication system also differs, in that the supercharger is self-contained whereas the turbocharger requires engine oil.

Because superchargers provide more fuel to burn, there’s a huge increase in horsepower and torque, at low and midrange engine speed. The output of a supercharged engine can be easily varied by simply changing the size of pulleys between the engine’s crankshaft and the blower.

Whatever charging system you use, whenever building an engine for big power, you must always remember, too many horses and bad tuning can destroy an engine, even if the engine is built up. Therefore, it is most important to increasing power and torque for reliably, rather than for peak power.

Heat is not good and compressing air produces heat!

Forced induction compresses air, and as a law of physics the temperature of the air increases as a direct counterpart to its compression. A lot of engineering goes into trying to compensate for this fact in supercharging and turbocharging design.

The word "adiabatic" describes a process in which no heat is gained or lost - 100% adiabatic efficiency would be the perfect forced induction device, creating no heat gain whatsoever, probably impossible to achieve ever. And the closest anyone can come yet is around 80% efficiency.

The problem with heat is it defeats the original purpose - the hotter the air, the lower the density possible, and the extra power comes from dense air. Another problem from heat is ignition - the hotter the inlet air, the more tendency the engine will have towards detonation and pre-ignition (knock and ping), which damages the engine, besides diminishing performance. Drivers of blown vehicles tend to keep an eye on the temperature gauge.

The goal of efficient charging is to compress the air and to keep it cool, for maximum power. The cooler the intake charge, the denser the air and the more horsepower produced.

The greater the adiabatic efficiency with which a supercharger compresses air, the less the heat that gets added to the intake manifold. Efficiency is measured by the discharge air temperature at a given pressure. For 6 pounds of boost, a supercharger with intake air temperature of 185 degrees is more efficient than another at 190 degrees. Boost itself is only the measure of pressure the intake air is under, not an indication of the power produced as horsepower.

Which has greatest adiabatic efficiency?

The Roots blower has the lowest adiabatic efficiency of all the forced induction designs (including the turbocharger, which has to start off with hot exhaust gases to deal with) - generally around 50 percent. The roots type is so inefficient because it doesn't compress the air directly, but delivers uncompressed air which wells into the intake manifold, becoming more compressed, but with additional heat gain from the turbulence and reverse flows of air mixing.

Centrifugal superchargers can vary from 60% up to perhaps approaching 80%+ efficiency, as can turbochargers; both are more efficient at higher rpm, which is another way of calling them more inefficient at lower rpm.

The twin-screw supercharger normally delivers lower output temperatures, for adiabatic efficiencies of 70-80%+ across the whole rpm range.

Twin Screw Supercharger - a positive displacement compressor

The twin-screw supercharger is a positive displacement air mover, in that it moves fixed amounts of air per revolution, like the roots type blower. Unlike the roots however, which is only an air delivery system, the twin-screw supercharger is also a compressor. The counter rotating lobes and chambers of the twin-screw are designed for a screw-like tapering effect which runs its intake air into a smaller space for output, thus compressing it. The rotors have very close tolerances yet never touch. Compressed air is delivered into the compression environment of the intake manifold with very little leakage or energy loss.

Because of the increased mechanical efficiencies of the superior design, the output air temperatures of the twin-screw positive displacement supercharger are radically improved from the roots type. The twin-screw quotes adiabatic efficiency of 70%-80%+ range across the whole powerband.

As with the roots, since the supercharger is under continual drive, and since it delivers boost practically from idle, overboosting is prevented by the use of an intake bypass system, which allows the engine to breathe normally at cruising or idle: the bypass closes on throttle use, delivering full boost.

Full boost by 2000 rpm

The twin-screw supercharger creates boost the instant the throttle is touched, and generally reaches full boost by 2000 to 2400 rpm. Full boost is then available all the way to redline. A positive displacement compressor is ideal for street performance cars.

Performance vehicles are very responsive with positive control using this type of supercharger. The instant torque for accelerating, passing, and hill-climbing diminishes the strain on the engine and increases the safety factor. The twin-screw compressor is especially useful at high altitude, where physics dictates that all engines lose power.

The twin-screw supercharger is essentially silent, producing discernible sound no greater than whisper strength. Of all the forced induction systems, the twin-screw compressor supercharger might be the most awesome direction for the guy who wants a sleeper.

Selecting an Intercooling System

Both air/air and water/air systems have their own benefits and disadvantages. Air/air systems are generally lighter than water/air, especially when the mass of the water (1kg a litre!) is taken into account. An air/air system is less complex and if something does go wrong (the intercooler develops a leak for example), the engine behaviour will normally change noticeably. This is not the case with water/air, where if a water hose springs a leak or the pump ceases to work it will not be immediately obvious. However, an air/air intercooler uses much longer ducting and it can be very difficult to package a bulky air/air core at the front of the car - and get the ducts to it! Finally, an air/air intercooler is normally cheaper than a water/air system.

A water/air intercooler is very suitable where the engine bay is tight. Getting a couple of flexible water hoses to a front radiator is easy and the heat exchanger core can be made quite compact. A water/air system is very suitable for a road car, with the thermal mass of the water meaning that temperature spikes are absorbed with ease. However, note that if driven hard and then parked, the water within the system will normally become quite warm through underbonnet heat soak. This results in high intake air temperatures after the car is re-started as the hot water takes some time to cool down.

Type of Intercooling
  • Efficient at constant high speed
  • Cheap
  • Cores readily available
  • Bigger is better
  • Longer induction air path
  • Packaging of large intercoolers difficult
  • Large pipes to and from intercooler required
  • Ambient air as the cooling medium, less efficient when in traffic: heat soak
  • Short induction air path
  • Easy to package
  • No heat soak
  • Excellent for short power bursts
  • Consistant efficiency for every day driving: stop and go
  • Heavier
  • More complex
  • More expensive
  • Heat exchangers harder to source

Technically, a water/air intercooler has some distinct cooling advantages on road cars. Water has a much higher specific heat value than air. The 'specific heat value' figure shows how much energy a substance can absorb for each degree temp it rises by. A substance good at absorbing energy has a high specific heat value, while one that gets hot quickly has a low specific heat. Something with a high specific heat value can obviously absorb (and then later get rid of) lots of energy - good for cooling down the air.

Air has a specific heat value of 1.01 (at a constant pressure), while the figure for water is 4.18. In other words, for each increase in temp by one degree, the same mass of water can absorb some four times more energy than air. Or, there can be vastly less flow of water than air to get the same job done. Incidentally, note that pure water is best - its specific heat value is actually degraded by 6 per cent when 23 per cent anti-freeze is added! Other commonly-available fluids don't even come close to water's specific heat value.

The high specific heat value of water has a real advantage in its heat sinking affect. An air/water heat exchanger designed so that it has a reasonable volume of water within it can absorb a great deal of heat during a boost spike. Even before the water pump has a chance to transfer in cool water, the heat exchanger has absorbed considerable heat from the intake airstream. It's this characteristic that makes a water/air intercooling system as efficient in normal urban driving with the pump stopped as it is with it running! To explain, the water in the heat exchanger absorbs the heat from the boosted air, feeding it back into the airstream once the car is off boost and the intake air is cooler. I am not suggesting that you don't worry about fitting a water pump, but it is a reminder that in normal driving the intercooler works in a quite different way to how it needs to perform during sustained full throttle. However, the downside of this is once the water in the system has got hot (for example, after you've been driving and then parked for a while), it takes some time for the water to cool down once you again drive off.

Which is better an air-to-air intercooler or a water-to-air intercooler?

It really depends on the application. In order for an intercooler to effectively cool the air that passes through it, the intercooler itself must be cooled by some external means. Most intercoolers are cooled just like your engine's radiator - air flows over the outside of the intercooler's fins, which in turn cool the air inside the intercooler - hence the name air-to-air Intercooler. Some intercoolers, however, are cooled by water instead of air, in which case they are generally called aftercoolers, or water-to-air intercoolers. The benefit to an aftercooler is that air passing through it can be cooled more than in a traditional air/air intercooler if very cold water and ice are used to cool the intercooler - in fact, some aftercoolers chill the air to below ambient air temperatures even after it has been compressed by the turbocharger/supercharger.

The reason aftercoolers are more effective in cooling the air charge is because water is a much better conductor of heat than air - in fact water conducts 4 times as much heat (energy per pound) as air! The obvious drawback is that with time, the water will heat up to the temperature of the air passing through it, and its ability to cool incoming air goes away. Some aftercoolers, however, use a small radiator to cool the water that runs through the system, making them ideal for street use as well as racing. The water is constantly pump whilst the ignition is on and is cooled as it travels through the water ratiator. The cool water travels into the charge cooler and cools the boost by absorbing heat energy. The hot water exits the cooler and back to the water radiator via the reservoir. This method of cooling is regarded as more efficient as the cooling action of the water is more consistent than air to air intercooling. The water drops to temperatures lower than ambient and therefore cools the boost with greater efficiency. However charge cooler systems require the installation of more components with a slightly increased cost. Charge cooling is commonly used for high compression engines where efficiency and temperature consistency are key requirements. For drag racing applications aftercoolers packed with ice work very well because they only need to work for around ten seconds or so before you shut down and head to the victory podium. For milder racing and street applications air/air intercoolers or aftercoolers with radiators are more practical as their ability to cool incoming air is not reduced with time.

Extreme Performance

For drag racing, autocross or performance street applications the Liquid/Air system can provide sizeable competitive advantages. Through the use of chilled coolant, you can reach intercooler efficiencies well in excess of 100%, unleashing dramatic performance gains.