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Engine speed naturally goes up when the throttle is punched opened, so for an engine of a given displacement the fastest route to more power is to increase its volumetric efficiency. A bigger carburetor, a better flowing intake manifold, a cam with more lift, duration and overlap, opening up the head ports, bigger valves and a free flowing exhaust are all traditional means by which an engine can ingest and pass more air and fuel. But even the best built naturally aspirated engine will never achieve 100 percent volumetric efficiency because atmospheric pressure can only do so much to overcome the restrictions and turbulence in the intake system. And as rpms go up, volumetric efficiency drops off.

WHICH WAY TO GO?

Forced induction can take one of two forms: supercharging or turbocharging. A supercharger uses a mechanically driven blower or compressor to push more air into the intake system while a turbocharger is driven by hot exhaust gases exiting the engine.

Those who say turbocharging is the way to go cite the fact that a turbo doesn't create a parasitic horsepower drain on the engine as a supercharger does. Because is turbo is driven by "waste" heat in the exhaust, it delivers boost for free. But there's a penalty to be paid.

There's no such thing as free lunch. Though a turbo is exhaust driven, it still steals some horsepower from the engine by creating backpressure in the exhaust. The other drawback is that because a turbo is exhaust driven, there's an unavoidable time lag between the instant the throttle is opened and the time it takes the turbo to "spool" up and start delivering boost. The delay can stretch out to as much as several seconds depending on engine speed, load, what gear the transmission is in, the amount of plumbing in both the intake and exhaust system, the size of the turbo, the size of the intercooler (if one is used) and even the inertia of the turbo impeller and turbine wheels. A turbo has to wind up to tens of thousands of rpms before it develops boost, and that takes time.

Turbo lag can be reduced by mounting the turbo close to the engine and using a smaller turbo so it spools up faster. But the tradeoff of using a smaller turbo is that it restricts power at the top end. Variable geometry turbos like the "dual scroll" housing that Mazda used their RX-7, or the Garrett turbocharger with moveable vanes can reduce turbo lag as can a lightweight, low inertia ceramic turbine wheel such as the one used by Buick in their 1987 Grand National. Yet in spite of all that's been done, turbo boost will always lag behind the throttle because a turbo is exhaust driven.

Another tradeoff of using hot exhaust gases to drive a compressor wheel is heat. Turbos run hot, and part of that heat is transferred to the incoming air. Heat is also hard on the turbo bearings and oil supply, which can shorten the life of the turbo if the oil changes are neglected.

Yet in spite of all the tradeoffs just mentioned, car makers have often preferred turbocharging over supercharging for upping the performance of small displacement engines. Some say turbos are more efficient, cost less or are easier to package under low profile hoods in cramped engine compartments. Yet here have also been quite a few OEM supercharged engines over the years such as the Ford Thunderbird Super Coupe, and GM12s supercharged 3.8L V6 engines in Pontiacs. The 2006 Chevy Cobalt is also available with a supercharged four-cylinder option.

Supercharged GM 3800 V6

HISTORY OF SUPERCHARGING

The primary advantages of supercharging over turbocharging are instant throttle response and more boost at low engine speed. Because a supercharger is belt driven off the crankshaft, it can deliver boost the instant the throttle is opened. Blower speed is directly proportional to engine speed, so if the blower is a positive displacement type (which most are), boost will increase at the same rate as engine speed. Thus there is no lag and the engine delivers right now power and tremendous low end torque.

A bolt-on supercharger can add anywhere from 30 to 50% more power to an otherwise stock engine. The hot setup is a small displacement blower (say 144 cu. in.) running at an overdrive of 1.75 to 2.25 that delivers 3 to 12 psi of boost pressure on a small block V8. This arrangement will deliver peak performance below 6000 rpm, which is where most street engines need the power.

Cost and reliability advantages are also claimed for supercharging over turbocharging. Unlike a turbo that spins at speeds up to 100,000 plus rpm, a blower operates at a much smaller multiple of crankshaft speed. The drive ratio between the blower and crankshaft pulleys determines blower speed (and thus boost), which generally does not exceed 12,000 to 15,000 rpm. Thus there is less wear and tear on the blower. There's also no hot exhaust gases to cope with so the blower bearings far cooler than their turbo counterparts.

Instant throttle response and low speed torque does have its price, however. Because a blower is belt driven, the amount of horsepower it takes to turn the blower depends on throttle opening and the blower drive ratio. The wider the throttle opening and the higher the drive ratio, the greater the amount of power required to drive the blower. A blower mounted on a top fuel dragster engine can siphon off as much as several hundred horsepower at top speed. But who cares if the engine is making thousands of horsepower? For street applications, the power drain is more modest. The horsepower power required to drive the blower on a typical OEM supercharger may be 50 to 60 hp at full throttle, but less than one horsepower at light throttle.

BLOWER SIZING & ENGINE MODIFICATIONS

For street applications, blower sizing is as important a consideration as turbo sizing. A small blower works better at low speed and develops power more quickly than a large one. A large blower is great for top end power, but isn't as well suited to lower rpm operation because of its greater power burden on the engine.

One of the most important advantages of supercharging street engines is that it can transform a relatively stock motor into a real performer without a lot of expensive modifications. Supercharging can be a straight bolt-on installation, and it works well with stock heads, cams and pistons. What's more, it delivers better low speed throttle response and torque than a naturally aspirated engine with a big carburetor, big valves, high compression and "hot" cam.

Dyno tests have shown that a blower can be much more "streetable" than a naturally aspirated engine that's been built up to deliver an equivalent level of power. A blown engine is more streetable because it performs better at low rpms. A blower kit may cost several hundred dollars more to install than what it would take in equivalent engine modifications to obtain the same level of performance with a bigger carburetor or throttle body, hotter cam and modified heads. But the blown engine will deliver about 10% more power and torque in the lower speed ranges where it does the most good on the street. Besides that, a blown street motor looks great.

The ultimate in performance, of course, is achieved by building a motor specifically for the blower. Using a "blower" cam that optimizes the breathing characteristics of the blower can improve both low speed torque and high speed power. A long duration cam with a lot of overlap isn't good with a blower because the boost pressure goes in the intake valve and right out the exhaust port at low speed. What works best is a cam with little overlap, a lot of lift and duration but more lift and duration on the exhaust valves (since the blower does nothing to help evacuate the cylinders). With a turbocharged engine, more lift and duration is needed on the intake valve to prevent backpressure (created by the turbo) from backing up into the cylinder.

One caution to keep in mind with any kind of forced induction system is compression. Because of the extra air and fuel that's rammed into the cylinders, detonation can become a problem if compression gets out of hand. Most blown engines work best with a static compression ratio of less than 8:1 (7:1 or 7.5: 1 is best). The lower the static compression ratio, the higher the boost pressure you can safely run with the blower and the lower the rpm range at which the blower can develop peak torque.

Roush supercharged Ford Mustang
BLOWER TYPES

Superchargers are available in a variety of street and race only configurations. Roots type blowers are the most common and are called "positive displacement" pumps because they pump a given volume of air into the manifold. They come in various sizes with a pair of two or three lobe straight cut rotors, or a pair of twisted flute three lobe rotors. The longer the blower case and/or the larger the rotors, the greater the volume of air the blower can deliver to the motor.

A blower with two lobe rotors can pump more air than one with three lobe rotors, but it will also experience more "backflow" which reduces pumping efficiency compared to a three lobe design. Straight cut two and three lobe rotors can also be quite noisy, so that's why some blowers use the twisted flute three rotor design (typical of the OEM superchargers made by Eaton that Ford and GM have used over the years). A twisted flute rotor opens to the manifold cavity more gradually as it turns, which reduces backflow and the pressure waves that create noise.

Roots blowers are available from a number of aftermarket suppliers, including B&M Automotive, Camden, Dyers, Hampton, Littlefield, Weiand and others. B&M was a pioneer in the development of practical street blowers and offers a wide range of blower sizes for various engine applications.

Paxton centrifugal supercharger
Another type of forced induction system is the "centrifugal" supercharger which uses a rapidly spinning impeller wheel to pump air into the engine. It works like a turbo, but because it's belt driven it runs much cooler than a turbo and delivers instant throttle response. This type of blower can be installed on almost any engine, including many of today's fuel injected engines such as GM's Tuned Point Injection, Ford Mustangs and others. Paxton Superchargers, ProChargers and Vortech have kits for many small block Chevys and Fords.

Latham axial flow supercharger
One unusual design is Latham's "Axial Flow" supercharger that resembles a fan compressor from a jet engine. A cylindrical rotor with dozens of little blades spins inside a bladed housing to compress the incoming air in stages. Like a centrifugal supercharger, it runs at high speed (470 percent overdrive) and is not a positive displacement pump but more of a high volume fan with an operating speed around 20,000 rpm! One of it's claimed advantages was a much lower outlet air temperature than a Roots type blower. The Latham supercharger was produced briefly from about 1955 to 1965, but never gained widespread acceptance.

Rotary piston and vane type superchargers have also been developed and used to pump air into engines. But this approach has yet to find much acceptance either at the OE level or on the race track.

Another innovate supercharger design is Performance Systems Inc.'s screw compressor blower (also called a "Lysholm" screw compressor or helix supercharger). The PSI blower resembles a three-lobed twisted flute rotor blower in appearance, except that one rotor is significantly larger than the other. Derived from a screw compressor originally designed for bus air conditioners, the blower is claimed to be smaller than a 6-71 blower but to breathe more efficiently than a 14-71 blower and produce less heat and drag.