Since the first internal combustion engine sputtered to life there have been a host of technological advancements made, but regardless of how many carbs, computers, injectors, cams, or valves they are equipped with, the amount of power produced is a direct result of how well they process the air required for combustion.
One limitation of most engines is that they rely on atmospheric pressure to fill the cylinders. As the piston descends on the intake stroke a vacuum is created and atmospheric pressure causes air to flow through the intake tract, past the open valve and into the cylinder. But atmospheric pressure varies with the weather and most notably with altitude. As a result a normally aspirated engine, one that depends on atmospheric pressure alone, will run better at sea level where there is more atmospheric pressure than it will going over a 10,000-ft. mountain pass where the pressure is less. But add a pump, or a supercharger, that forces air into the engine and it's a different story.
TorqStorm’s billet aluminum beauty is state of the art in centrifugal supercharger technol
How well the cylinders fill during the intake stroke is described as volumetric efficiency, or VE, and it's the difference between the theoretical maximum amount of air (or air and fuel) each cylinder can take in during the intake cycle compared to the actual amount taken in. For contemporary, naturally aspirated engines, a VE around 70 percent is close to the norm, over 90 percent is excellent, 100 percent is possible, and some competition engines can exceed 110 percent. Keep in mind these numbers are simply examples, but to put things in perspective, consider a modified 350ci engine running at 80-percent efficiency; the result is a powerplant that performs like it has 20-percent less displacement, or 280 cubic inches. Take that engine and add a supercharger and it's possible to get a denser air/fuel mixture into the cylinders. A little boost (the amount of air pressure created by the supercharger) can result in a dramatic increase of VE; at 130 percent, that same 350 is now being stuffed with the equivalent of 455 cubic inches of fuel and air. The bottom line is blown engines perform as if they had more displacement and as a result they produce more power.
Like the engines they're attached to, superchargers come in a variety of configurations. They may be crankshaft driven like a centrifugal or Roots type, exhaust driven like a turbocharger, and at one time superchargers driven by an electric motor were available. But regardless of the drive, there are basically two types of superchargers—fixed displacement and non-fixed displacement. Roots types pump a given amount of air per revolution. It should be noted that Roots superchargers do not compress air, they move it from the blower's intake port to its exhaust port—the increase in pressure, or boost, takes place in the intake manifold. But while Roots blowers are simple and reliable, the disadvantage is they heat the incoming air more than the non-fixed displacement designs.
Familiar to most hot rodders is the GMC Roots-style blower. Hot Heads offers complete kits
Centrifugal superchargers (crankshaft and exhaust driven) are not positive displacement because they do not move a fixed volume of air per revolution. The centrifugal supercharger impeller draws air into the center of the housing, it's then pushed to the outside of the impeller's blades by centrifugal force where it travels through a scroll that increases pressure. Centrifugal superchargers are simple and reliable and are ideal for "blowing through" fuel-injection throttle bodies.
Similar to crankshaft-driven centrifugal superchargers are turbochargers; the most noticeable difference is a turbo is driven by the engine's exhaust. On one side of the turbocharger the exiting gasses pass through a housing spinning an impeller on a shaft. The other end of that shaft has an impeller in a separate housing—that is the compressor section feeding the engine. While turbochargers are effective, they can suffer what is commonly called "turbo lag" or a delay in throttle response because the compressor wheel has to get up to speed to create boost.
The famous 1932 Ford drag car, the Orange Crate, used a Potvin front-drive GMC blower on a
Some of the fears of supercharging are that the engine will be difficult to drive on the street, unreliable, and will guzzle gas. The truth is a properly prepared blown engine won't be any more hassle than any other performance engine. About the only real concern is changing oil regularly, particularly when the supercharger's bearings are lubricated by pressurized oil from the engine. As for reliability, most superchargers will outlast the engine it's mounted on. And while no one builds a blown engine with economy as the primary goal, a Roots blower will usually cause a small drop in mileage, centrifugals and turbos little or none—of course if you've got the throttle pedal mashed to the floor constantly you'll pay to play.
When deciding to supercharge an engine, one frequently asked question is "how much boost will the engine tolerate?" The answer is—it depends. The biggest factor determining how much boost can be run is the engine's compression ratio. Generally speaking, the static compression ratio of the engine being supercharged should be in the 8.0:1 to 9.0:1 range. In most cases, the higher the static compression, the lower the boost level will have to be. On the other hand, compression can't be too low as it will result in an engine that is "lazy" when no boost is present.
Other common concerns are the engine modifications necessaryto accommodate supercharging. Not to oversimplify, but the more boost, the more modifications are necessary. For low-boost applications, 3 to 4 pounds on a sound, stock engine will suffice; cast pistons will even live. Upping the boost to 5 to 9-pounds will call for the next level of modifications, and 10 or more will call for some serious preparation. But let's look at it from a component standpoint.
In a Roots blower, the impellers draw air from the inlet forcing it to the outside of the
Simple in operation, the large beltdriven gear spins the smaller gear attached to the impe
9. These are the turbochargers found on Ford’s EcoBoost V-6. By using a pair of smaller tu