Superchargers
There are two basic types of superchargers: positive displacement and centrifugal. Both are most commonly driven off of the front of the engine via a belt and pulley system.
Roots, Lysholm, screw, Eaton, Whipple, GMC, Weiand, etc are variations on the basic positive displacement blower. They basically have two lobes that run a very tight seal against each other and are connected by two gears in sync with each other's rotation. These lobes compress air into the manifold and usually sit directly on top of it or in close proximity to it. Normally, air can not be sucked through the vanes if there is vacuum (decelleration).
The pressure rises in direct proportion to engine speed. Most consumer applications of this supercharger design are underdriven, meaning the blower turns slower than the crankshaft speed. Boost is governed by the diameter of the pulley and RPM.
The second kind of supercharger, centrifugal, is closely related to a turbo. It's actually a compressor wheel that's spun by the engine's crankshaft, but it's not a positive displacement system. It's boost also depends on engine speed, but at slower speeds the manifold still capable of engine vacuum. It creates boost via centrifugal force. Basically, the compressor wheel spins the air molecules out, away from the center of the blower into the volute of the compressor and then into the intake. Again, boost is governed by the diameter of the pulley and RPM.
Turbos
Turbos are essentially similar to centrifugal superchargers, as I mentioned, except that they use the force of exhaust (normally wasted energy) to drive a turbine impeller wheel connected directly to the compressor impeller wheel. The turbine energy is transferred to the compressor side and compresses the intake charge. This means that a turbo can have a speed totally independent of the crankshaft speed, but it will be proportionate to the exhaust volume.
Turbos are much simpler than superchargers, having just one moving piece. They aren't based on positive displacement, can operate at or below atmospheric pressure, and are much more fuel-efficient than positive displacement and centrifugal systems under normal operating conditions.
Turbos make boost on a pressure ratio. The exhaust pressure often is twice or three times the pressure going into the intake due to the turbine's restriction, but this can be remedied with different combinations and trims of turbos. In passenger cars, turbo boost is governed by wastegates that open when the peak boost signal hits their diaphragm, venting exhaust pressure away from the turbine.
Nitrous Oxide (N2O)
The use of a nitrous oxide injection system is one method of increasing power by 30% or more, depending on how it is setup. For street use with the stock bottom end (pistons, crank, rods, etc.), a 30% power boost should not over-stress the engine too much if reasonable rpm limits are observed. Nitrous comes in two flavors, U.S.P. for medical use and Nitrous Plus for "off road" engine use. Many speed shops keep Nitrous Plus in stock for refills, but be aware that, if you are tempted to "cop a high" by breathing nitrous, it contains a nasty toxin (sulfur dioxide) that is specifically added to discourage such use. Put it in your engine, not your lungs.
Nitrous oxide is an oxygen-bearing compound, or oxidizer, that allows additional fuel to be burned during combustion. That "additional" fuel must be injected along with the nitrous or the engine will become dangerously lean and may self-destruct. Several kits are available with varying sizes of jetting and nitrous storage tanks and most systems use a button to actuate a quick dose of extra horsepower. Refills can be a hassle, however, and certain electronics, fuel pressure regulator/fuel line and intake manifold modifications have to be made during installation to accommodate the injection of more fuel. Unless you have competent mechanical and tuning skills, the setup of a nitrous system should be done by professionals.
Nitrous tanks should be mounted in a location where they are protected from rupturing if car sites in direct sunlight, as bottle pressure can spike, causing a nasty explosion, or in event of an accident, the bottle will not rupture (the sudden release of a nitrous cylinder pressurized to 1,000 PSI could greatly complicate an accident).
Once installed and dialed-in, a nitrous injected Zetec can be a strong street machine, capable of surprising bursts of acceleration, especially if it is equipped with a free-flowing air filter, intake tract, mild porting, and an efficient exhaust system.
But these modifications will not make the Z engine scream just yet. That requires internal engine modifications but it can be done and still retain street reliability. It's mostly a matter of dollars..... like the speed shop sign says, "We sell horsepower, how many do you want?" The next step is more nitrous, a turbo or supercharging.
Differences in performance
A positive displacement supercharger will produce more low-end torque than a turbo, but you will quickly reach a limit where the blower will only provide so much air. The relationship between the blower and crankshaft is a fixed ratio. If it's an overdriven setup, then the blower might turn 1.5 times more than the crank, but it can never exceed that ratio.
With a turbo, you can obtain whatever rate of manifold pressure the wastegate is set for. It's possible to get, say, 15 psi of boost out of a turbo at 2000 rpm while a given supercharger might be able to achieve 15 psi only at full throttle (6000 rpm). Additionally, when the turbo is producing 15 psi at 2000 rpm's, it is also going to make 15 psi at 3000, 4000 and 5000 rpm's, and so on, until the compressor runs out of efficiency. Once you're under way, the blower is only going to give you X pounds of boost. At the point when you hit the rev limiter, boost cannot increase.
A positive displacement blower also superheats the air as it compresses it. And the fact that it's sitting on the intake manifold makes it a great heat sink. The centrifugal blower is a little better in that respect, but it's not going to give you as much low end torque because it, too, has to spool. Once you're up and running it's going to only give you a fixed amount of boost, again because it's tied to crankshaft speed. And most centrifugal setups are difficult to mount without custom mounting brackets and ducting.
That brings us back to turbos. You might already be thinking that if the positive displacement blower has heat problems, a turbo glowing under the hood is going to be much worse. It's true that the air going into the compressor is being compressed, therefore warmed, and it's right next to the red-hot (1500º-plus) exhaust.
This leads to the inevitable intercooled - vs. - non-intercooled debate. As long as boost pressure and intake charge temperature is kept in line, a car can take moderate levels of boost. The biggest keys are good quality fuel and avoiding detonation. This can be accomplished by injecting water, alcohol, or some other type of buffer (in addition to the octane in the gasoline).
If the boost level you desire is on the high side, you should probably start looking into intercooling--adding a device to the air tract that removes the heat generated by the turbo before it reaches the intake manifold. An intercooler is essentially a large block of tubes with fins that dissipate heat, like a radiator.
How much the intake charge is cooled depends on a multitude of factors from ambient temperature to core fin density to humidity. You'll note that a lot of the centrifugal blowers are now also going to intercooling. This allows them to build more horsepower with less chance of detonation.
Turbos have one clear performance drawback: lag, or slowness in spinning from idle speeds to the point where they generate meaningful boost. Driving technique can eliminate a lot of between shift lag as well as lag after takeoff. So can mechanical and electronic gadgets such as bypasses, and stutter boxes, which introduce unburnt gas into the turbo, which is quickly lit off, spooling the turbo.
For the typical racer, lag can be your friend (or enemy). One hot setup is using a two-stage boost system. These dial out the maximum boost while at the line, allowing the engine to get out of the hole cleanly and not bog. Once you are accelerating, you can hit the switch and get the full boost setting.
You can minimize lag in a couple of ways. One is to keep the turbo spooled during shifts. Some drivers do full-throttle shifts, not lifting for gear changes. This can be really hard on the driveline, but keeps the boost up. I wouldn't recommend this method for ordinary driving. The other method is a bypass valve. When you go to shift the throttle plate is closed, so the turbo is pumping against basically an impassable wall.
With a bypass valve, the pressure that builds up is bypassed back to the turbo to be recirculated and keeps the turbo spinning. Blow-off valves are basically bypass valves vented to the atmosphere and make a "pssfft" noise when venting.
Most of the time you can control boost lag by proper selection of the intake/compressor wheels. Big turbos have lots of horsepower due to their ability to move large amounts of air, but also lots of lag because of the added mass of the wheels. Small turbos have very fast spooling characteristics, but run out of steam quickly and can't attain the higher amounts of boost that big turbos do.
One attempt at a design solution to the lag problem is the variable nozzle turbo (VNT). Basically the turbine's nozzle changes diameter to vary the volume of the exhaust turbine side, and thus the turbo's A/R or area ratio. With less area, the turbo spools up faster, but then when you reach higher boost levels the nozzles open up to keep the turbo accelerating.
The result is you get the better low end spool-up of a small turbo and the high rpm power of a larger compressor. The VNT is a high tech piece, but accumulated soot can cause its vanes to stick and stop functioning. VNT turbos use the nozzles for the boost regulation in place of a wastegate. To learn more about the Garrett VNT, follow this link:
