POWER! It's all about POWER! Captain Kirk was always on poor Scotty's case about "more power," and Tim Allen's life seems to revolve around a never-ending search for it. For hot rodders, more power has never been easier to find. Of course, it costs money, but then, what doesn't? When it comes to power, the two easiest places to get it is with supercharging or nitrous oxide. Bolt on a blower or slip on even a budget nitrous kit, and 100 to 250 more horsepower is as close as the throttle pedal. So which one is better? Let's explore both of these power producers, and you can decide for yourself.
BLOWER BASH '96
There has never been a time in hot-rodding history when there have been so many ways to force-feed your engine. The theory behind superchargers is as old as the internal combustion engine. Normally aspirated engines are fed air by atmospheric pressure. One way to increase power is to compress air into a denser form using a compressor or air mover with which to shove more air into the engine. Combine that denser air package with the right ratio of fuel, and that engine will make more power—guaranteed. But like always, there's a catch.
Place your hand on the tank of a large air compressor when it's running, and you'll quickly learn that compressed air heats up. Hot air is also less dense, so you have two physical properties working against each other here. As the air gets compressed, it gets more dense, but it loses some of that density as it heats up. The best superchargers are the ones that heat the air the least. Rating how much the air heats up is called adiabatic efficiency. The second half of the power equation is how much air the supercharger can move at a given time.
Racers have long known that every supercharger has its own maximum boost point. Crossover occurs where the discharge temperature of the air, the manifold boost in pounds per square inch (psi) and the horsepower required to drive the blower converge. If you spin the blower any faster (by changing the drive ratio of the pulleys, for instance), the engine will actually make less peak power; the density of the air in the manifold is actually less than it was at a lower boost level, so the engine makes less horsepower than it did at a lower boost level. This combined with the additional power required to spin the blower faster actually reduces the peak horsepower.
There are currently three major styles of superchargers available for street and/or competition engines. The Roots or GMC-style blower is the most readily identifiable and comes in two- and three-lobe rotor designs. These superchargers are really just air movers.
They displace more air—even when spun slower than the crankshaft—than the engine can use and therefore "stack up" air in the intake manifold, creating pressure in the intake manifold that's read as boost.
Slightly different from the Roots design is the relatively new screw supercharger. Unlike a Roots, the screw supercharger is a true air compressor that squeezes the air between two intermeshing screws inside a blower case. You can get a closer look at the performance characteristics of a screw supercharger in the accompanying "Pressure Point" story (pg. 52), which outlines the new WhippleCharger. This supercharger cranks!
Centrifugal superchargers are the most resurgent of late. The acceptance of electronic fuel injection (EFI) has made centrifugals popular again. Older hot rodders may recall the use of a McCulloch belt-driven centrifugal blower on carbureted Thunderbirds back in the mid-'50s, but the blowers never caught on due to functional difficulties in a blow-through situation with carburetors. The combination of EFI and centrifugal blower's turbocharging technology for turbine designs has breathed new life into these blowers. The two leading companies in centrifugal blowers are Paxton and Vortech, each with internally gear-driven superchargers that are capable of serious power output. Ken Duttweiler, who has tested some of the most powerful centrifugally supercharged engines in the country, has used these blowers to crank power levels for 305ci Ford and Chevy engines up to 950 horsepower!
HORSEPOWER IN A BOTTLE
Just like superchargers shove more air into an engine, nitrous oxide injection makes this happen through the magic of chemistry. Nitrous oxide's chemical formula is N2O, as it incorporates two nitrogen atoms with one oxygen atom. It is an inert gas, meaning, among other things, that it is not a fuel. However, the one oxygen atom will oxidize additional fuel, which is one key to why nitrous works so well. By injecting measured amounts of nitrous oxide and additional fuel, you can safely make more power.
Another advantage of N2O is that it's stored at around 900 psi in a pressurized bottle that stores the nitrous gas as a liquid. As this liquid is introduced into the engine, it cools dramatically, also reducing the inlet-air temperature and the air and fuel in the cylinders. This cooling increases the air/fuel mixture density, which not only improves power but also reduces the engine's tendency to detonate.
How much horsepower can you generate on nitrous? The better the airflow potential of the base engine, the more horsepower you can make with either nitrous or blowers. It's not uncommon for nitrous to pump an additional 400 to 500 horsepower into an engine. This, however, is dangerous ground. Horsepower levels like these don't come without a price.
Any way you look at it, horsepower is easy to get and easy to make. Superchargers offer instant power anytime you hit the throttle. Nitrous is easily the best bang for the buck horsepower investment you could make. Either path will get you where you want to go—quickly.
