The classic hot-rod notion suggests there's no replacement for displacement. Detroit realized this in the early '70s as everyone except AMC, who made smaller cars, had an engine well over 400 cubic inches and Cadillac was up to 500.
But then the gas crunch hit in 1973, and the emphasis went from making cars bigger to making them more economical. Six-cylinder engines returned to full-size cars, but were often inadequately powered. A few years later, forced induction began a resurrection.
If you can't make the engine bigger, the next best thing is to make it think it's bigger, and since engines are little more than glorified air pumps, pushing more air in became the solution. Maximum engine output only required a small percentage of time, and smaller engines have less internal friction, weigh less, and generate fewer emissions off-boost. As a result, turbochargers began to appear on everything from Buicks to Porsches, and, by the early '90s, GM had installed them on the Syclone and Typhoon trucks. If you can make that boost at lower revs, you'll save a lot of engine wear and breakage since connecting-rod loading goes up exponentially with speed. The advent of fuel injection hastened the process because it eliminated "wetted wall" constraints from the airstream.
| Eaton Roots-style Supercharger
A supercharger or turbocharger can generate up to seven times the output of a same-size normally aspirated engine, assuming that the engine is constructed for such use. Top-fuel dragsters make over 6000 horsepower, F1 turbos made 1000 horses from 90 cubic inches, and current turbochargers can support engines over 4000 horses. As an added benefit for the western half of America, forced-induction engines maintain much more of their output as altitude increases.
In current worldwide engines, GM offers both turbos and superchargers on gas engines as does Mercedes-Benz, with forced induction available on almost any brand. We'll provide some information for you to consider, but won't pick which is best for your application. It's up to you to decide that a particular setup, like the Volvo marine diesel that uses both a supercharger and a turbocharger, is what you want.
Supercharger A supercharger is a mechanically driven positive-displacement pump that pushes additional air into the intake stream of an engine. Typically, the three types are identified as centrifugal, which has an impeller much like the compressor side of a turbocharger (as a result, it's occasionally called belt-driven turbocharger); screw-type (which has twin rotating screws); or roots-type (counter-rotating rotors).
Although the centrifugal type that runs up to 40,000 rpm is popular in the aftermarket and slightly easier to tune, most factory offerings are screw- or Roots-style systems that run up to 15,000 rpm and generate 5-9 psi of boost. These designs are utilized on the Ford Lightning, Nissan Frontier/Xterra, GM 3800 V-6, Jaguar V-8, and Mercedes I-4, V-6, and V-8 engines.
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When we asked our experts--including Bill Owen from GM Powertrain, Dave Dempster from Ford SVT, Sly Alviar from Nissan Motorsports, and a host of others--about the advantages of a supercharger, a primary reason was "faster response time" or "better low-end response."
Since a supercharger is mechanically driven by the engine, it's always on and running, capable of making boost, although this does mean some amount of power is being used to drive it at all times. For example, the GM 3800 V-6 can get to full boost in 2/10ths of a second, and low-end boost is usually what you want, especially with an automatic transmission. The GM 3800 makes 240 horses at 7.5-psi boost, but because of improved efficiency, the '04 Grand Prix will have 20 horses more at the same boost pressure and intake temperature as the '03 model.
A good supercharger matches blower size with the volumetric efficiency of the engine, and, because it operates parallel with engine speed, it can make a flat boost curve from 2500 to 5500 rpm. This in one of the aspects that makes a supercharger less complex from a control standpoint and why few of them are fitted with wastegates or pop-off valves--they simply can't overboost the engine unless you modify something.
Another advantage cited for the supercharger was the cost, as it's frequently the least-expensive way to get significant power increases. The Nissan Xterra/Frontier supercharged V-6 runs peak boost of less than 5 psi and no intercooler, so required changes included lower-compression pistons, a different intake manifold, and a belt-drive system, none of which is overly expensive.
| The GM Duramax turbodiesel mounts the turbo near the back of the valley; tighter space in a car or small utility would make this difficult. It looks neater here because none of the intercooler plumbing is pictured.
Supercharging the 3.3-liter engine raises peak output by 30 horsepower and 44 lb-ft of torque (for automatics)--and both occur at the same peak rpm levels as the naturally aspirated version, 4800 and 2800 rpm, respectively.
Those numbers don't make the 4000-pound Xterra fast by any measure, but the extra power is welcome for low-speed passes and many off-road conditions. On the other hand, the Lightning uses its supercharger for speed and instant throttle response. Although it generates copious amounts of power, excessive heat buildup underhood isn't a problem because, unless pulling a trailer, it takes less than 20 seconds of wide-open throttle to hit its speed limiter.
A few people mentioned lower underhood temperatures as another supercharger benefit, which means less stress not only on the engine and cooling systems, but also on ancillaries like power- steering pumps, wiring, hydraulic cylinders and lines, and so on.
Turbocharger Like a supercharger, a turbocharger condenses the air going into the engine by compressing it, but the energy used to drive it comes from exhaust-gas energy that would otherwise go out the tailpipe. It can't be called free power because the turbine wheel (exhaust side) causes a pressure restriction in the exhaust stream: It's not uncommon for the exhaust-gas temperature to drop 300-400* F as it passes through the turbine wheel and converts heat energy into the mechanical energy that, in turn, spins the compressor wheel (intake side).
The wheels inside a turbocharger usually rotate between 75,000 and 150,000 rpm when they're working hard. As such, the material must be balanced, stand up to considerable heat and pressure, and overspeeding may run into problems if the blade tips go supersonic. Most turbo manufacturers use a limit of about 1900 feet per second. In addition, any production turbo has to meet containment standards so that if it ever fails, shrapnel won't explode from it and damage surrounding machinery.
Unlike superchargers, turbochargers have just one moving part, and fewer parts often indicates fewer problems. However, because the lone moving part operates at such high speeds, lubrication is critical. Opponents of turbochargers have long held the oil-coking issue as a major detraction, but improvements in synthetic oils, ball-bearing designs, water-cooling, and electronic engine-management have made this issue less critical. Turbochargers have been used on almost every class of race car, including dragsters, F1, Pro Rally, and, in four years of 24 Hours of Le Mans, the winning three-car Audi R8 team hasn't had a single turbo failure.
Relative to superchargers, turbochargers tend to be quieter because the turbine is embedded in the exhaust system and not near a throttle butterfly. Where superchargers whine under boost, turbos whistle, and, given enough of it, either one can get on your nerves. Since some of the exhaust energy is driving the turbo, and it all has to pass the turbine, turbos have quieter exhaust tones; so much so that a few of them use no more than the catalytic converter for muffling tailpipe noise.
With the adiabatic efficiency (temperature rise versus boost pressure) of modern turbochargers approaching 75 percent, turbos tend to have an advantage over superchargers. As GM's Owen notes, "At the sweet spot in the A/R ratio, a turbo would impart less temperature addition to intake air than an Eaton-style blower at 10 psi." Advances such as Garrett's Variable Nozzle Turbine and electronics are making that sweet spot wider, and turbochargers have always been an efficient way to add boost in constant, stable rpm loads such as over-the-road diesels, locomotives, and marine propulsion.
Literature suggests it's easier to make more power with a turbocharger than a supercharger and that they're easier to tune, as well. However, as turbochargers get larger and capable of delivering more air, they can also suffer more from lag. Lag is the term used to describe the delay between when power is requested via your right foot and when there's enough exhaust driving the turbine wheel fast enough to make boost. Lag can be limited by electronic throttle and engine management, ball-bearing turbos, ceramic turbine wheels, wastegates, and variable nozzle and variable geometry turbine housings. These same parameters also mean that a turbocharger can be sized to provide peak boost pressure earlier than a supercharger.
Tuner John Lingenfelter reports his 2.2-liter Ecotec I-4 engine Cavalier goes from 0 to 35psi boost in 2/10ths of a second, although this isn't a mass-production automobile. In many production cars and most turbodiesel pickups, lag is noticeable only from a standing-start takeoff and not sending all that torque to the drive wheels at once helps longevity. Indeed, many small four-cylinder turbos develop peak torque below 2000 rpm, and some artificially limit output in first gear to avoid wheelspin and driveline breakage.
Ball-bearing (as opposed to sleeve bearing) turbochargers such as that in the MazdaSpeed Protege spool up 15 percent faster, use about half the energy to drive them, and are 2-8 percent more efficient.
Variable turbochargers have vanes that move, actuated by electro-hydraulic systems with low-current draw (0.4-1.5 amps on the 6.0-liter PowerStroke). Adjustable vanes allow the turbo to build higher air density at lower engine rpm, improve response, and help high-speed economy.
On the horizon are units such as Garrett's e-Turbo, which uses a motor/generator to spool up the turbine ahead of load and capture exhaust- gas energy in the "generator" mode, cast-titanium wheels, oil-less turbos that use air bearings, and multistage turbos. The new flexibility has spelled the demise of expensive sequential turbocharging as employed on the Porsche 959 and last Mazda RX-7.
From an installation standpoint, turbos are more complicated than blowers. Being an integral part of the exhaust system, they generate more heat in the engine compartment and require more time for the exhaust to warm up enough to light off the catalyst. In addition to exhaust manifolds that run at higher pressure and temperature than supercharged engines, the manifold has to hold up the weight of the turbocharger, and some manifolds, both stock and aftermarket, simply aren't up to it.
Although the average turbocharger is smaller than the average supercharger, packaging is more difficult because of added pieces. Amongst all these hot pipes, the turbocharger needs an oil supply to keep the one part moving, a drain for used oil, and ducting to get intake air from the filter to the turbo(s), the intercooler(s), and then back to the intake manifold. The longer the plumbing, the more likely lag is noticeable, but a turbo can be placed anywhere in an engine compartment and need not be near a drive belt.
A single turbo in a V engine fits nicely, but may be too large for a gas engine. Smaller turbos offer better response, so some import V-6s and the Mercedes V-12 use a single turbo for each bank, with associated plumbing. Finally, turbochargers in factory applications tend to add more money than superchargers, though the gap is not as wide in aftermarket or custom installations.
Pick One In everyday truck driving without a trailer, a forced-induction gasoline engine is likely to be under boost conditions less than 10 percent of running time. But if that time is on a big hill or at a dragstrip, a supercharger or turbocharger may be right for you. The turbo will be better for your diesel or Interstate travel, and the supercharger will be better for four wheeling, drag racing, or hilly areas. Note that centrifugal supercharger systems tend to build boost exponentially as rpm increases, so a wastegate or pop-off may be advised.
Although neither system needs a big cam and lumpy idle, bolting on horsepower might require some internal changes. In general terms, on 93 octane unleaded in an engine with standard compression, an aftermarket supercharger should develop 5-6-psi boost, although this is without an intercooler and may require retarding the timing. If the compression ratio is lowered, max boost could run 7-10-psi, depending on supercharger efficiency, and adding an intercooler will allow another 3-4 psi.
If you go too far in a gasoline engine, it breaks; if you go too far in a diesel engine, it melts. Fortunately, an exhaust-temperature gauge can monitor your turbodiesel for longevity. Power gains are impossible to predict, although turbodiesel pickup owners use a rule of thumb that one pound of boost equals about 10 horsepower.
An intercooler (also known as an aftercooler or charge air cooler) will decrease the temperature of the intake air close to what it was before the turbo; the most efficient drop it by 175-225 degrees F. Since the intercooler lowers the combustion temperature, it also lowers NOx and CO emissions.
Finally, regardless of which type you choose to add or upgrade, remember that the transmission, rear axle, and brakes may not be happy about the changes.
