Dropping The Boost Bomb - Tech - Super Street Magazine

Horsepower wins. You can score all the style points you want but without a heavy right foot you’re a poseur, a big dog with no bite. There are two mindsets when it comes to making power; go with step-by-step bolt-ons or drop the boost bomb. Boost fiends have three paths to glory: the turbocharger, the supercharger and nitrous oxide, known as boost in a bottle.

First, a little mythbusting. Boost does not make more power; it merely increases the potential for more power. An internal combustion engine makes power by burning fuel and doing so at a proper air/fuel ratio. Our Big Three power players, turbos, blowers and nitrous address the air portion of the air/fuel ratio equation while bigger injectors and tuning supply the additional fuel to produce big power.

Turbos and superchargers pressurize the combustion charge, forcing more air molecules into a given volume of air, so more fuel can be added at combustion resulting in more power. Visualize the combustion chamber as a Tokyo commuter train. Late at night there are only a few riders. This is a naturally-aspirated scenario. During rush hour (forced induction situation) the train car is crammed with commuters, which represent air molecules. The train car is the same size there are just more people (air molecules) occupying the space in the boosted scenario.

While both turbochargers and superchargers perform the same function, the big difference maker is how each is empowered to compress air. It comes down to drive.

Turbocharger
A turbo is driven by exhaust gases. A big part of its efficiency is that it recycles or harnesses exhaust gases that are on their way out of the engine anyway. The exhaust spins the turbine wheel in the turbine or hot’ side of the turbo, which can be identified by the turbo mounting flange. The amount of exhaust gases routed to the housing is controlled by a wastegate, that opens as pressure rises bleeding exhaust away from the turbine wheel.

The turbine wheel is on a common shaft with the compressor wheel. The middle ground between the turbine and compressor housings is the center cartridge, which houses the shaft bearings and is plumbed for oil to lubricate and cool these critical rotating parts. As the assembly spins the compressor wheel inducer draws air from the intake compresses it and blows it out the turbo nozzle and into the engine.

Supercharger
Where a turbo uses exhaust pulses to drive its inner workings, a supercharger uses a belt, driven off a pulley, to spin its compressor assembly. The big departure here is the parasitic drag of the drive system, i.e. how much power it takes to spin the supercharger. Another point of contention between the two methods of forced induction has to with the onset of boost, or boost lag. The argument being a supercharger, especially a positive displacement unit reaches its boost threshold quicker than a turbo. Proper turbo sizing can lessen the difference in response. The other side of the coin is a supercharger’s boost pressure is limited by pulley size while a turbo can be easily controlled on the fly to provide a wide variety of boost pressures.

There are two types of superchargers: centrifugal and roots/twin-screw positive displacement. The compressor section of a centrifugal blower resembles a turbo. The centrifugal design acts more like the supercharger’s blower’ nickname by using air speed to compress the air in the combustion chamber. The housing and wheels use turbocharger principles in their design and deliver boost in a similar manner. So a centrifugal supercharger needs to be pulley’d to attain its max boost at redline.

A roots/twin-screw positive displacement blower produces its boost differently than a centrifugal design. The positive displacement unit delivers linear boost because it pumps a fixed amount of air per revolution. So since engine speed rises in a linear fashion the supercharger can keep pace supplying the same maximum boost at all engine speeds. Behind the wheel, a positive displacement supercharged car feels like it has a bigger engine while a centrifugal produces a rush of power in a manner similar to a turbocharged engine. Generally speaking a positive displacement set-up will perform better below peak power at lower engine speeds than a centrifugal.

Looking at big power it is difficult to boost-up a positive displacement unit, you end up outgrowing’ the box that houses the screws. A centrifugal can be easily re-pulley’d to bump up the volume.

Nitrous Oxide Injection
Nitrous is called the chemical supercharger because it increases the amount of air in the combustion chamber by spraying nitrous oxide. Its boosting’ effect adds air molecules because nitrous oxide is 36 percent oxygen by volume while the atmosphere is 21 percent oxygen by volume.

There are two types of nitrous kits; dry manifold and wet manifold. A dry manifold system injects only nitrous oxide, usually at some point in the intake tract. Fuel enrichment is handled by the stock EFI system, sometimes via an adjustable fuel pressure regulator.

A wet system injects both nitrous and fuel at the nozzle which can be placed in either the intake tract via a fogger nozzle or aimed at the port from the intake plenum or the base of the manifold in a direct-port set-up, which features one nozzle array per cylinder.

A typical nitrous kit consists of a nitrous bottle, tubing, nozzles, flow jets, solenoids, an activation device, switches and miscellaneous wiring and fittings. Other items that may be standard or optional include gauges, bottle warmer, cooler spark plugs, remote valve opener and fuel system upgrades.

Nozzle jetting is the boost controller’ on a nitrous system. The size of the nozzle jets dictates how much nitrous is introduced to the engine. Bigger orifices mean more nitrous, which needs more fuel, which makes more power. The biggest advantages of nitrous is dollar-for-dollar the bottle it’s the cheapest way to get boost.’ The major drawback of nitrous is refilling the bottle, which is inconvenient and costs money.

Whether you choose the sound of screaming turbos, howling superchargers or stealthy nitrous oxide, dropping the boost bomb is the most direct route to big-power nirvana. So open those bomb-bay doors and let the ordinance loose.

IntercoolersMaximizing Power Potential
There is no doubt that an intercooler is an integral part of any successful boost system. An increase in charge-air heat is a natural by product of compression and cooling the charge packs more air molecules in the engine remember the Tokyo commuter train? Cooler, denser intake air opens the door to additional boost, more aggressive timing and more power.

Intercoolers reduce the engine’s detonation threshold by cooling the charge air anywhere from 80 to 200 degrees, which returns a good amount of the density to the charge air that is lost during compression. The detonation threshold is the point where the air/fuel mixture ignites by itself without the spark plug initiating the combustion event. The mis-timing’ of this event can cause catastrophic damage to engine internals.

Intercooler Types
There are two kinds of intercoolers; air-to-air and air-to-liquid. Typically air-to-air units are cheaper, easier to design/install, more reliable and need less maintenance than air-to-liquid set-ups. Air-to-liquid coolers can provide better response, better low-speed cooling performance and less pressure drop. The simplicity and low cost of air-to-air make them the predominant choice among today’s boost enthusiasts.

Air-to-Liquid Design
This style of intercooler uses liquid to dissipate heat. Water is roughly four times more effective at heat dissipation than air. The core is enclosed and water is pumped through the outer core. The charge air runs through the inner core and heat is transferred. The water is in a closed loop system with a reservoir, dedicated pump and in some cases its own integrated cooling system

The Chemical Intercooler?
Water/methanol injection kits from the likes of AEM, Snow Performance and Coolingmist increase intercooler efficiency by providing an additional conduit to cooling the charge and maximizing density. The highly-atomized mixture absorbs heat and vaporizes in the intake tract. When it enters combustion chamber it helps cool the cylinder walls. Systems can be set-up to come online at WOT or at a pre-set boost levels

Air-To-Air Design
The most popular design by far, air-to-air intercoolers use air as the cooling element in much the same way a radiator cools the water circulating through the engine. There are two different air-to-air types of construction; Bar-and-Plate and Tube-and-Fin. The differences between these designs include pressure drop across the core, frontal area, fin density, weight and manufacturing costs and explaining them is a story in itself.

Sources:

Garrett Turbochargers, turbobygarrett.com
Lysholm Technologies, 805.247.0226, lysholm.us
Nitrous Express, 940.767.7694, nitrousexpress.com
Procharger, 913.338.2886, procharger.com
ZEX, 888.817.1008, zex.com
Mackin Industries (ARC), 562.946.6820, mackinindustries.com
AEM, 310.484.2322, aempower.com