Supercharged - Garage.Tech

More Boost, Less Waiting
One of the biggest disadvantages of turbochargers is that they're essentially gas turbines driven by the exhaust stream. In order for a turbo to make boost pressure, the engine has to generate enough exhaust energy to spin the turbine wheel. This waiting period can take so long that it's actually been given a name: turbo lag. Turbo lag, especially with big, high-powered turbos, can be considerable. A turbo car's throttle response before the boost comes on can be lackluster. Much of the engineering that goes into turbo systems considers ways to reduce this. When a turbo does spool up, the power increase, especially with bigger turbos, tends to be rapid and non-linear. This just isn't the progressive, steady increase of power most drivers prefer.

A supercharger's primary performance advantage over the turbo is its progressive build-up of power with little discernable lag. A supercharger simply feels like a bigger, more powerful engine. In contrast, a turbo usually feels flat off idle with a sudden rush of power as things spool up. From a driveability standpoint, the steady, linear build up of power is, in many cases, more desirable than the wait and frenzied rush of a turbo.

Superchargers' response characteristics are inherent by design. Being driven directly off the crankshaft, there is a direct 1:1 correspondence between engine speed and how much additional air will be delivered to the engine. There's no lag and no waiting for the pressurized air to be delivered to the waiting cylinders.

In all-out racing applications, turbochargers are the undisputed power champs. The rules of thermodynamics dictate this. It takes a considerable amount of power to compress a large amount of air. To generate enough airflow to produce 1,000 hp from your EVO would require sacrificing about 80-120 whp just to turn the compressor. A turbocharger recovers most of this power as waste heat that would otherwise go out the tailpipe. A supercharger takes this power from the crankshaft, subtracting its power requirements from the total. This is called a parasitic loss and is why, especially with high-powered applications, the turbo always wins.

Well what about Top Fuel drag cars, you might ask. Since these undisputed power champions are supercharged with huge blowers, you might think there are exceptions to the rule; actually there are none in this case. The blower on a Top Fuel car can suck up to 800 hp from the crank! A Top Fuel car is chemically boosted from the oxidizer in its nitromethane-based fuel in addition to the blower. If turbos were allowed in Top Fuel, they would only make more power.

Interestingly enough, where a supercharger might reveal a power advantage over a turbo is in low-boost, street-type applications. This is because a street supercharger, despite its 10-20hp drag on the crankshaft, doesn't have the backpressure-inducing turbine that a turbocharger has in the exhaust stream. Street-type turbo systems typically have smaller compressor wheels and smaller exhaust housings in hopes of lessening lag and quickening spool up. Small turbines and housings induce a lot of backpressure, typically one and a half to two times the boost pressure. As a result, because of its much lower exhaust backpressure, a supercharger can yield better volumetric efficiency and a greater power potential than a small, quick-spooling, low-boost turbocharger. The turbocharger's big advantages of superior compressor adiabatic efficiency and waste energy usage usually do not come into play until larger, less-restrictive turbine turbochargers are fitted and higher boost levels (8 psi and up) are realized.