Throttle body spacers are aftermarket devices that purport to increase horsepower in a variety of ways. The efficacy of this modification depends largely on the engine it is applied to and the engine's factory shortcomings.

Placement

TB Spacers are designed to be placed between the throttle body (the large butterfly valve that controls airflow into an engine) and the intake plenum onto which it is bolted.

Marketing

According to manufacturers, TB Spacers make power through better fuel atomization, which is accomplished by creating a "tornado-like" swirl in the intake manifold.

The Tornado Effect

Air-swirl effects tend to benefit engines with throttle-body injection more than those with multiport injection, since multiport systems use a dry intake manifold that does not contain any fuel droplets to atomize.

Plenum Effects

Throttle Body spacers make the majority of their power by increasing the volume of space between the throttle body and intake runners. This gives the engine more unhindered air to tap under high-demand situations.

Cost and Consequence

Throttle body spacers are worth a few horsepower on some older cars, and those newer multiport systems that came from the factory with an under-sized plenum. Bear in mind, though, that a $250 billet-aluminum spacer probably isn't going to make much if any more power than a $35 plastic one.

Source:

4x4 and Off-Road Magazine

Rod and Custom Magazine

CarParts.com

Resource:

TB Spacer Install (Video)