What Is a Throttle Body Spacer?
A throttle body is defined as a valve of any type used to regulate airflow into an engine. These assemblies almost always use either one or two large butterfly valves, which modulate airflow into a large open space in the manifold called the plenum. The effectiveness of throttle-body spacers depends largely on the application, type and size of spacer used.
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Spacer Theory
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Most throttle-body spacers are designed to increase horsepower by causing the incoming air to swirl around like a tornado, which helps break up large fuel droplets for a more even burn in the cylinder. This "atomization" ideally entails the fuel tearing into droplets so small they become vapor, which evenly distributes itself in the incoming airstream.
TBI and MFI
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Almost all new cars use multi-point fuel injection (MFI). In this system, the intake manifold carries only air, and fuel is injected into the individual intake runners just ahead of the valve. Throttle-body injection (TBI) systems are found on many older engines, and are very similar to a carburetor in function and appearance. These systems use a throttle body that bolts on top of a manifold like a carburetor, but uses one or a pair of downward-pointing fuel injectors to spray fuel through the butterfly valves and into the engine.
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Plenum Effects
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The horsepower of most throttle-body spacers is generated when they move the throttle body a little further out, increasing the plenum volume. To a point, a larger plenum will help increase top-end horsepower in some cars, especially those whose plenums were undersized by the factory. In applications such as this, grooves cut into the spacer's body have little to no effect on power production, except possibly in TBI engines.
Atomization Effects
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Though many companies are loath to admit it, the spacer's primary job is to increase plenum volume, never mind all the "tornado effect" hype. This is especially true in MFI engines, where the last thing you want to subject your air-only (dry) intake to is highly turbulent airflow. The swirl effect is only known to benefit applications that use decades-old TBI systems, which may indeed need some help to adequately atomize the fuel.
Conclusion
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Throttle-body spacers really are worth a few horsepower on most cars, but bear in mind one caveat: Generally speaking, if your car has a stock MFI intake manifold, you're not going to make any more horsepower with a $250 billet-aluminum spacer than you are with a similarly sized, $35 plastic unit. TBI cars usually stand to benefit a little more, because the spacer adds plenum volume and can help to give the fuel more opportunity to vaporize.
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- Photo Credit Image by Flickr.com, courtesy of Reg Mckenna
