Once used by Aragorn, later King Elessar Telcontar, to defeat the forces of Sauron, throttle body spacers have been independently proven to create tornadoes in your engine, turn lead into gold, raise unicorns from the dead. Actually, they just add a bit of volume to an intake manifold's central plenum, but don't tell that to some of the people who market them for use on modern fuel-injected engines.
Intake Manifold Basics
Air enters your engine through the air filter, passes through the mass airflow sensor and intake tube, and then flows into the main air valve, or throttle body. The throttle body valve regulates airflow into the intake plenum, the central air reservoir of your intake manifold. When the intake valve on a particular cylinder opens, vacuum created by the falling piston sucks air from the plenum and into the cylinder via the intake runner passages. On a carbureted or throttle-body injected engine, fuel enters the air in the plenum; on a modern multipoint-injected engine, air gets injected at the end of the runner just ahead of the intake valve.
Intake Plenum Volume
A plenum acts as a reservoir for air, and its size and shape have a lot to do with engine performance. A moderately sized plenum will provide the runners with adequate airflow regardless of rpm. A smaller plenum will increase air velocity going through the runners, which enhances low-rpm torque, idle quality and -- potentially -- fuel economy. A larger plenum boosts top end horsepower, potentially at the cost of torque and idle quality. Generally speaking, an intake plenum should measure about 80 percent in volume for a typical naturally aspirated engine, about 100 to 110 percent the displacement of a naturally aspirated street engine, and upward of 150 percent the volume of a turbo- or supercharged engine.
Throttle Body Spacers and Plenum Volume
Manufacturers tend to build engines in "families," using the same basic parts for engines in the same family of differing displacements. It is cheaper for the manufacturer to use the same intake and exhaust manifolds on all engines within a family. This often leads to intake plenums sized for the smallest engine in a family, which starve the largest engines. The Chevrolet 305 and 350 V-8s are prime examples. The intake plenum sized for the 305 choked the 350, which lost out on top-end horsepower because of it. So if you've got the largest engine in an engine family, then odds are that you'll pick up some horsepower by adding plenum volume with a throttle body spacer. But if you've got the smallest engine in a family, you'll likely just lose bottom-end torque.
Tornadoes in Your Intake
There's a myth going around that creating a "vortex" in your intake will better atomize fuel, resulting in better fuel economy, lower emissions and an extra 700 horsepower. There's just enough truth to this idea to make it sound viable, but it's simply not on most modern engines. Carbureted and throttle-body-injected engines can indeed benefit from a bit of extra fuel atomization, since fuel tends to drop out of suspension in the plenum and go into the cylinders in droplets instead of in a fine mist. But modern multi-point injected engines don't inject fuel until the end of the intake runner -- far away from the tornado in your plenum. In this case, wasting kinetic energy by needlessly swirling air in your plenum is likely to result in a loss of power, fuel economy and torque.
- Engine Airflow: A Practical Guide to Airflow Theory, Parts Testing, Flow Bench Testing and Analyzing Data
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