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Advantages and Disadvantages of a Throttle Body Spacer
Throttle body spacers aren't quite the "fuel tank magnets" of the 21st century, but they're at least halfway there. While there is some solid science behind using them in some applications, most TB spacer manufacturers choose to gloss over the hard facts in favor of more eye-catching and easily digestible marketing gimmicks. The TB spacer isn't necessarily a bad investment, but do your homework before laying down the cash for one.
Claims and Theory
The manufacturing claims behind the throttle body spacer usually follow this form: "Improves fuel economy, horsepower, torque and unicorn emissions by" alternately "inducing a swirl in your air charge" or "straightening it out." Some manufacturers machine little divots or swirls into the spacer to help the airflow to assume a linear path into the engine or to swirl it in a "tornado" in your intake. The idea is that swirling the air imparts more energy to it, causing it to tumble and better atomize the fuel. Laminar-flow proponents claim that smoother airflow through the throttle body enhances net flow. We'll see.
Induction Basics
Air flowing into an engine doesn't just flow straight through in one big mass. When air flows along a surface, some of the air "sticks" to the surface, forming a very slow-moving boundary layer that acts as a sort of "lubricant" for air flowing over it. When air goes through your throttle body, a boundary layer of stagnant air forms on the inside of the throttle body bore and on the front of the throttle blade. Air flowing over this boundary layer goes into the intake manifold, where it sits in a central chamber -- the plenum -- waiting to be sucked into one of the intake runners.
Boundary Layer and Grooves
Here's a bold statement, backed up by about four centuries worth of physics: Any manufacturer that claims its spacer induces a helpful straightening or swirling of the air is -- wait for it -- lying. The boundary layer that builds up on the throttle body walls is thick enough to cover those machined swirls in the spacer bores. This is true even of spacers that use very thick, helical grooves, because the boundary layer will simply fill them like cement and the air will skip over them. So, the best-case scenario is that the airflow completely ignores the helical groove. The worst is that the groove makes the boundary layer even thicker, restricting flow. But that slight disruption in airflow will do one thing, at least: It'll produce a constant whistling or roar to remind you that you installed a throttle body spacer.
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