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.

Effects of Swirl and Flow

In the modern multi-point fuel injected engine, fuel enters into the airstream just ahead of the cylinder head, at the end of the intake runner. So, let's say, hypothetically, that you do have a vortex of air swirling in the plenum. When an individual cylinder's intake valve opens, it's going to pull a massive chug of air from the vortex. That air goes into the intake runner, where it establishes a boundary layer on the walls and flows through as it normally would. So, even a working "vortex generator" is pointless. If the divots could actually straighten airflow, it still wouldn't matter, because pressure reversion waves coming from the cylinders would stop it in its tracks, anyway.

Intake Manifolds

So, now that you know what spacers don't do, you may be wondering why they work at all in the applications where they do work. The likeliest reason has to do with plenum volume, or lack thereof. Engine families typically come in multiple displacements, and it's not unusual for the largest engine in a given family to run 25 percent larger than the smallest. Manufacturers typically prefer to save a few dollars by using the same intake manifold for every engine in a family rather than to create several optimized versions for each bore and stroke combination. So, if you've got one of the larger-displacement engines in an engine family, you could have wound up with a plenum about 25 percent undersized for the application.

Plenum Volume and Performance

Carburetor spacers have long been the go-to solution for the plenum-volume problem, and they're known to add horsepower and torque in many applications. The plenum acts as a sort of air reservoir for the cylinders, ensuring that the runners always have oxygen on tap regardless of rpm. Increasing plenum volume with a carburetor or throttle body spacer doesn't make power, per se -- rather, it's one way to correct a problem foisted upon you by corporate bean counters. You might see some improvement in fuel economy, but it's fairly unlikely unless the engine desperately needed the extra plenum volume. In fact, the engine may respond to the additional airflow by increasing the amount of fuel injected, which could cause a decrease in fuel economy.

What it Boils Down To

Throttle body spacers can work in some applications; all engines are different, and it's not impossible that the additional plenum volume could help to compensate for a deficit from the factory. So you may see a bit more horsepower and midrange torque, depending on the car. But trust it and take it to the bank: In this world of Corporate Average Fuel Economy standards, Detroit, Japan and Germany wouldn't hesitate for a second to add a $2.50 piece of plastic to the engine if they knew it would enhance fuel economy by any quantifiable amount. Top-end horsepower may be a sacrificial lamb on its best day, but tiny gains in fuel economy are liquid gold to auto manufacturers.