I Don't know anyone that has installed one and seen any performance or mpg gains. Sometimes they make a "cool" hissing sound that guys like but that is about all you will get from it. Read the first post in this thread. I am pretty sure that the same principles apply for a spacer.
http://www.gmtruckclub.com/forum/threads/what-do-cold-air-intakes-really-do.146280/
I found this searching the web. It is on the Internet so it must be true.. Just kidding
Before you run out and drop a C note for a gas saving, performance increasing, throttle body spacer for your rig, ask yourself a couple questions:
Does my truck have fuel injection?
Does a chunk of metal really need to cost $100?
Is the word "SUCKER" tattooed on my forehead?
If our third bullet point is true, then read no further because we have a gas saving, performance increasing modification for you.
With the cost of fuel these days, manufacturers are all looking for a way to capitalize on the American public but don't believe all the whoopla. Throttle body spacers, in theory, should work on a carbureted vehicle with minimal ducting and virtually no computerized control, but last time I checked, all of the late model trucks rolling off the assembly line are fuel injected unless its a Suzuki Sidekick, which probably sports a two cycle motor.
The Theory
The throttle body spacer is based on the theory that air entering a chamber will have a greater velocity and more mass if it spun. The helical groves on the throttle body spacer are designed to make the air start spinning as it passes through the grooves, thus delivering a greater volume of air to the ic engines cylinder and in turn, increasing the power of the detonation by adding more air to the fuel/air mixture. Additionally on older model vehicle with throttle body injection, the swirling air mixes the air and fuel much better, providing a more consistent spark and hotter detonation. More Air & stronger detonation = more Power. In addition to the increased power claims there is the claim that you will also experiance gains in fuel ecconomy as less fuel is required to make more power, meaning you will be able to save gas (up to 10% if you believe some of the manufacturers) if you don't alter your driving habits.
The Reality
Late model fuel injected cars and trucks have their fuel and air mixed just prior to being injected into the combustion chamber, meaning all the swirling going on in the intake manifold isn't happening with the fuel, so the "mixing" of the air and fuel doesn't happen. Older, carburated vehicles and ones with throttle body injection (older than a '95) can benefit from this swirling motion, however your brand new port injected Chevy Siverado isn't going to benefit at all from this action other than making a weird high pitch noise from under the hood that some kid might mistake for a turbo.
The Facts
We equipped a 2001 Ford Super Duty V-10 with an aftermarket throttle body spacer made by one of the name brand manufacturers out there. Prior to this test the truck had been modified with a cold air intake system and full 3.5" exhaust system from the headers back. When getting a baseline dyno, the best pull rendered 283.4 hp and 361.4 lb-ft torque (corrected for 93°F) at the rear wheels. After the addition of the throttle body spacer, the best we could pull 281.1 hp and 354.7 lb-ft torque. Now, we will concede that the engine was getting hotter and so was the day by the time we had the throttle body spacer installed (approx. 20 mins later) but according to the manufacturer we should have had an "noticeable increase in power" which is contrary to the reports generated by the Dynojet.
Summary
Purchase Price: $96.00
Horsepower Gain: -2.3
Torque Gain: -6.7 lb-ft
Minutes Installed: 32