I have had several requests to make this short write-up into a sticky so here it is:
Throttle Body spacers sound good in theory but do not work in practice. Assuming the spacer could impart enough spin on the incoming air to bring it to the level of better blending the A/F mixture it would still have to surmount a few hurdles.
First there is the throttle plate itself. In all but wide open applications this plate is sitting at an angle to the incoming air. This is kind of like having something lodged in your windpipe. The intake air enters through two crescent moon shaped openings at the top and the bottom of the throat of the Throttle Body. This means at best only a portion of the intake air is subject to any possible rotational action to be imparted by the Throttle Body spacer.
Second, let us assume that the air does remain spinning after passing the angled throttle plate. Now that air stream needs to be split into 8 separate paths - and mind you these paths are not continuous flow but rather start and stop spurts. As each cylinder draws air in through the open intake valves
air rushes down the intake runner into the cylinder. As it does so it rushes past the fuel injector (HOORAY!!!) which sprays fuel into the passing air stream. Hopefully the fuel is sprayed in rather than squirted as the idea is to have the fuel evenly dispersed as a mist in the air stream and not as droplets - as a side note clogged, blocked, or caked injectors result in coalesced fuel droplets rather than an even mist which decreases fuel economy so keep those injectors clean!
Now, back to what is taking place in the intake manifold and runners. The cylinder draws its air and fuel charge and then the intake valves spring shut. The air previously rocketing down the intake runner now slams to a halt. Simply stated, Newton's Third Law encompasses that for every action there is an equal and opposite reaction. Ooops. The air bounces off the now closed intake valves and the shock wave travels back up to the intake manifold where it collides with all the other air milling around in there. This is followed immediately and continuously by the air from the other 7 cylinders. Any spinning imparted by a Throttle Body spacer is systematically pummeled to death by the shock waves created when the air being drawn into the cylinders ricochets off the closed intake valves.
For this reason Throttle Body spacers do not provide improved fuel economy
. But the story does not end there.
There are ways to tumble the intake air as it enters the cylinders. A few years back I came across "intake tumblers" while researching a theory of mine. Audi did this awhile back by placing adjustable tabs in the intake runners just before the cylinder head. They would adjust according to intake velocity in order to provide the most beneficial mixing of the air and fuel as it entered the cylinder. They had some problems with the set-up and I am not sure if it was ever perfected but the idea is intriguing. It also brings to mind that any tumbling or spinning should be "tuned" to a particular RPM and Throttle Body spacers being a fixed object are unable to react to or adjust to changing intake velocities. If they did work it would only be in a very limited
RPM band - probably in the upper extreme RPM range where things got to moving so fast that each intake stroke of each piston would be so fast as to POSSIBLY
compensate for the reverse shock wave created by the intake valve
I have yet to have a manufacturer of Throttle Body spacers agree to provide their product for testing after explaining that I would post the results here in this forum.