Alright Rob....I don't have any dyno tests for you but I can explain what I'm saying so that it makes sense.
The throttle body spacers originated back in the day of carburetors to give more time to mix the fuel and air before the A/F charge entered the cylinder for combustion. This worked well especially at higher velocity airflows (mid-high RPMs) when the mixture time was small. The vortexes/turbulence added to mixing the air and fuel, but it did so at the expense of pressure drop (nothing is free). It DID increase power, increase efficiency, and decrease emissions because there was less unburnt fuel left in the exhaust stream. More of the fuel was burned when it was supposed to.
Then came the next generation of intake systems....the dreaded TBFI (throttle body fuel injection). This setup was like a carburetor but used a couple of fuel injectors in place of the fuel jets. My '80 Lincoln Towncar has this TBFI setup. The same things that made the TB spacers work for the carburetors made them work for the TBFI setup.....more time for the fuel to atomize and mix completely before entering the burn chamber. It still cost some energy in the form of pressure (more vacuum at wide open throttle) but again, the pros outwieghed the cons (if you could fit one under your hood).
Now we get to the next form of fuel injection....MPFI (multi-port fuel injection). This is what all of our Explorers have, even in '91 when they were introduced. One injector per cylinder (right above the head in the lower manifold) sprays/atomizes fuel into the airstream, giving almost no time for extra mixing. This is one reason why if you have a partially clogged fuel injector or one that has gummed up where the nozzle is streaming (like from a garden hose) instead of misting, you will see pretty significant decreases in economy and power. There is just no time for a stream/jet to mix when it is injected that close to the intake valve. There is no way a TB spacer can tumble/mix the A/F mixture in this scenario because it is only air at that point. It doesn't lengthen the mixing time at all (as it did in the two previous cases).
There are designs that tumble the air, spin it in a vortex, and generally make it more turbulent. As I said before, nothing comes without a price. It takes energy to tumble air, and that energy has to come from someplace. In this case it is supplied from pressure (vacuum). At idle, the throttle plate is closed and there is around 16-18" water column (WC, meaning it can suck a solid column of water up 18" in a tube with the vacuum applied at the top of the column and the bottom exposed to the atmosphere). When you start to open the throttle valve (basically a butterfly valve) air at atmospheric pressure (unless the engine is boosted by a turbo/supercharger) is pulled into the evacuated space. At partial throttle, the valve acts like a restriction to that flow, which could just as easily be a wad of cotton, a shop towel, a mouse, weasel, etc....just something to keep it from flowing "freely". When "full-throttle" or "wide open throttle" (WOT) is desired, the throttle plate is at 90 degrees to the flow, which is its least restrictive position. To add turbulence to the airstream you have to have some restriction, which means that it would never truely be WOT, as it would always have a restriction. So this is why I think that having a TB spacer could actually hurt performance (more to lose than to gain in this case).
Another thing that a spacer does is it increases the volume of vacuum space between the throttle valve and intake valves. When you crack open the throttle, more air has to flow into this space before you can have this higher pressure air reach the cylinders. What this dos is add a little bit more to the "lag" time between throttle position and additional air mass being introduced into the cylinders.....decreased throttle response.
If you wasnt to see these principals in action, just look at a modern crotch-rocket. Each cylinder has its own throttle plate followed by a fuel injector and then straight into the cylinder....they even angle it so that the incoming air comes almost straight down onto the top of the intake valves. This is the recipie for maximizing throttle response, performance, etc.....small volume between throttle plate and intake valve, unrestricted flow (into and out of the cylinders) at WOT (or even boosted), A/F charge at a perfect mixture for complete combustion, cold/dense intake air, high engine temperatures.
I hope I didn't lose anybody on that explanation. I'd say the money would be better spent on other things (but not a magnetic device for the fuel line
).
EDIT: Here's a site that actually tells which tells the truth about what the TB spacers are made for:
http://www.brandsport.com/airpowinsys.html
Also notice that Airaid's website that shows dyno results only shows them for the filter/intake tube setups. They have the capability of showing performance gains for just the TB spacers, so why don't they?
http://www.airaid.com/dyno.asp
