The reason for this page is not to actually try to sell you a Throttle Body spacer...it's to get you to ask some questions before you blindly throw away $60-something dollars on a modification that will supposedly improve the power of your engine.

First, let me begin by explaining my "gut-instinct", which means nothing in the grand scheme of things, but I want to be fair and start by saying that I have a HUGE suspicion that these things are more along the lines of a magical fuel ionizer which clips to your fuel line and gives you 25% better fuel economy.  In other words, I call B.S.  Here's my thoughts:

Back in the days of carburetors, there was a distinct benefit of keeping the carburetor as cool as you could, because cooler fuel was denser fuel, and you could pack more of a charge into the cylinder, giving you more horsepower.  Drag racers used to use PLYWOOD spacers to keep the heat transfer from the intake manifold to the carburetor throttle plate at a minimum, which kept the fuel cooler.  They even had "cool-cans" which served a similar purpose (cooling the fuel down), which was a coffee can with a long length of steel fuel line coiled inside, which they then added ice to, which cooled the fuel down before it arrived the carburetor.  I think this may have been where throttle body spacers got their reputation for "adding power", whereas they may well have been "reducing heat transfer" instead.

Another selling point the throttle body spacers guys use is that it increases the amount of air inside the intake plenum.  Ok, I buy that, but do they mean to tell me that EVERY single engine they sell a throttle body spacer for benefits from EXACTLY one inch of additional air space?  All of them seem to range from 3/4" to 1.25"...is it possible that EVERY auto manufacturer screwed up EVERY intake manifold they designed by making it exactly 1" too short, and that this missing one inch column of air returns the engine back to it's "optimum" efficiency?  Could this be possible in a world with CAD design, and computer aided flow-benches, and so on and so forth?  Again, I call B.S.

Let's talk for a moment about this "swirling" effect that some throttle body spacer manufacturers claim assists in "efficient atomization".  First off, if we are talking about a carburetor, then MAYBE this USED to have some effect...air swirling around and mixing the atomized fuel it pulled through the venturi  into the intake manifold may have made some positive effect.  However, in a fuel-injected vehicle (throttle body or port injection...it doesn't matter), like most all of our Jeeps in today's world are manufactured, the atomization is done by a high-pressure fuel pump supplying an electronically-timed injector which "mists" a certain amount of fuel into the intake manifold.  Would "swirling" air make an increase in power when the fuel is already atomized?  If so, how?  And, if that WERE the case, are we to believe that the air being "swirled" in one column down the throttle plate is supposed to split off into 6 or 8 separate "tornados" and each swirling tornado will somehow find it's way through several right and left turns, through the intake valve and into the cylinder while it's still swirling and carrying on?  B.S.!!!!!!!!  There is no way.  I'm no Doctor of Physics, so maybe I'm wrong, but until someone can prove to me that swirling air can not only be split from one column into 8 equal swirls and EACH of those will find their way into each of 6 or 8 cylinders past the intake valve, I don't buy it for a moment!!

So, that said, I have been told by other mechanics and retailers that the 1991-1998 4.0L HO engines DO indeed benefit from a throttle body spacer.  Maybe it's because AMC/Jeep designed the intake manifold too small, and that extra 1" column of air actually DOES help.  I don't have any other explanation.  Remember...there was very little CAD being used back in the 80s when that particular manifold was designed, so maybe it IS too small?? I have been told that the 5.2L and 4.7L engines benefit very little, if at all, from a throttle body spacer.  The 5.2L manifold was designed in the early-90s and the 4.7L manifold was designed in the late-90s.  AND, furthermore, since the 4.0L manifold was REDESIGNED in the late-90s for the 1999 model year, then THAT would explain also why the spacer seems to work better on the '91-'98 models.

Anyway, far less than scientific results, I realize...but I think I made some pretty good points above.  If you have any more-scientific results than mine, send me a link and I'll post it below in my Throttle Body Spacer link section.

Thanks, Kevin

So, if you are still bent on ordering one up after all that, I prefer you do it from us rather than someone else, so here's your link to do so, and thank you for your loyalty!