I have a '93 XJ w/ the 4.0 EFI engine. This engine has the old style intake manifold with straight intake runners, not the new type with the curved runners.
Recently I thought I'd experiment with a 1" aluminum spacer between the TB and the manifold, in order to increase the plenum volume and (hopefully) increase power. Aside from the 1" aluminum plate the same external size as the gaskets, all you need is two TB gaskets (use them as a template), 1" longer-than-stock TB bolts, a drill press, drills, and a holesaw the same diameter as the intake hole in the TB gasket. Put a gasket on both sides of the spacer and you're done. Took about two hours give or take. [Sorry, no pictures as that program in my computer is SNAFUed.]
It works awesome! There's a really noticeable low and mid-range power increase! If you try it, let me know!

 

can't wait to see pics. I have everything except the plate

 

i find it pretty hard to believe you noticed anything power wise. TB spaces tend to do next to nothing on 4.0's

 

N20Jeep,
It's your right to disagree, but I noticed a good improvement, and that's all that matters to me.

 

I am glad you noticed an Increase! However, with a fuel injection engine it will not increase anything. It is the same as adding an extra inch to your intake tube... The spacers are meant for CARB's engines where you have the air and fuel mixing there at the CARB-adding more air to the fuel.

With our engines its all electronically controlled and does nothing. I would Recommend taking it apart and boring it bigger =]

 

Hit the nail on the head!

 

Unless its TBI/Carbed, it will not do anything.

 

Reply:
IMHO I believe that adding the spacer increases the intake air velocity throughout the manifold and, therefore increases the "volumetric efficiency" at the intake port/injector/valve, i.e., packing a little extra mixture into the cylinder, which increases the power. [a buddy of mine - an ASE-certified Master Mechanic agrees w/me, BTW)].
I've been building/experimenting with street hot rods for more decades than I'd care to admit to, and although I don't have access to a fancy dynomometers, I'd say my seat-of-the-pants sensitivity to speed/power increases is as good as anybody's.
So, no offense, but I still say there was a noticeable improvement. Someday I'd like to try a bigger bore TB, but the vehicle is fine for now.

 

EDIT:...not arguing