What it does is move your power band, down, and increases torque slightly.
My engine is built up, after the new high compression, cams etc, My power was
coming in at 3200 and it would pull to about 6500. Without the spacer.
With the spacer it comes in at about 2400 and pulls to about 5500.
Much better off idle response as well.
On a stock engine with nothing done I would not expect to feel much difference,
with the 4.0 throttle body you should have a small bump in torque anda little flatter
curve so you can stay in a gear longer on a steep hill. Nothing earth shattering.
If your engine is not running good before it wont run better after you put it in.
ANd most of the folks who say it does nothing are bucking years of hotrodding where at
the drag strip a spacer will buy you a couple 10ths in the quarter...
Dave