As far as TB Spacers a smooth bore works better than one with screw design on the bore walls. Smooth bore gives you better air transitions. The one with the air blades in the middle also have a smooth bore and the air blades utilize all the air coming in as it gives it more velocity and pressure. It gives performance results though the whole RPM range. I currently run a 2.5L Throttle body that has been slighlty bored. It has a 60mm on top side and 52mm on bottom. I believe its taper is giving more air pressure into the manifold, she seems to be working pretty good along with TBS. I am soon going to swap to a stock 4.0L Throttle body and see what happens. My concerns is that I might lose torque. Its wierd I have a custom home brewed air intake in factory box that is adjustable and when I allow to much air in I lose torque at higher speeds. It has to be adjusted just right. Right now I have it pretty fined tuned and my lil 4 banger has plenty torque and top end as it is. Of course I have other mods. To list a few, catback, accel coil, accel wires, NGK plugs gapped at .037. TBS, Heat wrap on custom intake tube. Ram air from removing head light bezels. Magna flow muffler, etc, etc. Does anyone run a 4.0L TB, and what was your results compared to the 2.5L T ?