The primary function of a dual plane intake is to increase the air velocity through the carb venturi, AKA "carb signal" this can be done by dividing the intake so each cylinder sees only half the intake manifold (in any combination), to it has half the plenum volume to draw on thus increasing the air speed through the carb.
Increasing carb signal gets you much better throttle response, idle quality and off idle power (low and lower mid range torque is improved substantially) this is because the higher air speed means better atomization of the fuel, and also more accurate metering of the fuel. There's also a small "secondary" inertial ramming effect (tuning effect) just like there is when you lengthen the intake runner or make it smaller...
That is the famous Edelbrock RPM Airgap (for one of my Ford small blocks in this case) it is a dual plane, the air gap lets you more clearly see the runners coming off the split plenums. The two inner runners go across to the opposite bank than the end runners, so the banks are not actually split. Cross talk or reflected waves are so chaotic and unpredictable
IN PRACTICE for a street application that they are rarely a major part of manifold design in V8's... The RPM Airgap is hands down the best dyno'd dual plane intake for almost every V8 out there and it achieves this by lengthening the inner runners by crossing them over allowing them to be longer than they would be if the manifold just split each bank from the other. Basically wave tuning took a back seat to inertia tuning, which is the norm for most intakes. Excepting F1 and other ultra high dollar racing where they run in very predictable and steady RPM ranges ... Wave tuning is too hard to get right for very small power gains, while inertia tuning (think siphon effect) is very easy to get right and offers more power over a much broader RPM range.
That edelbrock is a so called 180* you can also split a single plane intake (a so called 90* intake) or divide the secondaries from the primaries for each cylinder by dividing the plenums and the runners all the way to the port, which is a so called 360* intake (you can also reduce the size of the intake runners or the total volume of the intake manifold in any combination to achieve the same goals with varying effects on upper RPM horse power)...
Offenhauser makes a "360*" intake for AMC 6's
this is one I have laying around, the entire manifold is split so that when the primaries are open only about half of the intakes total volume has air and fuel rushing through it, which means the air speed at low engine RPM's is dramatically increased. I mean DRAMATICALLY, I've never found good dyno comparisons of this intake but I would bet that it's worth 30-40 lb-ft of torque off idle and probably kills 20hp at the upper end of the RPM range. In other words it should be a great intake for a Jeep...
A few notes: making your intake runners bigger (matching the 4.0 ports) will improve UPPER RPM HORSE POWER while reducing lower RPM power... If you make a port flow more CFM by increasing cross section, air speed at lower RPM's will be reduced (which gets you less inertial ramming) while airspeed at higher RPM's will be increased. Also You really only need to pay attention to dual plane intakes for a 4bbl carb, a 2100 Autolite/motorcraft (I'm a classic mustang guy so that carb will always be an Autolite 2100) is a 2 bbl so the stock 258 manifold will be fine.
, I'm living in quebec, Canada
