If you use an open hole spacer with a dual plane manifold, it is no more a dual plane.
It become a single plane, so it makes more high end power...
Nope, it just allows the engine to see both sides of the carb, like most newer dualplane intakes do.
Actually, the answer is between those two, but teyerdahl is on the right track. The purpose of a dual-plane (or 180°) intake is to isolate half of the cylinders to half of the carb to increase charge velocity, throttle response, carb signal, fuel atomization, and a few other good things. While adding a spacer defeats the purpose of the dual-plane and increases it's peak effective RPM range, it becomes a divided single-plane. This is not necessarily a bad thing, and this technique has been used for many, many years to take manifolds with insufficient top-end power, and sacrifice low-end response and torque (the dual-plane benefits) to get it. This is why some manufacturers do it with certain designs, to give a better peak HP capability. Note, however, that the factories don't allow nearly the crossover of a typical 1" carb spacer, and it's generally more of a 'notch' to save some of the dual-plane benefits.
I would caution, however, that spacers will always
add power on a dyno. You can keep adding spacers and it will keep making more power - on a dyno
. It is a popular technique when dyno racing, but often falls on it's face in the real-world. An example is using a thick open spacer on a dual-plane, where the loss of lower RPM torque can result in a slower car, even with the increase in peak HP.
Dyno numbers aside, final effectiveness can only be established on the track, and will more likely benefit drag racers (mostly high RPM) than street drivers (mostly low RPM). If it's a street car, the loss of response and torque will be noticeable, and overall effectiveness can only be judged on the street by the driver. This technique is
a compromise, but fortunately, it's relatively cheap and easy to experiment with to see if it works for your setup.