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On sequential firing order, adjacent cylinders fire next to each other. The goal of any plenum is to fill the cylinders with air as much as possible on the intake stroke to maximize torque. With open plenum design, when one cylinder is on the intake stroke , the cylinder next to it can have both valves open (overlap) or or an exhaust valve open. So now the air distribution in the manifold is interrupted because feeding from the same volume and very close proximity of the intake ports . One cylinder can rob air from the other or dilute the air with exhaust. This reduces your volumetric efficiency . To get around this , you split the plenum like the way the stock Z manifold is design. All dependent on camshaft timing ,duration, firing order and engine rpm. On Supra, adjacent cylinder don't fire next to each other, not as sensitive with open plenum. On many cars, manifolds have a air valve that opens up like a throttle valve once a certain rpm is reached to maximize torque on higher rpm. The new Nissan VQ v6 manifold works like this as well as many others like Mazda , Cheverlet, Toyota . Having this design feature, you get a manifold that is strong on low and high rpm range. Engine designers spend hundreds of hours designing and testing intake manifolds to maximize torque across a broader rpm on street cars. On drag race only car, you don't care too much about low and mid range torque. I test drove a camaro with tunnel ram intake on street. Car was complete dog
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