I"ve got two extra intake manifolds laying around. One original and one aftermarket. I had them laying next to each other and noticed there is quite a bit of difference between the two. The opening where the carb bolts on is quite a bit bigger on the aftermarket manifold than the original one. The ports are also bigger on the aftermarket one than the original. From my observations the aftermarket manifolds will reduce the low end torque on a TO-20. They key to building low end torque is small ports which keeps up velocity in the air/fuel charge. The aftermarket one will hurt the low end performance on the smaller cubic inch Z120 engine. The aftermarket manifold would be ok on a Z129 or Z134 engine because of their increased displacement and better breathing because of their larger intake valve. I am going to measure tomorrow and give you all some numbers. Also by the design the aftermarket manifold will cause the exhaust to be louder than the originals.