Don is correct in that the slot across the dual plane divider originally started being used when with the three barrel carbs became popular. Although it’s now rare for the original three barrel carbs to even be found much less used, the ‘three barrel slots’ still remain in modern dual plane intake manifold design.
Beyond that, there appears to be some midrange power attributes that are enhanced by having the slot in place. The Blue Thunder intake manifold for the Y does have the slot while the Mummert intake does not. Each intake ends up wanting different carb spacers as a result. Because there is no slot in the Mummert intake, the manifold vacuum signal is actually stronger at idle making it the better manifold choice with an automatic transmission and especially when using increased camming. On the other side of the coin, having the slot fools the engine into thinking it has a larger plenum area. Although this tends to hurt lowend torque, it does enhance the midrange power. Even with the slot in the BT intake, the Mummert intake still outperforms it ever so slightly in all rpm ranges but that’s more a function of intake manifold port design rather than anything to do with having a slot across the plenum divider or not.
But this gives me an idea for another quick and easy dyno test. If I can come across an identical pair of large bore 1” four hole spacers, I’ll mill one with a slot across the secondary holes and run both on the dyno mule with the Mummert intake. And the spacer with the slot could potentially also be run with the ‘slot down’ versus ‘slot up’ which might also show some subtle differences. In theory, slot down or slot up should not make any difference but that's why you test. Either way, this would put some definitive numbers to this.
Lorena, Texas (South of Waco)