old gm aluminum intake

someone gave me an aluminum intake,it's a gm.
i notice the dividing between the barrels that divides the planes has
a large hole machined in it. does any one know why this is done?

I've seen those, too. I'm not certain what the philosophy is.

I think I read somewhere a while back that it's there to even out the vacuum so that if a cylinder misfires, it doesn't affect half of the carb's vacuum, sending everything out of whack and causing the other three cylinders on that plane to lean out.

That would make sense, but if you examine the throttle body/mounting flange on any old 4GC or more modern 4MV, the passages are "siamesed" there as well through the castings. Perhaps it was on the square bore intakes which were to accept a Holley, since the factory did use some of those on early cars and medium trucks.

After limited research (internet at that) I believe that the theory is that the equalizer hole some how allows a dual plane manifold to perform more like an open plenum manifold at high RPM while retaining dual plane advantages at lower RPM.

thanks alot ! thats kinda what i was thinking . now i'll experimet with this
theory , if ya like i'll keep ya posted.