Do those carburater spacers really increase hp?

Depends.. you have to fool around with different styles and sizes. A 1" open is worth 5hp for me, but I can't close the hood unless I buy a different air filter, so I don't run it..
Some people end up with open/4 hole combos, or 2" spacers. in my very limited experience, it really depends.

Maybe someone can fill in the why's here, I'd guess carb signal or charge velocity, maybe heat shielding to some degree, but thats a shot in the dark..

Matt

We always ran a 1in open spacer on any of the race cars with a dual plane manifold. They do run better. If you have a single plane that is really too big for your combo. a 4hole spacer can make up for a lot of low speed loss.