There are also several types avalible. the open spacer is just that, open, on devided plane intake, it`ll make it act as a open plane, 1 cylinder will draw off the entire mass of the carb, it kills low end torque in favor of high RPM horsepower.
A 4 hole spacer increases volecity and booster signal at really no cost in power at either end of the power band, it enhaunces low end torque.
I`ve seen dyno tests in magazines on the "super sucker" spacers, and it said they increase power by quite a gain over other carb spacers. But I`ve never tried one, so it`s hard for me to say if it`s true.