it ranges around 3/8" to 1/2" i think and it sits between ur plenum and intake manifold, idealistically the same thing as the old-style intake spacers. supposed to give you more HP by creating more air volume per time interval.
i got a 1/2" one for 20 bucks just for teh hell of it. doesnt do a damn thing, didnt think it would either. maybe 1 hp