What does it mean when people are describing their motors and they "double stack" the IM. What exactly is it and how doest help with performance?

 

below is a pic of the IAB "spacer" that can be found in H23 and F22A6 manifolds. it fits between the plenum and the runners. for either a larger plenum, or longer runners, you can add a second one "stacked" and modify as needed (gut it for a big plenum, trim it up for long runners...)

http://i38.tinypic.com/2ihtbf5.jpg
Credit: NASOHC
heres a pic o what an assembled one looks like. i think this guy actually had those spacers machined new.

 

What are the benefits of doing this?

 

the double stacking specifically, it "tunes" the runners (i think shifting the torque peak in the power band).

beyond that you'll want to research first hand. i know i have seen a thread or two on runner length and plenum sizing.. but i can't seem to find it at the moment...

 

Blacktrax sells them for the H series, though I'm sure they fit on others.

http://www.blacktrax.net/products/In..._Manifold.html

There's some info on there as well.