Here is a link to Jegs - a large American retailer. http://www.jegs.com/cgi-bin/ncommerce3/ ... rmenbr=361
The phenolic spacer works the same way under the manifold halves as it does under a holley - it stops heat soak into the upper manifold. Because the air is not getting heated by the manifold - it stays denser. Denser air has more oxygen molecules = more air pumped into the motor per cycle = more horse power achieved. I could fry an egg on my uppermanifold after a long trip.
The other thing the spacer does is lengthen the intake tract. The HO motor gets is mamouth torque from having a long intake tract and the 3/8" spacer helps slightly lowering the revs at where peak HP and torque are achieved. If you could make the same power using less revs you would have to be happy wouldn't you?
Shane

f**k oath! I love getting more power out of my engine for less throttle! ( and it appears to be reasonably cheap too )