What is the purpose of the notch between the left and right holes in this intake?  

Started out years ago for the Holley Three barrel carb with the single butterfly in the secondary.  Later, folks thought it helped with high rpm power, but in dyno testing, it causes a torque dip, and in some instances, it causes turbulence and loss of flow in the rear ports on that plane. Joe-JDC 

JDC

Joe has summarized both why and what effect the plenum slots have on the performance.  Here’s the link to the article that was published in the Jan-Feb 2014 issue of The Y-Block Magazine that goes into much more detail and also gives dyno testing results on those intake manifold plenum slots.

Intake Manifold Plenum Slots – Eaton Balancing  

Lorena, Texas (South of Waco)

im surprised they never included something with that manifold to fill in that gap if you didnt want it.

No surprise there as when the Blue Thunder intake was first introduced, it was done so mimicking what the rest of the industry was doing in regards to plenum slots.  Having a plenum slot was thought to be the best option so there was no option allowing the purchaser to use the BT manifold out of the box as no slot version.  It was only when John Mummert introduced his manifold did it become apparent that maybe the plenum slot was not such a good idea in all instances.  Once it was proven that the plenum slots were detrimental to low end performance, Blue Thunder simply did away with the slot.  There are those on this forum that have taken the early production BT manifolds and filled in that slot area.

Here are some additional links to threads on this forum regarding the Blue Thunder intake manifolds and plenum modifications.  Doing a search may bring up some more.

4 Barrel intake differences (y-blocksforever.com)

New Member & Intake Manifold Question (y-blocksforever.com)

Blue Thunder intake manifolds (y-blocksforever.com)

Lorena, Texas (South of Waco)

Thanks for the info.  This is the first I've even heard of the Holley 3 barrel.  Probably a reason for that!

What would be a practical way to fill that slot?

See if this link to a past thread helps.
http://forums.y-blocksforever.com/FindPost133096.aspx

Lorena, Texas (South of Waco)

Sounds like an improvement I need to tackle. Nice explanation. Thanks!

       A  L  A  N   F  R  A  K  E  S   ~  Tulsa, OK    

Will J B Weld hold this piece in place permanently? Don't really want to pull the intake if I don't have to. Concerned about the JB weld someday failing and the piece being sucked into the engine.