In addition to changing from 915 heads to the Edelbrock Victors, I am trading my Torker for the Victor 2886 to better compliment the new heads. Besides the obvious differences in the design of these manifolds, I have noticed that the plenum area directly below the carb flange is quite different. With the Torker you can look down thru the throttle bores of the carb(in my case the throttle-stop bores)and it is wide open. On the 2886 manifold there is material that sticks out into the plenum entry and just barely clears the throttle blades as they open. My first thought was to clean out the excess metal and open it up like the Torker, but Edelbrock apparently engineered the manifold to be like that. Anyone got an idea on it? I have posted pictures, the Torker first.

Here' the 2886.

My guess is that the guys at Edelbrock designed that intake to work with fairly small engines. When I sent my intake to Wilson, the engineer there said that all that plenum material needed to be removed. My intake was a Super Victor which has a much larger plenum than the Victor. The Victor plenum is tiny but it does seem to work on smaller big-blocks.

Thanks for the help Andy--my grinder-trigger-finger has been twitching since I got a good look inside this manifold. I see that the runner dividers on the Super Victor extend well into the plenum. Is that as-manufactured, or have the dividers been extended?

Here is what the out of the box Super Victor looks like.

Andy, that intake work is a dyno test i would love to read about. just that with no other changes

alittle off topic,but i'd like to see a dyno test with and without a turtle just to see if they make an improvement.

I upgraded from a 440 Torker to 440 Victor but just for Eddy RPM heads on a 496 rb and the Victor was the same as yours. I had a spacer and put that over the mounting surface and basically cut the extra material away. In my mind to make it flow better because it protruded in a fair bit. Then i tried the car on the chassis dyno with the Torker and then with the Victor. Picked up 30hp with the Victor back to back test. Far from an expert but i would think it helped.

there are a bunch of things going on in that plenum.
There are 3 worth mentioning.
1. Is distribution...getting 4 carb bores to supply 8 intake runners. not to mention the odd locations and geometry forced by the intake runner window where it enters the plenum.
that is why you see those dividers, its to coerce distribution.
then you have to factor the multiplicity of carburetors that could be used to install on to the intake plenum flange...that dictates the intake flanges shape has to accommodate all of them thats why the shape final looks the way it does...this creates the next issue..
2. Is boundary layers..At the interchange of the carb flange and the intake base mount is some void areas that create dead spots. these dead spots are different depending on who's carb is bolted on top..
The intake manifold has to accept many different carb plates.So that dictated the shape of the plenum entry and base shape.. is a compromise from/for a mass produced part.
You can custom shape the carb base on the intake to perfectly fit your carb's base architecture/shape..this is done to minimize the boundary and dead or turbulent areas directly under the carb..
This issue also brings to light the #3 which is "Coanda"..
Those factory bulges under the carb matting surface are to create Coanda.. to pull the air on the roof of the runner into the runner.
its very effective..think of it exactly like the short turn on the floor of the intake runner on the heads..
those bulges in the intake manifold plenum are there to maximize coanda
When you go to a 4500 carb base..this "bulge"area is "in the way.. so its removed..but its removed due to architecture and not because the material or shape is corrupt..its removed because its physically in the way of a 4500 carb bore.
if there was enough material outside of the dimensions of the 4500 base shape...you could include or keep it..and you would see benefits..
However there just isnt any material left in the upper plenum to include this shape..so its remiss
you can see it clearly in the pictures..

sometimes and in fact often spacers show improvements..but i feel its a mask of another symptom ...the spacer allows time and distance for the after carb air to "find its way into the runners and distribute based on intake pulse. and or velocity changes..

its a partial remidy or band aid thats used because its an easy fix..but its not the best fix..

you want good clean transitions from carb plate architecture to intake plenum architecture.
and you want some coanda built in on the roof of the intake tracks as they meet the plenum.
and you want smooth radius-ed dividers to begin to segregate..
The cheap easy band aid improvement to all this is volume...just make the plenum a little bigger so the air has room to figure it all out based on demand...and it will reach the intake runner.
but its not the best way to dont confuse the two issues...its easy to make that conclusion because spacers seem to show gains..
but you have to understand what the problem was or is...fwiw.
cheapst

The mods shown to the plenum on this Victor 440 intake were worth about 10 HP on a 600 HP 440 over an unmodified Victor tested back-to-back on PRH's dyno.

It wasn't simply an increase in plenum volume, either, since adding a 1" open spacer to the unmodified intake still didn't show the same gains as the plenum-ported intake w/o a spacer. The only mods to the intake are what you can see since I didn't even bother to port-match the intake at that point.

FWIW, here's what Wilson did to an Indy 4150 intake's plenum as part of their "economy" porting service where they update the plenum and match the ports.