Holley 2BBL Air Cleaner Fabrication & Flowtesting 

Bill Jones' Photo Gallery Page 27

Fabricated two barrel air filter base, 2" tall spacer

-This air fliter base was located in a wrecking yard and chosen for it's size to allow a 14" diameter filter---and because of the offset center hole, and because it is steel and therefore fairly easy to braze or torch weld.
-the offset allows the two barrels of a 4412---500cfm or a 7448---350cfm to be installed dead centered in the middle of the 14" element.
-Some rules don't restrict modifications to the airfilter-----and some tech inspectors never look there---so this filter base was modifed with nice smooth curves entering the choke horn.
-These curved pieces are made of steel and brazed in place---in a manner that they snug fit to the OD of the choke horn, locate and center the base to the choke horn.
-Typically would use the tallest filter here that can fit----BUT----if a tall carburetor spacer can be used such as the one shown in example E----then air filter height
    may have to be slightly reduced.
    -an extra inch of carburetor spacer height is usually slightly more important than an extra inch of filter element height if the element is already 4".
-Both the 2" tall spacer and this sort of filter base seriously help get air thru the restrictive two barrel carburetors.
-I flowtested 5 different carburetor spacers and 8 combinations above the air filter mounting flange on one legal carburetor.
---air flow is corrected to 3" mercury and to sea level--
         A            B          C          D         E-------------2 barrel adapter/spacers
1------441---------467--------497--------470-------501--------------naked choke horn---no filter
2------459---------481--------511--------485-------507--------------with a K&N stubstack-----no filter
3------472---------484--------522--------496-------538.9------------with hand made filter base only
4----------------------------------515--------------------522--------------3" x 14" K&N & handmade base that centers the two barrels within the confines of the filter element.
5----------------------------------478--------------------487--------------3" x 14" K&N with common drop base (the 2 barrels are mounted forward inside the element)
6----------------------------------494---------------------------------------3" x 14" K&N with drop base and K&N stubstack                              "
7----------------------------------502---------------------------------------4" x 14" K&N with drop base and K&N stubstack                              "
8----------------------------------518--------------------533.7-----------4" x 14" K&N with handmade base and a lid that filters air thru a foam element like a MR Gasket #1476
9----------------------------------532--------------------557--------------same pieces as test 8 except for using a different LEGAL box stock 4412
A=a simple #1933 MR Gasket adapter with on open oval hole under the carburetor.
B=same type but with two holes and a divider bridge for about 1/4" below the gasket.
C=a 2 hole spacer adapter, fully divided when on a 180º manifold, the holes are machined with 7º tapers front & rear--then hand blended to divider
    wall and to the outer plenum walls.
D=a different 2 hole spacer adapter with 19º tapers---that cause an intersection in the center which is above the divider wall of a 180º manifold
E=the 2" tall adapter shown here---has a full divider wall bridge and three 7º step angles--about 5/8" long each----for each carburetor barrel exit.
    -and then radiused right at the bottom.
NOTE: all these spacer/adapters set the carburetor directly above the center of the manifold plenums----any offset either forward or rearward has been found to USUALLY hurt performance on the race track.
-The only time moving the carburetor forward helps is when the top of the manifold is tilted forward in relation to the floor of the plenum---quite common with Edelbrock open plenum manifolds.