I have noticed that the bottom surface of the factory dual quad manifolds sometimes has two large set screw type bolts while some manifolds have a completely smooth bottom surface. What is the significance of these differences? Pictured below is a two-bolt type.

 

Steve

Here are pictures of a 1956 EDB manifold modified as you describe.  The four independent throttle holes have been converted to the kidney shaped holes as found on the 1957 ECG 2X4 intake manifolds.  This modified manifold was dynoed along with the other manifolds on an aluminum headed engine and it was down on power when compared to a stock EDB manifold.  Here’s the link again for the dyno test results.
 
Dual Quad Intake Testing

  

 

Lorena, Texas (South of Waco)

Thanks Ted. There is a definate difference but to me its not enough that you would ever be able to tell in driving the car. Would probably show up on the quarter mile times but again i'm thinking not much. Driver could make more difference than that. I would still got for the stock .57. Not enough difference between the stock '56 to worry abour. The aftermarket manifolds would be an improvement though. I port matched my '57 to the heads but no idea how much if any help it was.

Looks like the two 56 castings are the 2 different ones. Casting # on the top of one and the bottom of the other. The "8" shaped one also is flat where the carbs bolt and the other has indent/open areas between the studs unless one has be trimmed a bit. The 56 did not need an indented valley cover either as I understand it like the stock 57. Did any aftermarket dual quads need one?  I love reading and learning about these early factory and aftermarket performance parts.

56 Vic, B'Ville 200 MPH Club Member, So Cal.

Both styles of the ’56 Ford 2X4 EDB-C intakes I’ve had in my hands required a ’depressed’ valley cover to allow the intake to bolt in place without any valley cover clearance issues.  On the flip side of this, the 1956 Mercury 2X4 intake does not require a special cover but that intake looks to have come out of the same basic molds as the Edelbrock FM255 intake.  The Ford intakes were cast by Schmeller Aluminum and are a completely different design than the Edelbrock intakes.  Even the carb spacing is different between the Ford and Mercury 2X4 intakes.  To date, I haven't come across any aftermarket or non-Ford dual quad Y intakes that required an indented valley cover to facilitate their installations on the engine.

Lorena, Texas (South of Waco)