Thermal properties are different.  As a rule, less dense would have superior insulating properties over more dense.  With that in mind, then fibrous phenolic would be better than plastic.  And what Charlie mentions about wood is also correct as when talking to the Nascar crowd, even different wood types for the carb spacers comes to the forefront.

Lorena, Texas (South of Waco)

Although a four hole design can supply a maginally stronger signal for the carb over a slotted version, Steve illustrates a good point about the shrouding that can take place under the throttle blades.  With this in mind, when the carb base is slotted the intake is still separated down its middle so that the carburetor is halved.  This divides the intake plenum so that the draw thru of fuel within the carburetor alternates from side to side and allows for a higher intake manifold vacuum during part throttle situations.  An open plenum under both halves of the carb essentially makes for a reduced vacuum as the carb now sees open exhaust valves on cylinders that would normally be isolated from the throat of the carb that’s trying to draw fuel.  As a general rule, keeping the primary carb bores separated are good for peak torque values while opening up the plenum under the carb either with an open spacer or cutting on the intake gives a boost to the peak horsepower number while hurting the peak torque numbers.

Lorena, Texas (South of Waco)

I believe that wood is actually the best at insulating the carb, followed by plastics and probably phenols, then metals.


Lawrenceville, GA

There is a particular problem with the four hole style spacer that the slotted version avoids. It maintains the side for side division of the "cross-H" style manifold - but prevents the primary butterflies from being shrouded (?) as they swing down into the cylindrical passage of the four hole spacer.

 

This illustration was created to show a particular problem that occurs when the 4 hole spacer below the carb is stepped (smaller bores) and the primary butterflies swing near the step. On a manifold that has the slot style opening, there is nothing behind the primary to interfere with smooth passage out of the venturi throat. 

My tests with the infared thermometer shows the phenolic spacers to be way superior to the aluminum or pot metal versions when it comes to preventing temperature rise of the carb baseplate.

 
 Steve Metzger       Tucson, Arizona

Answers the same question I was wondering about.







Does someone have an answer for this? A difference between plastic, fiber, or metal? I'm sure they transfer heat differently and therefore one is more desirable than the others?

Heber City, UT (15 mins outside of Park City- basically it's in the mountains)

55 Effie

On the Mummert intake and my blue thunder you will notice a divider down the middle of the carb opening. This separates the two plane of the intake. A four hole spacer has material down the center that seals and preserves this separation ( two oblong holes front to back would also work but spacers are not generally made this way). A single square hole in the spacer allows for the free flowing of air between the two planes. Does this help explain? Mike

1956 Fairlane Victoria (ORREO)

 

Overlooking Beautiful Rimrock AZ

Then why does the Mummert manifold and carb spacer have two holes?

Ol'ford nutCentral Iowa

56 Vic w/292 & 4 spd.

Is there a significant difference in insulation between the plastic and fiberlike phenolic spacers?

Mike, located in the Siskiyou mountains, Southern, OR 292 powered 1946 Ford 1/2 ton, '62 Mercury Meteor, '55 Country Squire (parting out), '64 Falcon, '54 Ford 600 tractor.

The Y factory four barrel manifolds are a "dual plane" - and "air gap" - style manifold - both are well known concepts that build torque at low(er) rpm ranges. Use of the four hole spacer not only provides a thermal break between the iron and the carb base plate - but preserves the dual plane chambers of the manifold. 

 
 Steve Metzger       Tucson, Arizona

Regarding the carb spacer: Why is the four hole preferred to an open spacer.

Thanks,

Richard