In a , for instance, the single plane manifold is deep and contains a centrally located plenum with runners that lead from the top of the manifold to the port entries in the cylinder head. The common plenum allows each cylinder to draw from all four of the carburetor venturis when the throttle is wide open. When the partially vaporized air-fuel mixture leaves the base of the carburetor, it forms four individual mixture streams.
As the cylinders place a demand on the plenum chamber, the mixture streams physically bend in order to meet the demand. The runners within the manifold help to direct the stream and withdraw a larger volume of air-fuel mixture during the available induction time span. As each cylinder withdraws a charge from the plenum, the mixture streams constantly change direction. Unless you are operating at or above 6, - 8, RPM and are using a single plane intake manifold, you "shouldn't" need a carb spacer.
Intake manifold manufacturers aren't stupid. If they wanted the plenum to be larger, they would design it to be larger. One of the only manifolds where a spacer is commonly used are small block Chevy Edelbrock Victor Jr's because so many guys use those on hot street engines and mild race engines that see a lot of upper RPM track time, yet still a lot of lower RPM street use.
That manifold works well on the street, but at the track, in much higher RPM use can use a little bigger plenum because it isn't a "serious" race manifold like a Super Victor is , so carb spacers are pretty common on those, AND to top it off, Edelbrock KNOWS this and offers that manifold with a 1" taller plenum right out of the box so you don't have to buy a separate spacer IF you are going to be running in the upper RPM range more often than normal.
Again, manifold manufacturers know what works and what doesn't, and if they thought they needed spacers, they would modify their manifold design to have it, like Edelbrock did , or would recommend one for certain applications higher RPM use, larger cubic inch engines, etc , or they would supply you with one.
But the Victor Jr is the only manifold I know of that has that option. Spacers and Heat Insulators: Which one do I need? Spacers come in different designs and materials. The most common types of spacers are so-called 4-Hole, divided wall, or open Spacers. They are available in aluminum, wood fiber laminate, plastic, or rubber gasket material.
Which one you need depends on what you want to use it for. Spacers are used for various purposes. The 2 main reasons are to adjust the power band of your engine or to prevent heat problems; such as vapor lock. Is your fuel boiling in the carburetor Vapor Lock or is it hard to restart the engine at operating temperature?
First, check the operation of your cooling system and check if fuel lines and components are too close to any heat source. If there is still a heat problem, a heat insulator or spacer can be the solution.
The heat insulator is a thick rubber gasket to prevent heat transfer towards the carburetor. The insulators come with longer studs and are made of gasket material, so no extra gaskets needed to install. If the heat problem is more persistent, a spacer of wood fiber laminate or plastic should be used. These are available in different types, but the best heat blocker is the 4-hole variant. I want to adjust the powerband of my engine, which spacer should I use?
With the help of a spacer, you can certainly improve your performance goal. The most common spacers are 4-hole, divided wall, and open spacers.
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