A Guide to Intake Manifolds for Carburetors
There are countless options for choosing an intake manifold. Some intakes will help you make power down low, while others will help with top end power. Choosing the right intake is a matter of your application and the kind of horsepower your engine will potentially make. We'll give you Intake Manifold basics and then get more technical at the end.
Intake Manifold Format
You've heard the terms: High rise manifold, dual plane, single plane, tunnel ram....Each manifold type has a purpose in the performance world.
Dual Plane Intake Manifolds
Dual plane intake manifolds are named for their split plenum opening in the intake where the carb sits. Each side of the opening feeds 4 cylinders on a V8. Dual plane intake manifolds are the most popular for high performance street and mild racing because they generally build power across a wider range and start at 1,500 RPM, depending on the dual plane manifold. Each intake manifold has its own performance characteristics. It's best to know how you'll use your vehicle and select from there. To be clear: Dual plane has nothing to do with the number of carburetors that the intake will accept. You can have a dual plane manifold that accepts 1 or 2 carburetors.
Single Plane Intakes
Single plane manifolds are named for their intake opening where the carb is bolted on. A single plane intake has one "hole", in the plenum where the carburetor sits on the intake. Fuel from the carburetor enters the intake through one opening with no separation. That single hole feeds all 8 cylinders on a V8. They are typically less restrictive and work best to build power between 3,000 and 8,000 RPM's. Because of the RPM range, the single plane intake manifold is best suited for racing applications.
Square Bore Intakes
Carburetors have venturis that open and close as you apply the throttle. A 4 barrel square bore carb has 4 equal sized venturis that you can more easily see from the underside ofthe carb. Square bore intake manifolds match the square bore shape of the carburetor base and venturis.
Spread Bore Intakes
Spread bore carburetors have 2 small venturis up front that are the primaries and 2 larger secondaries on the rear of the carb. Spread bore intake manifolds match that shape to accomodate the larger venturis toward the back.
Low Rise Intakes
Low Rise refers to the height of the intake. Low rise is a general term used to describe intake manifolds. There is no clear distinction between a low rise and a medium rise intake. It's pretty easy to describe an intake manifold as low or high rise. Low rise intakes fit under hoods better and offer certain performance advantages over taller intakes.
High Rise Intakes
High rise intakes are taller than low rise. High rise is a general term to describe an intake. There is no standard height where low rise intakes end and high rise intakes begin. High rise intakes are better at building horsepower in the upper RPM range and usually have a wider power band.
Tunnel ram intakes are extreme Hi Rise intakes that accomodate one or two 4 barrel carburetors. They are made for high RPM and big horsepower setups. You see tunnel rams on the street, but they are best suited at the track where you can really get into the higher RPM's.
So, How do I select the right manifold for my application?
As with many situations in building an engine, it should match your intended purpose. Everyone wants the big tunnel ram dual quad intake sticking out of the hood, but it may not be practical, or best, for maximum performance. All intakes advertise an RPM range that identifies where they are most efficient. As an example, the intake may advertise "1,500-6,500 RPM". This RPM range must be considered and is the easiest guide to choosing the right intake for you. Street cars work best with a dual plane intake, most advertised "idle to 6,000 RPM" or "1500-6500 RPM". Race cars that work more in the upper RPM ranges will require a single plane, 2,500-7,000 RPM or "3,500-8,000 RPM". Next, the intake and camshaft selection should go hand in hand. The camshaft will also state an RPM range for its best performance. The RPM range of the camshaft and intake should match or be very close. A minimal low RPM difference of 500 RPM is acceptable, but should not exceed 1000 RPM. If this is the case in your selections it is better for the intake RPM range to start lower than the camshaft. The reverse may cause low end instability, thus an off throttle hesitation. Lastly you need to consider the physical fitment. Most obviously, be sure to select the appropriate part number for the engine and cylinder head design. Consideration for engine vacuum ports, water coolant ports, carburetor flange fitment, and be sure to check for proper hood clearance.
For all of you automatic transmission hot rodders out there, be sure to see our tech section on torque converters too. Once you�ve selected your intake manifold and camshaft, be sure you have the right torque converter too!