Posted: August 2, 2011 in Induction Systems


     The plenum area is where the intake runners meet. There can be one plenum that all runners meet, or two smaller plenums with 1/2 the runners meeting in each. The plenum volume is a very importing tuning aid. As high velocity gasses flow through the carburetor or throttle body, the plenum give the gasses a chance to slow down, as the velocity is reduced the pressure rises. Higher pressure means that the air will be denser, and of course that means more power. As rpm goes up you need a larger plenum, but a larger plenum will reduce throttle response and low-end power. A plenum also reduces peak air velocity through the carburetor (or throttle body). The induction pulses in an intake cause velocity to rise and fall with every pulse. The plenum helps to reduce them by acting as an air capacitor. Average velocity will remain the same, but the highs and lows will be closer together. Since you need a carburetor that will flow enough air at peak velocity, a larger plenum will allow you to run a slightly smaller carburetor, but it will also reduce the peak signal strength, which is why large plenums tend to reduce low-end power.

Helmholtz Resonator

     A Helmholtz Resonator is the theory behind what happens in the intake (and exhaust systems). For more information see the Helmholtz Resonator Tech article.

Intake Runners

     These are the connection between the cylinder head and the plenum area. They must flow enough air at peak rpm to support the horsepower your engine is capable of, but not be so big that they have extremely low velocity at low rpm. The runner length is also very important if the induction pressure waves are to be used to increase volumetric efficiency (see Tuned Port Basics for more info).

Port Matching

     Not to be confused with gasket matching, never grind a port to the size of the gasket. Port matching is when the intake and head ports are ground to match each other, but sometimes a mismatch can help

Carburetor Spacer

     These are probably the most misunderstood things there are. It seems that almost everyone installs one on there car. Most people know that it helps top-end power, but they don’t really know why. The answer is, it increases plenum volume, which reduces the induction pulses at the carburetor and brings the peak velocity through the venturi down. Most manifolds are made with plenums that are too small, so adding a spacer will usually help. Manifold companies know that the plenums are too small, but it is easier to add a spacer if it’s too small, than to remove space if it’s too big.

Individual Runners (IR)

     IR manifolds have no plenum. There is one throttle bore per cylinder and nothing connects with anything. These offer the best signal strength at low rpm, because the have the highest peak velocity through the throttle bore, but are very hard to tune in and induction pulsing at high rpm is a big problem. Due to the high peak velocity, IR set ups need a lot of air flow capacity. The basic carburetor sizing formula does not apply here. There could be 2500 CFM on top of a 350 cubic inch engine and it could run fine. This is because each throttle bore gets an induction pulse once every two engine rotations, so it’s only in demand about 25% of the time. Plenum type set ups will allow other cylinders to use that throttle bore while other cylinders do not need it, so you don’t need nearly as much airflow capacity.

Tuned Port

When a port is the correct length to add volumetric efficiency by utilizing the induction pressure waves, it is said to be tuned. This can only help over a narrow rpm range (see Tuned Port Basics for more info).

Manifold Heat

     Most production manifolds will have some sort of exhaust or coolant passage in it to heat the intake. This helps fuel atomization, but hurts power. Cooler air is denser and denser air makes more power. Any kind of performance engine should not use manifold heat.


     An hour glass shape in a carburetor that cause the air to increase velocity as it passes through the narrower section. As velocity increases, pressure decreases. This is how a carburetor flows fuel. The pressure in the venturi will be lower than the pressure in the fuel bowl, so the higher pressure will push fuel through the carburetor.

Booster Venturi

     This is where the fuel enters the venturi and it is fact another smaller venturi it self. Its main purpose is to further increase the speed of the air and in turn lower its pressure even more to gain more signal strength. There are many kinds of booster venturi; the ones that give the best signal strength and atomization are usually the most restrictive to airflow.

Signal Strength

     This directly related to venturi size, shape, booster venturi, and air speed though the carburetor. The signal strength is how much the venturi can reduce pressure. A large venturi will have less signal strength than a smaller one, but will also flow more air. If the venturi is too big, it will have a hard time metering fuel at low rpm, if it is too small, it will be a restriction at high rpm.

Dry Flow Intake

     With fuel and air traveling through the intake, sharp corners are a bad thing as velocity increases. Air is lighter that fuel and can take sharper turns. As an air fuel mixture goes around a sharp turn, the fuel separates and flows along the outside of the turn. Getting intake runners long enough to help low to mid range torque is hard to do with limited hood clearance. Multi-port fuel injection lets us inject fuel right at the intake port of the head, which leaves the rest of the manifold flowing only air. By doing this, we can have some sharper bends. Air still flows better in a straight line, but not having fuel separation is a big plus. The GM TPI manifold is a good example of a dry flow manifold. There is no fuel in the runners until right before the heads. The runners come out of the plenum and cross to the opposite side of the engine, making them long enough to help low-end and still give hood clearance.

Wet Flow Intake

     They flow air and fuel of course. Carburetors and throttle body injection are wet flow systems. The intake runner shape is much more critical because it must minimize fuel drop out.              

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