Manifold Destiny

An invisible science

Manifold Destiny
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Feature Article from Hemmings Classic Car

January, 2013 - Ray T. Bohacz

When viewing an automobile engine, it can be easy to think of the intake manifold as not much more than a connection between the carburetor and the cylinder head. It has no moving parts and appears to have little, if any, function beyond being a mooring. But as with many aspects of the automobile, a part that does not move and seems boring can have a paramount effect on how the engine runs.

The Intake Manifold
Air, in scientific terms, is considered a gas. By definition, a gas is any substance that is in a rarefied state. Physics has proven that a gas does not like to make turns and will experience flow losses from friction when asked to travel in a nonlinear path.

The ideal intake path would have no bends; it would be a straight run to the port of the cylinder head and intake valve. This is not possible on a production engine, due to hood interference and the packaging of accessories such as the alternator, air conditioner and power steering pump.

Types of Intake Manifolds
Wet Flow: An intake that has both fuel and air coursing through the runners. The best example is a traditional carburetor application. Throttle body injection systems are also wet flow, since the fuel is mixed with the air prior to entry into the manifold.

Dry Flow: The runner carries only air, with the fuel administered near the union of the cylinder head and manifold. Port fuel-injection systems are considered to be dry flow, even though the injector is fitted to the end of the runner.

Plenum: The McGraw-Hill Dictionary of Engineering describes this as, "A condition in which air pressure in an enclosed space is greater than that of the outside atmosphere." The definition needs to be altered when discussing an intake manifold. During part-throttle operation, the plenum is under vacuum; at wide-open-throttle (WOT), it is near atmospheric pressure. The purpose of the plenum is to allow for distribution of the charge to the individual runners and, depending on the intake manifold and engine design, to allow the pumping action of other cylinders to impact the fill rate on the companion bores.

The plenum capacity has a direct impact on idle stability. As the plenum volume increases, so does idle instability in most cases. A general consensus is that the plenum volume should be approximately 1.5 times the displacement of the engine. As an example: A modern General Motors LS engine with 346 cubic inches of displacement has a plenum volume of 10.1 liters, or 1.77 times the displacement of the engine.

To be considered a plenum, the chamber must be after the throttle plate and before the intake manifold runners.

Single-Plane: Most commonly found in performance applications with an open plenum, it is easily identified by its short, nearly equal-length runners. Intended for continual high-engine-speed use, this design tends to be less driveable than a dual-plane configuration. The large capacitance undivided plenum suffers from a weaker venturi signal to the carburetor. This is usually met with low-speed fuel metering inaccuracy, as a function of exposing all of the cylinder's pumping pulses to the barrels of the carburetor.

Its benefits are the equal runner length, straight flow path and consistency in fuel distribution. This allows for efficient operation at higher RPM, albeit with a trade-off of low-speed torque, driveability and fuel economy. During the late 1960s and early 1970s, some small-displacement Chrysler V-8 engines enjoyed a single-plane intake manifold from the factory.

Dual-Plane: The most common manifold design on a domestic V-8. In order to be classified as a dual-plane, one of the following criteria must be met:

The intake runners need to be divided into two distinct groups, i.e., taking an eight-cylinder engine and treating it as two four-cylinders. Each group then needs to alternately receive induction pulses with these pressure differentials being evenly spaced. Commonly equipped with a divided plenum, low-speed torque is enhanced.

The firing order of the engine needs to be evenly spaced for a dual-plane application. Odd firing sequences (such as on early Buick V-6 engines) will not benefit from the strengthened pressure waves during filling of the cylinders and are not well suited for this design.

Tunnel-Ram: It could be thought of as the big brother to the single-plane. Intended for high-RPM racing applications, it offers a significant advantage over a single-plane design by providing a large plenum that will support multiple carburetors. The runners are moved up and off the valley of the engine, which allows for a cooler charge air temperature. The short, straight runners also offer less frictional flow loss. Hood clearance is an issue.

Individual Runner (IR): In carburetor form, this is best represented by multiple Webers or similar designs. This manifold is connected to each cylinder separately. It offers a single throttle plate and fuel metering device per bore, but is usually air-flow limited due to the need to balance velocity throughout a broad RPM range. Another important feature--or downfall, depending on the application--is the isolation of the pressure wave pulses from the companion cylinders. Single cylinders provide strong signals at low RPM, so fuel metering at slow to medium engine speeds is excellent.

Tuned: When an intake manifold is categorized as being "tuned," it uses the natural dynamic effects that occur in the cylinder to feed more air into the bore, raising the volumetric efficiency. It actually works under the auspices of dynamic flow effects along with inertia supercharging.

When the intake valve closes, a fast-moving high pressure pulse hits the valve and bounces back up the runner of that cylinder. By modifying the length and cross-section of the runner, the returning pulse can be timed to arrive just when the valve opens for the next event, filling the cylinder with additional charge that is pulled along.

The camshaft profile needs to open the valve and catch the second pulse or harmonic for it to be effective. This is Detroit logic, while some dedicated race manifolds may be designed to work with the third, fourth or fifth harmonic. Concerns for valve closing are also expressed. The charge needs to stay trapped in the bore. Since the air column has inertia created by its rebound back from its original collision with the closed valve, it is called "inertia supercharging." When this occurs, the manifold is said to be resonating or tuning.

One of the best examples of tuning is the mid-1980s General Motors TPI systems and, before that, the 1950s Chrysler cross-ram system. The Mopar design placed one carburetor by the radiator and the other near the firewall to achieve a balanced tuning effect. The GM design was constructed with an extremely long intake runner of almost 24 inches to the valve seat. It worked well to produce high rates of volumetric efficiency at low RPM, but was too long to fill the cylinder at high engine speeds, thus limiting horsepower.

Not all intake manifolds are designed to capture this pulse. A short runner is often the hallmark of an untuned manifold, which can be referred to as anti-tuning. A manifold of this style will not offer a definitive point of resonance. This results in lower peak torque, but provides more area under the curve with a wider power band. Most dual-plane intake manifolds are designed to enjoy the benefit of resonance tuning, while the runner configuration and open plenum of a single plane will not apply this theory.

When Detroit engineers design a cam profile for a production engine, the intake manifold style is paramount in the final specifications. Valve events that need to work with a resonant (tuned) effect need to have the intake valve opened to catch the desired harmonic pulse. Any later, and the inertia energy is consumed, limiting the benefit. This dynamic response in the intake manifold has been studied and defined under different theoretical approaches. Among them are the Helmholtz resonator, ram charging and organ pipe theories.

Intake Manifold Design

An engine in a passenger car has a number of contradictory requirements that make an intake manifold designer's job very hard. It needs to operate under transient throttle and varied speed conditions. Single-speed manifold design is much easier. Packaging considerations are paramount. The ideal manifold does not exist for a multi-speed engine, so the goal of the engineer is to limit the compromises.

A well-designed manifold will offer fast mixture delivery, low flow loss when compared to an open intake port on the cylinder head, uniform mixture distribution, low fuel film accumulation and uniform temperature distribution.

With a carburetor, temperature is an extremely critical issue. Heat is required when the engine is cold to help vaporize the fuel. Once warm, the same heat now becomes an enemy. For every 10 degrees Fahrenheit the temperature is changed, the power is altered by one percent. Heat also presents a problem by lowering the octane tolerance and making the engine more prone to detonation.

The volume of an intake manifold is defined by the length and cross-section of its runners and plenum. The capacity of a transient intake system with single-point fuel metering has been proven to significantly influence the engine response and emissions behavior. Runner cross-section should be as small as possible for fast mixture delivery times, minimal wall wetting, reduced weight and maximum rates of heating when cold and rejection when hot. Runners are usually designed for a balance of flow loss versus velocity.

It is common for an intake designer to incorporate taper into the runner. The cross-section of the runner would then be larger near the plenum and, at a predetermined point, start to close up to the size of the intake port in the head. Working under the venturi principle, this would accelerate the charge.

Intake runner length on a wet-flow system is usually limited by packaging concerns and the need to keep the mixture from falling out and separating from the air column.

The length of the runners for a single-point mixture formation manifold is determined by factors completely different than used when calculating the length of a runner for a multi-point EFI system. EFI systems can enjoy the torque benefit of long runners for resonance tuning, while a wet-flow induction system needs to shorten the length for acceptable delivery times and to minimize fuel film deposits.

A proper intake manifold design will allow the automobile to meet the expectations for performance in its intended market. Idle quality, tip-in feel and full throttle power are found in the simple but complex intake manifold.


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