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An intake manifold is the part of an engine that supplies the fuel/air mixture to the cylinders. Its primary function is to evenly distribute the combustion mixture (or just air in a direct injection engine) to each intake port in the cylinder head(s). Even distribution is important to optimize the efficiency and performance of the engine. It may also serve as a mount for the carburetor, throttle body, fuel injectors and other components.

The design and orientation of the intake manifold is a major factor in the volumetric efficiency of an engine. In some engines the intake runners are straight for minimal resistance. In most engines, however, the runners have curves, and some are very convoluted to achieve desired runner length. These turns allow for a more compact manifold, with denser packaging of the whole engine. However, abrupt contour changes provoke pressure drops, resulting in less air (and/or fuel) entering the combustion chamber. High-performance manifolds have smooth contours and gradual transitions between adjacent segments.

Short, wide inlet runners move the torque curve up the rev range whereas long, narrow runners move the curve down the rev range. But, there is a whole lot more to it than that. The effect is caused by “pulse waves,” waves of relatively high and low pressure in the runner. The opening of an inlet valve causes low pressure at the engine end of the runner, as the engine sucks air out of the runner. Air starts to flow down the runner into the cylinder until the valve shuts, at which point all that air (traveling at high speed) crashes into the shut valve and creates a relatively high pressure “slug” of air. This is reflected and starts to move back up the runner until the valve opens again, at which point it heads back towards the port. If the dimensions of the runner are calculated well, the high pressure slug or pressure wave will make its way through the port before the valve shuts again, when the next slug is created. The dimensions are critical to the operating speed of the engine. If the runner is short, the reflected pulse might fall out of the end before the valve opens to suck it back in. If the runner is too long, the larger mass of air may react too slowly to perform well at high engine speeds.

This is the reason car manufacturers have invented variable or twin length inlet manifolds, which change the dimensions of the inlet runners at a certain rpm to give improved torque across the engine's rev range.

Variable-Length Intake Manifold

A variable-length intake manifold (VLIM) consists of four common implementations. First, two discrete intake runners with different length are employed, and a butterfly valve can close the short path. Second the intake runners can be bent around a common plenum, and a sliding valve separates them from the plenum with a variable length. Straight high-speed runners can receive plugs, which contain small long runner extensions. The plenum of a 6 or 8-cylinder engine can be parted into halves, with the even firing cylinders in one half and the odd firing cylinders in the other part. Both sub-plenums and the air intake are connected to a Y (sort of main plenum). The air oscillates between both sub-plenums, with a large pressure oscillation there, but a constant pressure at the main plenum. Each runner from a sub plenum to the main plenum can be changed in length. For V engines this can be implemented by parting a single large plenum at high engine speed by means of sliding valves into it when speed is reduced.

As the name implies, VLIM can vary the length of the intake tract in order to optimize power and torque, as well as provide better fuel efficiency.


Cast Iron Manifolds

Up until the 1990s, most intake manifold assemblies were made from cast iron when lower cost was a predominant factor, or from aluminum, when lighter weight was judged more important for performance reasons. "180-degree intake manifolds" were originally designed for carburetor V8 engines. The dual-plane, split plenum intake manifold separated the intake pulses which the manifold experienced by 180 degrees in the firing order. This minimized interference of one cylinder's pressure waves with those of another, giving better torque from smooth mid-range flow. Such manifolds may have been originally designed for either two- or four-barrel carburetors, but now are used with both throttle-body and multi-port fuel injection.


Dual Plane Intake Manifold


383 Engine- Cast Iron Intake Manifold

Aluminum Manifolds

The 6.1 Hemi is an example of an engine with a classic high performance light-weight aluminum intake manifold. It has shorter, larger-diameter and tapered runners for high-speed tuning. Its internal runners are core-dipped to smooth the runner finish and improve air flow. The engine was designed to replicate the 426 Hemi’s iconic 425 hp. figure. It also produces 420 ft.-lbs. torque.


6.1 Hemi Engine Cast Aluminum Intake Manifold

Composite (Plastic) Manifolds

Intake manifolds molded from plastic began to gain popularity during the 1990s because they offered both lower weight and cost. Early designs, however, were biased toward keeping the overall manufacturing costs as low as possible, so these original plastic intake manifolds suffered cracks, warpage and coolant leaks. Some of the first flawed designs, subject to early failure, were on GM’s 3.8-liter V6s, Ford’s 4.6-liter V8s, and Chrysler’s 4.7-liter V8s.


Cracked Composite Intake Manifold- 4.6 Ford Engine

However, after decades of testing and studying common failure points, optimum blends of plastic with 35% fiberglass or related glass elements have been perfected to enhance both strength and elasticity. As a result, plastic is now the preferred material of choice for most intake manifolds. Today, all of the current Challenger engines use composite intake manifolds (see below).


3.6 Pentastar Engine- Composite Intake Manifold


5.7 Hemi Engine- Composite Intake Manifold


392 Hemi Engine- Composite Intake Manifold

Active Runners- 392 Engine

Originally, the first new generation 392 Hemi engine tested didn’t feel as strong as the numbers said it should be. So the engineers took a look at the SRV dual runner intake that was being used in its Dodge trucks, and designed a new active runner manifold to increase the engine’s torque curve (Note- Short runners are used for power and long runners for torque).

The active manifold on the 392 Hemi, ideally, allows it to be physically sized for optimal velocity for best low/midrange rpm performance, but still allows for free-breathing response on the top end, as the "active" gate opens up the short runner paths at upper rpm. As a result, the engine produces an impressive 485 hp. and 475 ft.-lbs. torque.
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