While it’s the cylinder head itself that puts the introduced air to work within the engine, it’s the job of both the intake manifold and the throttle body to direct said airflow into the head as evenly dispersed as possible and to do so in a controlled fashion. The intake manifold then, is essentially an extension of the intake ports of the cylinder head with provisions to allow fuel to be introduced into the combustion chambers. The throttle body simply dictates how much air will enter and for how long as a result of how much pressure is applied to the accelerator. Intake manifolds typically feature tube-shaped runners – one exiting each individual port on the cylinder head – that mate up to a horizontally opposed plenum designed to equally distribute airflow to each runner. The throttle body generally attaches directly to the intake manifold’s plenum and features an electronically controlled or cable actuated plate to control the amount of air to be let in.

As mentioned earlier, many engines fall short in regards to what their volumetric efficiencies are in reality and what they may be potentially. Substituting or modifying an engine’s inferior intake manifold is one of the easiest ways to bump up an engine’s volumetric efficiency and, in turn, its power.

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Intake Manifolds: What They Do

Designing an intake manifold that works is not terribly difficult; designing a tuned intake manifold that works well is a calculated effort, requiring both skill and an in-depth understanding of the four-stroke engine process. The intake manifold comes into play once the intake valve opens during the intake stroke of the engine cycle (the four-stroke engine cycle process is detailed in chapter 3). Air enters through its respective runner and comes charging toward the cylinder head, through the port and into the combustion chamber. Most manifold designs can’t help but accommodate this scenario; the more mysterious part occurs once the intake valves close at the end of the cycle’s intake stroke. With the valves closed, the incoming air charge slams against both the cylinder head’s ports and the backside of the intake valves. The incoming air charge then reverts back upon itself forming a high-pressure area within the manifold’s runners. This high-pressure area, also referred to as a wave, ultimately heads back toward the intake manifold’s plenum. The process repeats itself, and if the manifold is tuned and designed properly, the next time the wave reaches the intake valves, they’ll be open, just in time for the next intake stroke.

While a well-designed intake manifold will increase horsepower and torque, it can also improve fuel efficiency. As pressure is applied to the accelerator, the intake manifold, through the throttle body, adjusts the correct amount of air that will enter the combustion chamber in order to not waste fuel. But modifying or swapping intake manifolds can be tricky business; what works for one engine combination might not work for the next. By simply swapping out something as simple as a set of camshafts, a manifold that once worked well may not now.

The Vacuum Effect

As the piston travels downward in the engine block a less-than-atmospheric phenomena occurs in the intake manifold – a vacuum is created. This vacuum effect poses several benefits and lends itself toward accomplishing a number of tasks. Many vehicles still rely on intake manifold vacuum to power braking systems while older cars rely on vacuum to control ignition timing, cruise control, windshield wiper motors, and power windows. More conventionally, intake manifold vacuum is used to suck piston ring blow-by gases out of the crankcase via a PCV (positive crankcase ventilation) system. The gases are released back into the intake manifold and are reintroduced into the combustion chamber to be burnt again. Recognizing less than ideal vacuum conditions is one way to tell whether or not a number of engine related problems exist like improper ignition timing or a blown headgasket.

Fuel Injection in All its Forms

Since the early 1990s, nearly all production vehicles sold in the United States have been outfitted with some form of electronic fuel injection in lieu of carburetion. The benefits of fuel injection are clear, the most important of which is the fact that air/fuel ratios may be adjusted electronically per any given RPM. But while the carburetor may have been replaced with a more emissions-minded and more accurate fuel metering system, the number and types of electronic fuel injection systems offered over the years are, at the very least, many and diverse. With that being said, we’ll forgo laying out the argument of why fuel injection is preferred over carburetion and get right into the different types.

Multi-port injection, sequential injection, throttle body injection; the ideas are all similar – to meter and introduce fuel into the cylinders for combustion – they just all go about accomplishing this in their own way. Today, most fuel injected production vehicles introduce gasoline into their respective combustion chambers through the intake manifold’s ports, although direct injection seems to be a viable alternative now. Through a computer-controlled process, fuel is released from each injector at the ideal point of the four-stroke engine cycle, specifically when the intake valves open.

Various Plenum Shapes, Volumes and Their Effects

The intake manifold plenum is best regarded as a reservoir. Air enters through the throttle body and resides in the plenum prior to entering the individual runners. When any individual cylinder requires additional air it’s pulled from the plenum. As is the case with most things engine related, bigger is not always better and plenum size can affect torque and horsepower. Generally speaking, smaller plenums allow for quicker, low-end torque production at the expense of high-RPM, top-end horsepower. The opposite is true of large plenum designs in which high-end horsepower is easier to come by here.

The Ideal Runner

With carburetors, air would mix with fuel long before it made its way through the intake manifold. This would make for a change in air velocity and would often times produce unpredictable results. Modern-day intake manifolds featuring fuel injection are much different. In these cases, air meets fuel once it’s been released from the manifold into the cylinder head’s ports. When air is first introduced into the plenum and finally into the runners, fuel is yet to be sprayed from the injectors; in fact, it won’t come into contact with the air until after the air has been released from the intake manifold. It’s at this point that it’s most ideal for the manifold to feature a smooth and laminar design. Either friction or bends can both inhibit the amount and quality of air that may actually be used once fuel is introduced.

Long vs. Short Runners

The length of the intake manifold’s runners determines exactly how much air will enter the cylinders during the intake valve cycle based upon the speed of the engine. And similar to how the camshaft dictates the engine’s power curve, so does the length of the intake manifold’s runners. Due to the nature of the air waves created within the runners, high-RPM engines tend toward manifolds featuring shorter runners while engines that spend more time in a lower RPM range will benefit more from longer runners. The key when tuning intake manifold runner lengths is determining the appropriate amount of time it will take for the intake charge to bounce back and forth within the runner and ultimately arrive back at the intake valve upon its opening. This is often trial and error process. Since the intake valves open more frequently and faster on the high-RPM engine, it’s only logical that the distance the wave must bounce across within the runner be shorter. Of course, the opposite also holds true for the low-RPM, long runner setup. With that being said, ideal runner lengths may still vary significantly between two engines of equal RPM limits when comparing those of differing displacement.

Length is not the only important characteristic when concerning intake manifold runners. The shape of the runners themselves is perhaps equally as important. The intake air charge requires a high velocity in order to reach the combustion chamber and ignite. Runners designed too large in diameter will decrease air speed; conversely, runners designed too small in diameter will increase air speed but sacrifice the amount let in. A compromise must be made and generally lends itself toward the small side, but with a twist. The trick is incorporating a runner design that tapers down in diameter as the air gets closer to the cylinder head. This helps promote both an acceptable air speed and volume. Equally important is to ensure that each runner features equal lengths and diameters. This will ensure that each cylinder receives the same amount of airflow, at the same point during the intake valve cycle, to help promote equal combustion within each cylinder.

Multi-Stage Manifolds

Multi-stage intake manifolds were first introduced to both widen the engine’s powerband and to lessen the contrast between low-speed and high-speed power production. Remember our discussion of volumetric efficiency [link] and how it doesn’t remain constant? Variable stage intake manifolds are meant to optimize an engine’s volumetric efficiency at all points in the RPM range, not just one. They accomplish this by inserting computer-controlled butterfly or rotary valves within the manifold’s individual runners. Runner lengths may be adjusted for optimum cylinder filling by simply opening and closing the butterfly valves. The factors of which determine the time at which the valves will open vary depending on the engine but almost always include engine speed. The results are impressive. Engines featuring multi-stage manifolds offer the benefits of short-runner, top-end power and long-runner, low-end torque all from the same package.

Multi-stage intake manifolds are especially beneficial in that engine builders are no longer limited to constructing a manifold that works optimally within only one narrow RPM range. Dual-stage manifolds, for example, help promote optimal air swirling and pressurization at two varying points across the RPM range. When constructed and tuned properly, the multi-stage manifold will increase low-end torque without sacrificing top-end horsepower production.

Aluminum, Carbon Fiber and Plastic Manifolds

Since intake manifolds conduct less heat than, say, exhaust manifolds, the materials in which they may be constructed in vary almost as much as the numerous sizes and shapes offered. But despite more exotic materials such as carbon fiber, both aluminum and plastic remain the most popular of materials for intake manifolds destined for production vehicles.

The intake manifold is not only responsible for directing air into the cylinder head’s ports, it must also keep the incoming air cool, provide a certain amount of rigidity so as to not break or warp, and some must also seal off oil, coolant and exhaust recirculation passages. Depending upon which characteristics are most important, the material of choice may be of little or no significant difference. Where overall engine weight and a low intake air temperature are key, plastic manifolds are well suited. These, however, are typically not suited for turbocharged engines with higher manifold pressures. Manifolds that reintroduce burnt exhaust gases back into the plenum for emissions reasons or that circulate coolant to or from the cylinder head are typically constructed of aluminum. While metal is certainly heavier than plastic, aluminum is more suited for taking the abuses given off by the heat of burnt exhaust gases and is less likely to corrode when introduced with engine coolant.

Injector Location Matters

The rules for where a fuel injector should be placed in order to function most efficiently are simple: First, its spray pattern must aim as straight down the port as possible; and second, it should be placed at the point where air velocity is at a maximum. Most OEM intake manifolds do a fairly decent job at meeting both of these criteria, placing the fuel injectors as close to the cylinder head as possible at a steep, downward angle. This reduces the likelihood of fuel meeting any sort of restriction and also helps promote proper atomization.

Porting and Port Matching

Sometimes, there’s more work involved than just bolting on that new, oversized intake manifold. Often times, the port openings at the end of the intake manifold’s runners don’t match up exactly with those of the cylinder head. In other cases, the cylinder head’s intake ports may have been opened up slightly, requiring the intake manifold’s runners to be further enlarged to match. In either case, port matching is the solution. Neglecting this important step will lead to a major obstruction to incoming air, resulting in decreased velocity and less power. Be sure and check and resize the intake manifold gasket as well to the appropriate size.

Throttle Bodies: What They Do

Throttle bodies aren’t too complex; their job is to provide the driver with a means of managing airflow into the intake manifold and ultimately into the combustion chamber. Inside the throttle body lays a butterfly plate. The plate is controlled by either a cable or electronically via the gas pedal, but since the throttle body’s job is to introduce air into the equation, perhaps it should have been named the air pedal. At any rate, once the driver depresses the pedal and additional air is introduced, either a throttle position sensor or mass airflow sensor communicates the change in airflow to the ECU. At this point the ECU instructs the injectors to supply the appropriate amount of fuel in order to maintain the correct air/fuel ratio.

Throttle bodies come in a number of configurations: single, dual and, in some cases, one per cylinder, with the single being the most widely used for production purposes. Upsizing the throttle body in and of itself often times won’t reap huge horsepower gains, if any, but, when combined with a freer flowing intake manifold, high-lift camshafts and a ported head, it’s a must-have since the ability to bring in a larger volume of air has just been increased.

Written by Aaron Bonk and Posted with Permission from CarTech Books