When you think about the parts that make an engine perform, the intake manifold probably isn’t the first thing that comes to mind. Although it might not have the sex appeal of a camshaft, ported and polished cylinder heads, or a set of big-tube headers, the intake manifold plays a critical role in getting an engine to make power. Sitting between the throttle body or carburetor and the cylinder heads, the intake manifold's job is to distribute the incoming air charge (the air or air/fuel mixture) evenly to the engine's individual cylinders.

That might sound simple, but how that air gets to the cylinders can have a big impact on how an engine runs and where in its powerband it makes its power. Inside the manifold are individual passages called runners, and their length, shape, and size all influence how efficiently the engine can fill its cylinders as engine speed changes. In other words, the intake manifold isn't just a collection of tubes bolted to the top of the engine. It's a carefully engineered part of the airflow system.

Runner length is particularly important because it can influence the engine's torque curve and powerband. That's why two engines with similar displacement and cylinder heads can have noticeably different personalities depending on the intake manifold they're using.

Understanding the principles behind intake manifold design can help you make smarter choices when selecting one for your own engine. Whether you're building a street cruiser, a weekend warrior, or an all-out race engine, understanding how the intake manifold effects engine performance can help you choose the one that's best suited to your goals for the build.

The Anatomy Of An Intake

Intake manifolds can look quite different depending on the engine and application they're designed for. Based on the induction system, an intake may feature mounting points for fuel rails, a throttle body flange, or a carburetor mounting pad. Other features, such as provisions for accessories and sensors, can be found on manifolds regardless of whether the engine in question is running carburetion or electronic fuel injection. Beneath those application-specific details, however, every intake manifold shares two fundamental features: the plenum and the intake runners.

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The plenum is the chamber within the intake manifold that serves as a central gathering point for the incoming air charge. On a typical multi-cylinder engine, the air charge enters the manifold through the throttle body or carburetor and collects in the plenum before being distributed to the individual cylinders.

The plenum's job is to provide a common space where the incoming air charge can be distributed among the engine's cylinders. Its size and shape can influence how the charge moves through the manifold, which makes the plenum an important part of the intake's overall design. A well-designed plenum helps maintain a consistent supply of air charge to the runners as the engine demands more or less airflow, depending on engine speed and load.

While the plenum provides the central space where the incoming charge is collected and distributed, the runners are responsible for carrying that air charge to the individual cylinders. Their design is just as important to the way an engine performs, and that's where things start to get particularly interesting.

How Intake Runner Length Affects The Powerband

“Using a port-injected engine as an example, the intake runner is what’s going to connect the plenum to the backside of the valve,” Jake Anderson of Holley Performance explains. “And every time an intake valve closes, it creates a pressure wave that travels at about the speed of sound. That pressure wave will travel all the way back up the runner and into the plenum itself. When that wave reaches the plenum, it then reflects back toward the valve.”

Because of that, changing the length of distance between the valve and the plenum can alter the timing of that return wave. When it returns, it can actually encourage more air to come into the cylinder through the valve if the timing is optimized properly. And engine speed is going to affect the timing of when that wave reflects back.

“Imagine you’re standing in a hallway, and you clap your hands,” says Anderson. “There’s a delay between when you actually clap your hands and when you hear the echo come from the end of the hallway back to you. And by changing the width and length of the hallway, you can delay or accelerate the time that the echo comes back to you. So, by adjusting the runner length, we’re optimizing the ‘echo timing’ of that pressure wave.”

Generally speaking, an intake manifold with long runners is going to be more ideally optimized for low-end torque, while an intake manifold with short runners will do its best work at higher RPMs. These characteristics are important to keep in mind when selecting an intake manifold for a specific application because they can positively – or negatively – impact the engine’s performance in the RPM ranges that it’s going to live in.

“With a longer runner it’s going to take a longer amount of time for that pressure wave to get back to the valve,” he says. “So, the longer your runner is, the faster that charge is going to have to travel, so you will get a higher port velocity very quickly with a long runner intake versus a short runner. The Sky Ram manifold (pictured below) we did as an April Fools’ joke a few years back is a perfect example of this. Somebody actually put that intake on a dyno, and it absolutely followed the theory. I believe that intake made peak power at around 3,500 rpm, and by 3,800 rpm it was gone.”

Because of this characteristic, longer runners tend to work best in street cars and trucks, as the engines in these vehicles spend most of their time in low and mid-range. And that’s precisely where long runners, like those on the Holley Hi-Ram, offer the biggest advantage over a short runner intake manifold design.

“A longer runner intake is going to give you more of that ‘seat of the pants’ kind of power whenever you crack the throttle,” Anderson notes. “It really wakes up in that 3,000- to 4,000-rpm range, which is what the engine’s going to see most of the time on the street.”

Conversely, intake manifolds with short runner lengths, like the Holley Lo-Ram and Ultra Lo-Ram, are slower to develop port velocity – they require more engine speed to find their most efficient operating range. That behavior makes them better suited to sports cars and race cars built for drag strips or road courses, as these engines tend to spend more time in the higher RPM ranges due to the nature of those use-cases.

“The benefit here is that you can rev the engine out very, very high and not worry about getting so much velocity that the runner is actually creating some kind of restriction. Those restrictions can create turbulence that negatively impacts atomization, and it can generate more heat simply due to air friction. So, because we have these short runners and we don’t have to worry about those potential restrictions, these manifolds tend to do their best work above 6,000 rpm.”

In other cases, a balanced relationship between low-end torque and high-RPM power may be the goal. Anderson points to manifolds with fixed-length runners as good example of this approach.

“It’s basically a compromise so that the mid-range is really where it’s optimized, and that makes it a good jack-of-all-trades solution. OEs can also tune intakes for multiple RPM ranges by using Intake Manifold Runner Control systems. With IMRCs you can have a manifold with two different sets of intake runners and use a stepper motor to flip back and forth between those two sets at different RPM ranges. The Coyote V8s in the latest Mustangs have this technology, and automakers like BMW have been utilizing this design strategy for a long time.”

Another factor to keep in mind when it comes to runner length is the packaging constraints of your application – or, more specifically, how much hood clearance you have to work with.

“With a racing application, this is probably less of a concern,” says Anderson. “In those use-cases, we will optimize an intake and it’s really up to you to figure out how to make it fit. In a motorsport use-case, most of the time the engine combination is going to come first, and the hood selection will be decided last, so clearance isn’t as much of a priority. But with a street car application, the story changes substantially – in that situation, packaging is really the first concern. That’s where the Lo-Ram and Ultra Lo-Ram really have a lot of appeal.”

To put these packaging constraints in perspective, while the Holley Hi-Ram stands almost 12.5 inches off the flanges where intake rests, the Lo-Ram drops that height to about nine inches, while the Ultra Lo-Ram brings that down to about six and a half inches. Those clearances can be the difference between your new intake neatly fitting under the factory hood and having to break out a Sawzall to make room for it.

Optimizing Your Combination

Although the presence of forced induction has a significant effect on the pressure and velocity of the charge, Anderson says that you can expect different intake runner length designs to behave similarly to those of a naturally aspirated engine.

“Even if you have a turbo, the powerband will follow similar trends. Though it’s not quite as pronounced as it is with a naturally aspirated engine, a longer runner intake on a turbocharged is still going make more power down low as compared to an intake manifold with short runners. But you can also kind of ‘Band-Aid’ a short runner intake by changing your boost curve to come in a little sooner.

If you’re running a belt-driven supercharger, the boost curve is essentially fixed, so you don’t really need to worry as much about speeding up the turbine. As long you get the RPMs up, it will make boost. You can optimize this a bit for the moments when you’re out of boost at low RPMs for some extra punchiness, but with a supercharger you can use rear gear or a torque converter to force the RPM to flash higher and essentially skip the laggy portion of the powerband.”

When selecting an intake manifold for a given application, he offers a few guidelines to help narrow things down.

“The first thing you need to consider is the vehicle’s usage. If it’s a street car, I think packaging is one of the most important aspects to keep in mind. With a race car, that’s probably going to be much less of a priority. The second thing you really need to keep in mind is the performance range of the other components in the engine. What RPM does your torque converter stall at? What RPM is your camshaft the happiest at? What about your cylinder heads? All of those components need to play in the same ‘window’ as the intake manifold if you want the engine to reach its full potential.

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A great example of this are two very similar cars that come into my shop periodically. The combinations are very similar – six to seven-liter naturally aspirated Corvette engines – but one makes 400 horsepower, and the other makes 575 horsepower. And the main difference between them is that all of the components were specifically selected to work together in the same window with the engine that makes the higher amount of power. That person bought an intake that was designed to work that type of camshaft, which was designed to work with those cylinder heads, and so on. It’s all designed to live in the same range of the powerband. And as a result, it’s a much more enjoyable car to drive as compared to the other vehicle, where the owner didn’t really consider the bigger picture and just grabbed whatever performance part was on sale at the time.”

With that in mind, he tells us it’s imperative to have a game plan for your engine combination going into the build – one that not only takes power goals into consideration, but other real-world factors as well.

“The whole thing really needs to be thought out before you click ‘Buy’ on a single component.”