The Structural Dynamics of Flow: How CID Builds A Better Intake

Street cars are a collection of compromises. When an OEM sets out to design a new vehicle, performance-oriented or otherwise, a set of targets are established, and engineers embark on finding a way to reach those targets. Some results are fantastic, others mediocre. But ultimately, the path to those targets is a give and take between cost, reliability, performance, and compliance with federal regulations. And the designs of the engines that go into these vehicles are no exception to this rule.

For those who place performance above all else, the appeal of race-derived designs is obvious. Many of the compromises that are inherent to street cars simply don’t apply in the racing world, and those that do are often significantly deemphasized in motorsport applications.

Case in point: intake manifolds. Compare manifolds from decades ago to those being used in mass production applications today, and you’ll notice that one of the fundamental differences between the two is the design and location of their inlets. You’d be forgiven for assuming that the front-mounted inlets that are popular today provide significant performance and efficiency benefits over the top-mount inlets of yesteryear. Still, the truth is a bit more complicated and primarily rooted in concerns over clearances more than anything else.

CID’s BE 4.0 LS7 intake manifold was developed to yield unmatched horsepower-producing characteristics amongst all of the cast single plane manifolds that are available today. It features a square carb pad that’s designed to suit a 4500 Dominator carburetor or a throttle body.

And that’s where companies like Competition Induction Designs (CID) come in. Leveraging the latest technologies in computational flow dynamics, this niche builder of cylinder heads and intake manifolds is the brainchild of John Konstandinou. Konstandinou’s previous venture, CHI Cylinder Heads, produced 3V Cleveland heads and intake manifolds that went on to win no less than five Engine Masters competitions over the years. So yeah, it’s safe to assume that Konstandinou knows what he’s talking about when it comes to making power. Here we’ll take a closer look at the science behind intake manifold flow and see how CID produces data-driven designs that minimize those compromises.

The Forward-Facing Problem

“The factory LS manifold is designed to package well in a variety of production cars,” Konstandinou points out. “It has long runners because it caters toward power at lower engine speeds, but the biggest consideration is packaging, because it needs to fit under a factory hood. Considering that, it’s actually a really good design for those purposes, but those requirements differ from what a lot of enthusiasts and racers are looking for.”

As aerodynamics and federal regulations continue to have an increasing influence on vehicle design, these days it’s not often you’ll find a significant surplus of free space under a factory-installed hood. Image: GM

It’s a trend that isn’t exclusive to OE design, though, as many aftermarket companies have followed suit. “There are a lot of people who don’t want to mess with the hoods on their cars – even racers,” he tells us. “But when you’re trying to make horsepower, that design is counterintuitive. Air doesn’t like making sharp turns. It’s that simple.”

When you’re trying to make horsepower, a forward-facing inlet design is counterintuitive. Air doesn’t like making sharp turns. It’s really that simple.

And if you look at how air enters a manifold with a forward-facing inlet, it’s not exactly a straight path to its destination. “It has to make a series of 90-degree turns,” Konstandinou explains. “And that’s why it doesn’t work as well as a spider-style intake manifold. The air still has to turn in that kind of design, but the turns are nowhere near as abrupt to get to the cylinders, and that makes a difference.”

Konstandinou worked with John Schmidt, a design engineer whose resume includes companies like Siemens and Space X, to better understand the behavior of air in different intake manifold designs through Computational Fluid Dynamics (CFD) testing. Schmidt posited the following and notes that while this serves to explain the design considerations for forward-facing air-entry for an intake manifold, the same rules apply to tops for tunnel-ram style manifolds and intake designs with an elbow that attaches to the top of a manifold which has a carburetor flange.

This CFD data shows the change in air pressure in intake design with a 90-degree elbow as the air is introduced into the system and encounters various elements of resistance on the way to the cylinders.

“The purpose of a manifold top or elbow is to connect a throttle body to a plenum with a flow path that delivers the air to the plenum with balanced flow to all regions of the plenum with the least drag possible. The most important design feature for the transition of air moving horizontally from the air entry to vertically into the plenum is the radius of the turn. The smaller the turn radius, the greater the drag on flow.”

Schmidt also explains that the image below quantifies the flow through a bend with a flow coefficient. The vertical scale of the graph is r/d (the bend radius centerline/duct diameter).

“The horizontal scale of the graph is the coefficient of flow. In this case, the coefficient ranges from 0 to 1, meaning that a flow coefficient of 0 is the most restrictive. Notice that the flow loss coefficient of a duct with a bend radius that is 1.5 x duct diameter is more than twice that of the same diameter duct with a bend radius that is 2.5 x duct diameter.”

This chart illustrates the cost of flow loss due to a bend. Notice that the decrease in flow loss is the greatest from a sharp turn (0.5 X diameter) to about 1.5 x diameter, improvements from a larger bend radius are more gradual above 1.5 x diameter. Image: Engineering ToolBox, (2008). Air Ducts Minor Loss Coefficient Diagrams.

He also adds that when flow through a bend is important, a bend radius that is at least 1.5 times the diameter should be chosen. For a throttle body that is approximately 4-inches (101.6mm) with a bend radius that is 1.5 times diameter, the inside radius of the turn is 4-inches (8-inch diameter). Notice that when the bend radius is 1.5 times diameter, the inside radius of the turn is the same as the diameter.

Going Top Down

Schmidt illustrates that since airflow in a turn follows the inside radius, the inside radius is the most critical dimension regardless of the shapes surrounding it. “Some elbows and tunnel ram tops have an entry that directs the air to flow upwards before entering the plenum. CFD does not show any advantage to flow from this sloping trajectory. The only advantage is that it makes for more compact air inlet ducting.”

The image illustrates the critical radius.

And this one illustrates the packaging differences of the two radii.

The goal was to utilize CFD in order to test a cut-down version of our BE LS intake with a fabricated elbow that fits the low hood design criteria VS a properly designed elbow on top of the CID intake and see which had the best distribution and the least amount of resistance, Konstandinou says. CFD clearly showed us that a fabricated low elbow with a forward-facing throttle body on a cut-down version of our BE LS intake corrupted the inherent even distribution of the original design. This can be seen clearly in the CFD streamlines closest to the entry of the throttle body. Air doesn’t want to follow the tight radii at the front created by these styles of elbows and in turn, favors the rear of the plenum whilst adding considerable resistance compared to a properly designed elbow on top of the existing intake carb pad.

You can see in this image that the front runners suffer from uneven airflow. To tune an engine with this manifold would require more fuel in the rear runners and less in the front since the airflow is vastly different.

So the purpose of our testing shifted to determining what the ideal elbow design would be in order to create the least amount of resistance.

He’s quick to point out that many high-horsepower applications often overcome the resistance created by low forward-facing intake designs with additional pressure from a power adder like a supercharger or a turbo. However, the truth is a bit more complicated. “In an ideal situation, you’d never cut a manifold down for a naturally-aspirated application to make more power. When you cut down a good spider-style intake the air has less time to transition to the cylinders and therefore has to navigate sharper turns, ultimately robbing you of horsepower, and the same applies in a boosted application. Generally speaking, you’d really want to do the opposite and add a spacer to a spider-style manifold to make more power.”

It’s clear that using a spacer provides some direction to the airflow. Cutting down the top of the manifold makes the turns after the inlet sharper, reducing that airflow, and horsepower in turn.

And when you ask that airflow to make a sharper turn, the disruption to the flow is increased. “On a top entry spider-style manifold, air doesn’t have to make as sharp and abrupt a turn as it does with a forward-facing entry throttle body. With a properly designed spider intake, the air will have time to transition to all the runners,” Konstandinou says. “By adding height and giving it the correct radii – especially at the front and the back – you’re encouraging air to not separate and leave the walls, but rather follow the walls to cylinders. And that’s the best way to get the air there.”

Schmidt’s CFD tests on elbows also provided additional information on dividers within a properly designed elbow: adding a divider within the elbow can make a big difference. Konstandinou says. “From the perspective of needing elbow in the system, a divider within an elbow that has the correct radius design considerations helps fix any lingering distribution issues. But it does create a reasonable amount of resistance within the elbow”

CID’s Approach

Using extensive CFD data analysis, along with critical input from a long-standing customer and one of our industries greats, Tony Bischoff Owner of BES Racing Engines, Konstandinou designed a manifold that benefits the most from what the team had learned. “It’s not feasible to make it ‘perfect, but we did everything we could to shape the internals of the manifold to minimize the disruptions we found in CFD.” But CID also wanted to ensure that the air was directed along the way. “A big hole might create the least amount of restriction, but without direction, the distribution suffers.”

Here we see the divider’s influence on the direction of the air as it travels toward the runners.

After nailing down runner length, CID started looking at how the air goes from the carburetor pad to the rest of the cylinders. “With our manifold, a lot of it came down to determining what the best way was to get the air to distribute itself from the opening to the cylinders with as little disruption as possible,” Konstandinou says. “So it’s basically about maximizing flow guidance to the runners with as little restriction as we can manage.” And that resulted in a manifold design with wide dividers to channel the air in a way that encourages even distribution amongst the cylinders while also keeping the air suspended as much as possible.

CID’s new LS intake design is available and can be adapted to various applications. “With this one casting, we can accommodate multiple deck heights. The manifold can be ordered to fit 9.24-inch OEM blocks as well as aftermarket 9.45-inch Mid deck and 9.75-inch tall deck blocks, carburetors, and EFI with -12 Fuel Rails as standard. The manifold accepts LS1 style “long” injectors with two different injector locations, 8 or 16 injectors and it’s also available to suit 4150 and 4500 carb and throttle body designs. There’s a lot of different options, so it just comes down to what the customer needs.”

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About the author

Bradley Iger

Lover of noisy cars, noisy music, and noisy bulldogs, Brad can often be found flogging something expensive along the twisting tarmac of the Angeles Forest.
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