Common Plenum vs. Individual Runner Intake Manifold

Author: Hilborn | 11/17/2018 < Back to Motor Life Home


For maximum power to be realized, all cylinders of the engine must do the same work, or produce the same power. For many years, research has developed new designs of cylinder heads, intake manifolds and other engine internals to provide increases in horsepower. In fact, gains in engine efficiency are some of the factors in these large-scale power increases. Some of these efficiencies can be defined as improved distribution. Distribution, in relation to the internal combustion engine, can have several definitions. For the purposes of this report, we will concentrate on distribution with regard to equal distribution of air and fuel at a given air/fuel ratio, along with distribution in regard to equal amounts of the air-fuel supplied to each cylinder. This helps us in our goal to make peak power in each cylinder and therefore maximum power from the engine.

In the extremely competitive world of prostock racing, engine development advances have contributed to 500 cubic inch engines making in excess of 1300 horsepower. Many of these advances have also redefined the distribution in the engine and therefore its efficiency. For example, siamesed intake ports gave way to symmetrical intake ports, factory valve angles led to reduced valve angles for increased line of sight to the intake valve, and then from rectangular ports to oval ports. Fabricated intake manifolds with huge plenums, along with short straight runners, replaced cast intakes. These developments were years ahead of their stock counterparts and contributed to the massive power levels seen today. So how does one increase engine output without a prostock racer’s budget? The easy answer starts with fuel mixing and intake manifold selection. The eight-stack injector disposes of the concerns of the carburetor and the v-style common plenum intake manifold. And because of this, efficiency and distribution are improved along with power output and throttle response. The eight-stack fuel injector originally pioneered by Stuart Hilborn in 1948 was simple in design, yet it produced fantastic results in the racing world against carburetors, making the carburetor extinct in some racing venues even today. Today, advancements in carburetor and intake manifold technology still waver in comparison to the original design of the fuel injector from many years ago.

What makes the Hilborn injector superior to the carburetor and superior to other EFI systems available today? I will explain.

The Carburetor

The carburetor can best be described as an air/fuel mixer that uses a differential in pressure to provide fuel at an established metered amount. Although easy to define, the actually workings of a carburetor are complicated, enough so that very few people are able to maximize its potential.

With air movement toward the intake valve in the intake manifold, a result of piston movement and valve timing, the main venturie of the carburetor has air flowing past the booster creating a pressure drop. This pressure drop causes fuel to be pushed into the booster supplied by the fuel bowl via the main well of the carburetor. The shearing of the fuel as it enters the air stream out of the booster causes the fuel to separate into smaller particles, where it is picked up by the air and carried into the intake manifold.

The legendary Smokey Yunick states, “The carburetor is a big restriction in the intake system”(1). This is because in a round column of airflow, flow speed is fastest towards the center of the column (2).The design of the carburetor places the booster towards the center of that column in order to receive the strongest signal, or pressure drop at the booster, for maximum booster performance. Since airflow is impeded, the direction of part of that flow is diverted into eddies of spinning air that will disrupt the rest of the airflow around it.

Float bowl volume is essential for correct air/fuel ratio. With a needle and seat size a nominal .110 of an inch for gas, coupled with fuel pressure of six to eight pounds per square inch at the needle and seat, it is difficult to keep the bowl filled. Carburetors use atmospheric pressure to provide lift of the fuel, therefore as the bowl empties, an increase in pressure drop is required to lift the fuel up the main well into the booster, compromising consistent air/fuel ratio. Fuel bowl problems also manifest themselves in applications that incur aggressive changes in direction, as in road-race and autocross applications.

CFM ratings for carburetors were introduced as a way to identify correct carburetor sizing for specific applications. In actuality, only the throttle blade size is needed to identify size, since a venturi will only flow so much air at a certain depression. Any published CFM ratings, regardless of product, should also include their depression in inches of water. Unlike the cylinder head aftermarket where the standard for CFM ratings is 28 inches of mercury for pressure drop, there is nothing published for the carburetor aftermarket. As an example, all of Holley’s 750CFM carburetors use a 1.375 primary and secondary throttle blade, yet there are many companies who will claim up to 950CFM with the same throttle blade size. These types of exaggerated specifications only lead to confusion of the end user on what size carburetor will fit his needs. Qualified carburetor shops will sell a carburetor by throttle blade size and not by exaggerated CFM ratings.

At idle when air speed is low, the fuel droplet pulled from the booster is much larger than the one pulled when air speed is greatest or wide-open throttle. This has a profound effect on mixture distribution

If a carburetor’s venturi is sized for low to midrange torque and strong acceleration, it will be too small to produce top end power; conversely, if the venturi is sized for top end power, low to midrange torque and throttle response will suffer (3). The correctly sized carburetor for some applications will be a compromise of low to midrange torque and top end horsepower.

The V-Style, Shared Plenum Intake Manifold

As stated earlier, for maximum power to be realized, all cylinders of the engine must do the same work, or produce the same power. Although many factors control this quotient, none are more important than the intake manifold, specifically the shared plenum style used today.

First it is important to keep in mind that the air and fuel have not mixed into a homogenous mixture in the venturi of the carburetor or in the runner of the intake manifold. A homogenous mixture is defined as a mixture whose physical properties are uniform throughout. Fuel in the manifold is not uniformly mixed as this only happens under the extreme heat and pressure contained in the combustion chamber. In reality, fuel in the intake tract uses air as a carrier; therefore, it is relatively easy for fuel to fall out of suspension, causing mixture distribution concerns.

Accelerating air has passed through the carburetor venturi, picked up its metered amount of fuel, and then is deposited into the plentinum. As the air/fuel enters the plenum, a reduction in air speed is created due to the large increase in plenum area, which causes the outer boundary layer of air to slow dramatically. This reduction in air speed creates eddies in the outer layer causing fuel to fall out of suspension and onto the port walls and floor. Meanwhile, the faster moving center column of air also slows, but is still on a collision course with the manifold floor. The first concern happens when the column of fuel and air collide with the floor. The law of physics states that an object in motion tends to stay in motion comes into play, since air and fuel have weight. Since fuel is much heavier than air, fuel falls off the carrier and puddles onto the floor, although the air carrier has corrected and is entering the intake runner. Correspondingly, this same air will also change direction and creates additional eddies into the plenum. In some applications turbulent air will now disrupt the incoming air/fuel affecting carburetor metering (4).There is also the air/fuel that has made the transition and is proceeding into the intake runner. Part of the distribution concern is evident by the raw fuel on the floor and plenum walls. This fluid will be picked up by passing air, put into suspension, and carried into the cylinder. It is important to note that these fuel particles will be of different size than the ones still in suspension and will contribute to distribution concerns especially on the four inner cylinders of a single plane intake where the runners are shorter and are provided a stronger signal, or faster airflow, than the four outer runners.

The advent of carb spacers and shear plates have been used for years to combat some of the concerns associated with air/fuel transitioning from plenum to intake runner and reversion concerns of this type of intake.

But a lesser-known approach of solving the problem of plenum floor transition was the introduction of the “Turtle,” a concept pioneered by manifold designer Jean Dittmer. As you can see at left, the Turtle, with multiple channels and a raised center, is used to help reduce the sharp transition of the plenum floor along with helping direct the air/fuel in a more efficient means into the intake runners (5). The Turtle is an excellent example of the methods required to fix the poor distribution concerns associated with the plenum floor. Ultimately, due to its specific intake applications and concerns with installation, the Turtle never made an impact in mainstream racing.

Although the effects of reversion have been mentioned, it is important to understand the impact it has on this type of intake. Reversion is the reverse pumping of the air/fuel in the intake, due to the intake valve opening while the piston is approaching TDC. This timing event happens two times in a four-cycle engine. The first reversion pulse is during the exhaust stroke as the piston is about to reach TDC. The intake valve opens as the exhaust valve is closing, commonly know as valve overlap. Air, rushing out of the cylinder, due to a low pressure at the exhaust valve, is enhanced by the movement of the piston towards TDC. Theory states that when the intake starts to open, the air/fuel will be drawn into the chamber exiting with the burnt gases to flush the cylinder of the remaining spent air/fuel and provide the momentum for the air/fuel in the intake manifold to start filling the cylinder. It is at this time that some reversion makes its way into the intake runner, diluting the fresh intake charge with the spent air/fuel mixture.

The second reversion pulse is much more significant and happens on the intake cycle. As the piston travels past BDC, heading towards TDC, the cylinder continues to fill. The momentum of the air/fuel continues to pack the cylinder even though the piston is moving up the bore. When the intake valve closes, the piston has already moved past BDC by up to 20 degrees or more. This reversion pulse is very obvious in the lower rpm range, and typically the larger the camshaft, the more reversion in the lower rpm range.

Reverse pumping in the intake can be identified as the bounce commonly seen in the vacuum gauge, or engines that do not “clean out” until higher rpm’s.

Reversion, fuel falling out of suspension, eddy currents, long runners versus short runners, high-speed right angle turns. All of these conditions in the intake manifold do little to promote good distribution. It then becomes impossible for all the cylinders to do the same amount of work and therefore the engine to make maximum power.

With regard to Multiport Fuel Injection that uses a convention shared plenum intake, much of the same negatives apply. Regardless of how and where the fuel is introduced into the intake tract, concerns with air direction changes, sharp runner turns and the negative pulses from all cylinders will also negatively affect any mulitiport EFI system.

The Eight-Stack Manifold

In a test I conducted with a common plenum intake against the same engine with a Hilborn eight-stack manifold, with both adjusted to provide the same air/fuel ratio, the eight-stack manifold made more power (6). There are numerous reasons why.

Without a carburetor’s booster in the way, the eight-stack manifold will flow more air than a comparably sized carburetor venturi. Without the need for a pressure drop to supply fuel for the engine, the bore size of the eight-stack can be increased to supply the required air for top end horsepower, yet have increased throttle response along with increased low to midrange power. The mild turning radius of the eight-stack intake helps promote line of sight for the air/fuel into the cylinder head. Since there is no common plentinum, the cylinders no longer need to digest an air/fuel mixture that is contaminated with pulses from companion cylinders. Since each cylinder is separate, there is no dilution to companion cylinders from reversion, eddy currents, fuel puddles and the tight bends for the air/fuel to follow causing mixture distribution concerns.

The eight-stack manifold employs a converging tract design, meaning the larger ram tube top is reduced in size as it enters the lower portion of the manifold before entering the opening of the cylinder head runner. With this design, air/fuel speed is increased when going from the larger opening to the smaller opening, ensuring that the air/fuel stays in suspension, unlike the tract of the common plenum manifold, which allows the air to slow when going from the small venturi of the carburetor to the larger opening of the plenum. Around the point that the intake tract is constricted, fuel is injected under pressure, allowing it to take maximum advantage of the air speed. In the converging tract manifold, most of the fuel that falls out of suspension as the wall is constricted is picked back up and put into suspension instead of being pushed along the walls.


Fuel supplied into the nozzle of an EFI Hilborn injector can be as high as 60 pounds per square inch, allowing the injector to spray the fuel in a fan pattern. It also allows a consistently smaller sized droplet, which is much more consistent than that of a booster. Therefore, the airspeed will not dictate the size of the droplet, which in turn allows the fuel to mix in better proportion with the air, providing improved engine efficiency and therefore making more power.

Since a Hilborn injector does not require a bowl for fuel storage, it is an ideal system for any application that will create excessive fuel slosh as in road race and auto cross applications.

Separation caused by the fuel crashing into the plenum floor is reduced due to the individual throat design of the Hilborn injector. Also important is the reversion from one cylinder has no effect on the other cylinders. This allows each cylinder to act on its own, greatly improving the engine’s ability to produce equal power from the fresh intake charge each cylinder receives on the intake stroke. Since each cylinder has its own throat, the line of sight to the intake valve is improved, resulting in a straighter shot for the air/fuel mixture. The fewer turns the mixture has to make, the more the fuel stays in suspension, providing enhanced distribution into each cylinder.

The Hilborn injector has numerous advantages over carbureted systems. A carburetor requires a correctly sized venturi for its application in order to pull fuel out of the booster. However, the Hilborn, with its injected fuel, can maximize airflow potential by removing the restriction associated with the booster while enlarging the throat diameter allowing maximum breathing for the engine. This increase in throat size does not impede low speed engine performance but enhances throttle response, and engine acceleration. This allows for maximum engine rpm output without sacrificing low speed performance.

For maximum power along with razor sharp throttle response, it is quite apparent that the Hilborn Fuel Injector is the right choice.


1 Smokey Yunick, Smokey Yunick’s Chevy Engine Guide, Hot Rod High Performance Series Vol. 4 Number 3 (1987): 59.

2 Heinz Heisler, Advanced EngineTechnology, (Warrendale: SAE International, 1995) 233

3 Yunick 71.

4 Yunick 68.

5 Brodix Catalog, May 1998, 44. Used without permission.

6 Andrew Starr, “Field Evaluation Report,” December 2002.

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