Many engine builders say that individual-runner intake manifolds are more forgiving of long-duration camshafts than a common-plenum intake, and the reason for this is very simple. With an IR induction system, you have a column of air that is always available to each cylinder. In most cases, this volume of available air is considerably more than most street engines will need or even be capable of using. The upside is that the air dedicated to each cylinder doesn’t have to be shared with the other cylinders, so you don’t have to be concerned with the pulses or draw from the other seven cylinders affecting the airflow within each of the eight plenums. From a camshaft signaling standpoint, long-duration cams tend to have more overlap within their design. Overlap is a major contributor to the reversion pulses seen in common-plenum manifolds and can—in some instances—play a diminutive role in vacuum signal. Consequentially, the reversion pulses from one cylinder can compromise the airflow of an adjacent cylinder, reducing low-speed torque. Since the airflow into each cylinder in an individual-runner system is isolated from that of the adjacent cylinders, it gives you the ability to tune each cylinder individually. This makes the entire engine combination more efficient because it’s now working more like eight single-cylinder engines rather than eight cylinders working together.
In a traditional common-plenum intake manifold, the intake charge must sharply change directions several times while traveling from the plenum to the intake runner, and then into the cylinder heads. On the other hand, an individual-runner intake provides a much straighter flow path for the induction charge, which can have a substantial effect and airflow and performance. In a typical four-barrel–type application, you’re using the closing side of the exhaust valve in conjunction with the opening of the intake valve to draw air into the combustion chamber. With an IR system, there is a continual column of air there just waiting for the intake valve to open. This allows for an incredible amount of throttle response and torque when coupled with the correct cam profile.
Most EFI motors inject fuel directly ahead of the intake valve. Conversely, an individual-runner intake—whether it’s carbureted or fuel injected—atomizes fuel at the very top of the intake runner. The biggest, most evident benefit we found with this arrangement is the ability to develop a point of entry for the fuel that complements the torque, horsepower, and driveability of our target application. When we decided to design our own throttle bodies for the Inglése systems, we took into account their intended usage as a whole. Many, many hours and thousands of dollars were spent modeling systems with various injector locations to determine the best overall placement. If you measure the entire intake tract from the point of entry to the intake valve, you’ll find our injector placement—and angle of fuel entry—optimal for most street applications. Knowing things like spray patterns, fuel pressure, and rate of atomization relative to air speed all play important roles in knowing how things work and react within the available air tract. Most important is the condition of the fuel and air mixture when it arrives at the intake valve. While you don’t have a lot of velocity within the column of air, you can create your own velocity based on how you introduce the fuel into the intake charge as the air travels toward the intake valve. Optimizing injector placement and increasing charge velocity has allowed for huge performance improvements over previous IR system designs.