Runner length is a key design element of the intake manifold. What if you could adjust the runner length to suit your combination?

Photos and Words By: Richard Holdener

One of the key design elements in any intake manifold is runner length. How is the runner length tuned to actually increase airflow to the motor, you ask? In addition to providing a simple airflow path to the combustion chamber, the intake runner (combined with the head port) provides additional charge filling through three different methods.

The inertial wave cylinder filling happens when the downward movement of the piston (and open intake valve) creates a vacuum, initiating movement of the air column. This air column continues toward the combustion chamber, even past the point of intake valve closure. The inertia of the air column continues the movement of the air stream against the closed intake valve. This creates a build-up of pressure against the closed valve. If the build-up of pressure is present when the valve opens, additional cylinder filling will be realized.

The reflected wave provides additional cylinder filing as well, but by a different means. The reflected wave happens when the intake valve opens to create a negative pressure wave. This negative pressure wave travels out away from the valve until finally arriving at the common plenum. The negative pressure wave expands out into the open plenum (rarefaction). The expansion of the negative pressure wave creates a low-pressure area, which is quickly filled. The filling of the void creates a positive pressure wave that then travels back down the intake port and into the combustion chamber, improving cylinder filling.

While both inertia and reflected wave ram tuning can improve cylinder filling, the key is timing. The pressure waves must arrive at the proper time to maximize cylinder filling. Obviously, they must also not overlap or work against each other (where a positive pressure wave is effectively cancelled out by a negative one). The runner length (and diameter) help determine when in the rpm range these forms of cylinder filling become effective. Since the pressure waves run through the intake runners (and head port) at the speed of sound, altering the distance changes the travel time through the port. If the runner length is properly tuned, the motor will realize not only the airflow drawn in by way of the vacuum created by the downward moving piston, but also the mild supercharging effect that comes from the inertial and reflected wave tuning.

A third form of wave tuning is called Helmoltz resonance. This deals with the resonance of the common plenum and tuned length of inlet tubing (from the air filter to the intake manifold). Once excited, the common plenum resonates at a given frequency. These resonance waves can be used to bombard the intake valve with additional pressure, once again assuming the tuned frequency coincides with the effective operating speed of the motor.

While we have simply scratched the surface of the intake manifold, it should be clear that the proper design is much more complicated than simply building runners connected to a common plenum (or plenums) fed by a throttle body. Obviously, computer simulations can be used to design an intake manifold for a given application, but given the complex nature and sheer number of pressure waves involved (most intake modeling is done with a single cylinder motor), the final testing almost always involves actual dyno testing.

Though it is possible to determine (or design) the effective operating range of the intake manifold runner length, diameter, and plenum, what effect does the design have at other engine speeds? Is the area under the curve sufficient? Are there any undesirable dips or peak? These are questions that only the dyno can answer effectively. Thus, a proper intake design will likely see plenty of dyno time before being finalized.

Obviously, it would be nice if the same intake fit properly, bolted up to all the original hardware, and looked like a proper performance piece, but for now, let’s first take a look strictly at the effect on power production.

The best way to illustrate the effect of changes in runner length is to make an adjustable intake. The author did just that for a number of applications. This test involved one such system designed for the 5.4L, four-valve Modular Ford motor. In addition to adjustable runner length, the intake also featured dual plenums, dual throttle bodies, and even variable connections between the two plenums (which affect Helmholtz resonance). We will stick to the changes in runner length for this discussion, but know that major shifts in the power curves are available with changes in the plenum connections.

The test motor started out life as a 5.4L, four-valve Navigator, but was modified by Sean Hyland Motorsports for performance use with forged internals, Stage 2 cam, and ported heads. This combination easily exceeded 1,000 hp with a pair of small turbos, but for this test, we ran the combination sans boost. The 5.4L was run on the engine dyno using a FAST XFI management system controlling Holley 50-pound injectors.

The custom variable-runner, dual-plenum intake was welded together using a pair of 4-inch plenums fed by dual, 75-mm Accufab throttle bodies (originally designed for a 5.0L Ford). (photo 8) Eagle-eyed readers will no doubt recognize the four injector bungs, located on top of each plenum, designed to top-feed the runners. These were not used during this testing, but were part of the massive adjustability of the design. The plenums featured radiused air entries feeding 2-inch diameter runners. The runner length was adjustable using slip-fit aluminum tubing (duct taped to eliminate vacuum leaks). Adding runner length was as simple as installing longer or additional sections of tubing. The design of the intake allowed us to adjust and test runner lengths ranging from 11 inches to 20 inches. Naturally, the difference in runner length made a sizable change in the overall power curve.

For reference, know that shorter runners tend to optimize power production higher in the rev range than longer ones, but there is almost always a trade off at the opposite end of the rpm scale.

To illustrate overall efficiency of the dual-plenum design, we also tested it against one of the only aftermarket performance intakes available for the 5.4L, a single-plane intake offered by Sullivan Racing.

To start the test, we set up the variable runner intake with the full 20 inches of runner length. The long runners greatly enhanced low-speed and mid-range torque production. Equipped with the 20-inch runners, the 5.4L produced 430 hp at 5,800 rpm and 442 lb-ft of torque at 4,100 rpm. Stepping down in length to 16 inches shifted the peak power up to 448 hp at 5,900 rpm, but pushed torque down to 439 lb-ft at 4,400 rpm. This same trend continued once we installed 14-inch runners, as the 5.4L produced 456 hp at 6,400 rpm and 433 lb-ft at 4,300 rpm. The final runner section measured 11 inches, and the peak numbers checked in at 464 hp at 6,500 rpm, but torque was down to 430 lb-ft at 4,500rpm.

As is evident by the numbers (and supplied graph), each successive reduction in runner length realized a higher peak power but a lower peak-torque value. The peak numbers certainly changed, but so too did the shape of the entire curves (see graphs). Remember, man does not live by peak numbers alone.

As if testing the different runner lengths wasn’t exciting enough, we decided to see how the custom intake compared to a known performance manifold. The single-plane intake has long been a favorite among performance enthusiasts, so it was no surprise when Sullivan Racing offered the design to Modular Ford enthusiasts. (photo 13) There are a lot of fast mod motors out there sporting the design, so we figured it was a solid bench mark to compare the adjustable manifold to.

The supplied graph compares the 11-inch runner length to the single-plane design with impressive results. The dual-plenum intake matched the peak power output of the high-rpm, single-plane design, but managed to greatly enhance torque production through the rest of the curve. Gains were as high as 40 lb-ft down near 4,500 rpm, but the consistent gains showed what happens when you can give your mod motor an Attitude Adjustment and dial in the runner length for your combination.

Sources: Accufab, accufabracing.com; FAST, fuelairspark.com; Holley/Hooker/Weiand, holley.com; JE Pistons, jepistons.com; SCAT, scatcranks.com; Sean Hyland Motorsports, seanhylandmotosports.com.