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Whether you have a stock, street, or strip LS application, the intake manifold is one of the three major players in terms of power production. The aftermarket has produced intake combinations for performance LS3 and LS7 applications. Intake designs do more than just allow airflow into the ports; they actually provide a tuning effect that aids in power production over a given RPM range. Not surprisingly, factory LS3 or LS7 intake manifolds were designed with a combination of peak and average power combined with ease of production and even fuel mileage.
The right intake can help you produce impressive power, especially when used in conjunction with the right cam and ported cylinder heads. More than any other single component, the intake manifold (most specifically the runner length) determines where the engine makes effective power. Match the runner length to produce power in the same operating range as the cam profile and you are a long way toward making an impressive LS combination.
For any engine (including LS3 and LS7), intake manifold design may be broken down into three major elements: runner length, cross section (and taper ratio), and plenum volume. These elements are listed in the order they most affect the performance of a given manifold. By this I mean that changing the runner length has somewhat more of an effect than altering the cross section or plenum volume. This is not to say that all of the elements are not important, it is just that proper care should be given to the elements in accordance with their eventual effect on performance. Take note, intake designers often spend countless hours altering the plenum volume in an attempt to change the effective operating range when they should have simply increased (or decreased) the runner length. Also, manifold design is sometimes limited by production capability or rather ease of construction. Building a set of runners with a dedicated taper ratio and a compound curve is difficult, if not impossible, for the average fabricator. Despite the fact that this design produces the best power, it simply isn’t going to get produced unless a major intake manufacturer (like FAST, Holley, or Edelbrock) steps up to the cost of such a complex combination.
Fabricated, short-runner intakes such as this unit from Speedmaster are popular among LS enthusiasts, but know that the design lends itself to power production higher in the rev range than the stock (long-runner) LS3 or LS7 design.
The first element in intake design is the runner length. The overall intake runner length actually includes the head ports, but the discussion will be limited to those in the manifold. Fuel-injected intake manifolds seem to be broken down into two distinct groups, long and short. Obviously not very scientific, the terms “long” and “short” do not properly describe intake manifolds. The reason for the long and short designations is that, generally speaking, the longer the runner length, the lower the effective operating rpm. Obviously the opposite is also true because shorter runner lengths improve top-end power. Production LS intake manifolds are typically of the long-runner design to help promote torque production. It is possible to design an intake that offers more low-speed or top-end power than the stock LS3 intake, but doing both has proven to be difficult. It should be pointed out that the “ideal” intake design varies with engine configuration as well because the power gains offered by a given design on a stock engine are most likely different on a wilder combination. This is why FAST designed its adjustable LS3 intake manifold to allow adjustment for individual combinations. Since the reflected wave is determined by the cam timing, its initiation point changes with different cam profiles. Thus, changing the cam timing may well require a different intake design.
The next element in intake design is cross section, or port volume. A related issue is taper ratio, but I will cover that shortly. The port volume or cross section of the runner refers to the physical size of the flow orifice. Suppose you have an intake manifold that features 17-inch (long) runners that measure 2.00 inches in (inside) diameter. It is possible to improve the flow rate of the runners by increasing the cross-sectional area. Suppose you replace the 2.00-inch runners with equally long 2.25-inch runners. The larger 2.25-inch runners flow a great deal more than the smaller 2.00-inch runners, thus improving the power potential of the engine. From a reflected wave standpoint, the increase in cross section has no effect on the supercharging effect, but it alters the Inertial Ram and Helmholtz resonance.
For the ultimate in LS3/LS7 induction systems, look no further than an individual-runner intake system.
Related to the cross section, taper ratio refers to the change in cross section over the length of the runner. Typically, intake manifolds feature decreasing cross sections, where the runner size decreases from the plenum to the cylinder head. The decrease in cross section helps to accelerate the airflow, thus improving cylinder filing, but the real difference is the effective change in cross section brought about by the taper.
The final element of an LS intake manifold is plenum volume. This refers to the size of the enclosure connecting the throttle body to the runners. Typically the plenum volume is a function of the displacement of the engine. Most production intake manifold applications feature plenum volumes that measure smaller than the displacement of the engine (somewhere near 70 percent), but this depends on the intended application. A number of manufacturers have recognized the importance of the plenum volume and incorporated devices to alter the plenum volume to enhance the power curve, but the LS3 and LS7 manifolds rely on a fixed volume.
Contrary to popular opinion, increasing the plenum volume does not increase the air reservoir allotted to the engine as much as it affects the resonance wave. When excited, the area in the plenum resonates at a certain frequency. Changing the plenum volume changes the resonance frequency. The Helmholtz resonance wave aids airflow through the runner (acoustical supercharging). Where this assistance takes place in the RPM band is determined by a number of things but primarily by the plenum volume. The air intake length, inside diameter, and a portion of the cylinder (when the valve is open) are also used to calculate the Helmholtz resonance frequency (and why air intake length and diameter have a tuning effect on the power curve).
LS applications also run very well with carbureted intake systems such as this dual-quad Holley Hi-Ram.
Test 1: Holley Single- vs Dual-Plane Intake on an LS3
When it comes to carbureted engines (including LS), the choice basically comes down to single- or dual-plane. That particular induction argument predates the LS engine family by multiple generations, but carbureted LS owners must ultimately choose. We all know that the LS was originally equipped with factory fuel injection, but MSD made the carb conversion ultra simple. Carb swappers were soon faced with the same induction question that plagued previous small-block Chevy owners. Choosing the proper intake design is critical for maximum performance, but just what defines the term maximum?
In most cases, it doesn’t mean peak power, but rather maximized power through the entire rev range. Now throw in things like drivability, fuel mileage, and even torque converter compatibility, and you start to understand the dilemma. You see, despite similar peak power numbers, the two Holley (carbureted) LS intakes tested here offered decidedly different power curves (and likely street manners). We all like to brag about peak power numbers, but the reality is that the vast majority of carbureted LS engines spend most of
