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 their time well below the power peak. In fact, street engines spend most of their time well under the torque peak and even during hard acceleration, the engine operates primarily between peak torque and peak power.
Holley’s single-plane intake was designed to optimize power production higher in the rev range than the dual-plane. Just make sure to apply it to the proper combination that can take full advantage of the top-end power production.
The choice ultimately comes down to where you value power production. For those new to LS performance (though this applies to every type of V-8 regardless of generation or manufacturer), the intake debate between single- and dual-plane manifolds is a simple matter of operating (engine) speed. The dual-plane was designed to enhance power production lower in the rev range than the single-plane. This simple fact makes the dual-plane ideal for the vast majority of street applications.
Run on the LS3 test engine (with mild Comp cam), the Holley dual-plane produced peak numbers of 544 hp at 6,900 rpm and 471 ft-lbs of torque at 4,300 rpm. After installation of the single-plane intake, the peak numbers changed very little to 552 hp at 7,000 rpm and 463 ft-lbs at 5,200 rpm. Despite minor changes in peak power, the power curves were decidedly different. Check out the curves and decide where you want your LS3 power production.
Most street and street/strip (carbureted) LS3 applications prefer the dual-plane design because of improved throttle response at lower RPM. The dual-plane was designed to maximize low- and mid-range torque production where it can be enjoyed most often.
Holley Single- vs Dual-Plane Intake on an LS3 (Horsepower)
Holley Dual-Plane: 544 hp @ 6,900 rpm
Holley Single-Plane: 552 hp @ 7,000 rpm
Largest Gain: 14 hp @ 7,200 rpm
The horsepower curves show a number of things, including the fact that the single-plane intake did indeed make more peak power than the dual-plane design, but not by much. Starting at 5,000 rpm, the single-plane pulled ahead, but the power difference was minimal. Run out to 7,200 rpm, the single-plane showed its worth by besting the dual-plane by 14 hp.
Holley