you have a stock, street, or strip Hemi application, the intake manifold is one of the three major players in terms of power production. Unlike the LS, the aftermarket has not stepped up with an abundance of aftermarket Hemi intakes, although we did manage to test just about everything available. Contrary to popular belief, intake designs do more than just allow airflow into the ports, they actually provide a tuning effect that aides in power production over a given RPM range. Not surprisingly, factory Hemi intake manifolds for the truck, Magnum, and SRT versions were not designed with peak power production in mind, but rather a combination of peak and average power combined with ease of production and even fuel mileage. Just as a poorly designed manifold can (literally) ruin an otherwise good Hemi, 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’s runner length will determine 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 a powerful Hemi.
All of the factory Mopar intakes perform very well, including the aluminum SRT8 manifold.
For 5.7L, 6.1L, and 6.4L Hemis and any other engine, intake manifold design may be broken down into three major elements, runner length cross section as well as taper ratio and plenum volume. These elements are listed in order of importance or, more specifically, in the order they most affect the performance of a given manifold. By this we 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. Intake designers, take note of this perspective, because fabricators 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. It should be stated here that 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, such as FAST, Holley, or Edelbrock, steps up to the cost of such a complex combination.
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. Unlike their carbureted counterparts, fuel-injected Hemi intake manifolds seem to be broken down into two distinct groups, long and short. It’s obviously not very scientific, but 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; shorter runner lengths improves top-end power. Production Hemi intake manifolds typically have a long-runner design to help promote torque production even though the SRT8 intake differs in length compared to the Ram truck or Magnum. It is often possible to design an intake that offers more low-speed or top-end power than a stock Hemi intake, but doing both can be proven difficult. The SRT8 intake was designed to produce power at a higher engine speed than the Magnum or Ram truck, but the trade-off is a loss lower in the rev range (also see Test 8 in this chapter where the author adjusted the runner length).
The next element in intake design is cross section or port volume. A related issue is taper ratio, but we 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 has 17-inch-long runners and measures 2.00 inches in inside diameter. It is possible to improve the flow rate of the runners by increasing the cross-sectional area. Suppose we replace the 2.00-inch runners with equally long 2.25-inch runners. Keep in mind, the SRT8 head ports and intake runners are larger than 5.7L truck or Magnum runners. Accordingly, the larger 2.25-inch runners would flow a great deal more than the smaller 2.00-inch runners, thus improving the power potential of our engine. From a reflected wave standpoint, the increase in cross section will have no effect on the supercharging effect, but it will alter the Inertial Ram and Helmholtz resonance. 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.
Plenum volume is the final element of any Hemi intake manifold. Plenum volume refers to the size of the enclosure connecting the throttle body to the runners. The plenum volume is typically 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 early Magnum, SRT8, and truck intake are all fixed volumes. The 2009-up truck intakes and 6.4L SRT manifolds offered dual runner lengths. Contrary to popular opinion, increasing the plenum volume does not increase the air reservoir allotted to the engine as much as it does affect 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). A number of things, but primarily the plenum volume, determine where this assistance takes place in the RPM band. 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.
The single-plane intake from Mopar Performance likes to rev, but it does drop torque production lower in the rev range.
Test 1: Truck vs Mopar Perfomance Single-Plane Intake for a 5.7L Stroker
When it comes to Hemi applications, both carbureted and EFI intakes are available. For carbureted applications, one of the common intake designs is the single plane. That particular induction system predates the modern Hemi engine family by multiple generations, but carbureted Hemi applications are becoming more commonplace. We all know that the Hemi was originally equipped with factory fuel injection, but MSD and others made the carb conversion ultra simple. In truth, the single-plane intake tested here may also be run in EFI configuration, but this test was run with a simple Holley carb. The real choice here has less to do with carburetion versus injection, and more to do with intake design. 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 entire rev range, where acceleration actually takes place. Now factor in drivability, fuel mileage, and even torque converter compatibility and you start to understand the dilemma.
In this test between the factory 5.7L Hemi truck intake and a single-plane MP intake, we saw peak power numbers that differed by a scant 8 horsepower, but the power curves couldn’t be more different. We all like to brag about the peak power numbers, but the reality is that the vast majority of Hemi engines for street and strip spend most of their time well below the power peak. In fact, the vast majority of driving is spent well below the torque peak and even during hard acceleration, the engine spends most of its time accelerating between peak torque and peak power. Tested on our 370-ci
