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 stroker Hemi combination equipped with TEA-ported 5.7L heads, a Comp XFI 260H cam and internals from JE, Scat, and Speedmaster, the long-runner truck intake easily outperformed the short-runner single-plane intake up to 5,800 rpm. For 5,800 rpm on up, the single-plane intake was tuned to optimize power production and offered 8 more peak horsepower, but the difference grew to 25 hp at 6,300 rpm. Intake manifolds are optimized for power production in specific rpm ranges, a point illustrated perfectly by this, and every other test in this chapter.
Despite being designed for low-RPM truck applications, this factory intake offered surprising power production.
The single-plane design was designed for high-RPM power and was a tad out of place on this mild 5.7L stroker.
Truck vs MP Single Plane for a 5.7L Stroker
Stock Truck Intake: 436 hp @ 5,700 rpm
MP Single Plane: 444 hp @ 6,300 rpm
Largest Gain: 25 hp @ 6,300 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 long-runner truck intake. Measured peak to peak, the single-plane, carbureted intake produced 444 hp to the 436 hp offered by the truck intake. The difference was a healthy 25 hp out at 6,300 rpm, where the power output of the stock intake fell off rapidly.
Truck vs MP Single Plane for a 5.7L Stroker
Stock Truck Intake: 440 ft-lbs @ 4,800 rpm
MP Single Plane: 410 ft-lbs @ 5,000 rpm
Largest Gain: 40 ft-lbs @ 3,500 rpm