The real restriction in these 6.7L Cummins intake elbows is not just inlet size. It is the whole path from the hose connection to the outlet, especially the outlet shape and how the casting handles the turn into the intake. The stock elbow is heavily restricted, and the BD 4-inch elbow still gives up flow because the wall thickness, outlet shape, and rectangular cross-section work against it. We opened up our Monster-Ram from the inlet forward, including CNC work at the 4-inch entrance, and carried that improvement through the outlet. Tested at the same pressure drop, that is why the stock piece came in at 432 CFM, the BD at 697 CFM, and our Monster-Ram higher still. We also kept the practical details in view with provisions for sensors, dipstick mounting, and water-meth placement that targets the air stream instead of spraying a wall. Better shape, less restriction, and cleaner packaging are what move more air.
This segment of the Banks Power Elbow shootout focuses on the BD 4-inch power elbow and compares it against the stock 6.7L intake elbow assembly and Banks Monster-Ram alternatives. All parts are tested on a flow bench at the same pressure drop: 10 inches of water, or roughly one-third of a PSI through the casting. That common test condition is important because it allows direct comparison of airflow performance between designs.
The host emphasizes airflow in both CFM and pounds of air per minute because air mass is what ultimately supports horsepower. In his explanation, horsepower comes from combining pounds of air with pounds of fuel, so a higher air-mass flow rate means greater power potential. In prior testing, the Banks Monster-Ram 3.5-inch version flowed 906 CFM, equivalent to 69.22 pounds of air per minute, while the Banks Monster-Ram 4-inch version reached 1,008 CFM and 77.0 pounds of air per minute.
On this test, the BD 4-inch power elbow measured 697 CFM, which the presenter converts to 53.33 pounds of air per minute. That places it well above the stock intake elbow but substantially below both Banks Monster-Ram versions. Compared with the Banks 4-inch unit, the difference is about 300 CFM, a gap the presenter attributes not to inlet diameter alone but to the internal geometry from the hose connection to the outlet.
The comparison is framed as a casting and transition-shape problem rather than a simple diameter contest. Although the BD part uses a 4-inch inlet, the measured result shows that inlet size by itself does not guarantee high flow if the internal path and outlet remain restrictive.
The stock 6.7L intake elbow assembly establishes the lower end of the range. Tested with the throttle held wide open and without the intake grid heater assembly underneath, it flowed 432 CFM, or about 33.0 pounds of air per minute. The presenter notes that this is effectively a race-oriented comparison because the grid heater, which would normally be present, also restricts flow. In other words, the stock number shown here is already somewhat favorable compared with a full street configuration.
The stock assembly includes the throttle blade commonly found on highly EGR-equipped diesel applications. It also carries emissions-related hardware on top, along with provisions for the T-MAP or MAP sensor depending on model year, separate intake temperature sensing on earlier versions, wire-loom mounting points, and a dipstick mounting location. Looking at the gasket opening, the presenter points out a large opportunity for improvement in inlet area and overall passage shape.
The host briefly addresses the common idea of replacing the stock elbow with a billet piece that deletes the throttle blade. He agrees that removing the throttle blade would likely recover a few CFM, but he does not believe the blade is the primary restriction. His view is that the real choke point is farther downstream at the outlet of the stock elbow.
If that assessment is correct, deleting the butterfly would have only a minimal effect because the outlet geometry would still dominate the pressure loss. He notes that a flow bench is the proper way to confirm that theory, but based on the shape of the stock casting, he believes the outlet is more restrictive than the throttle area itself.
Beyond raw airflow, the BD elbow is criticized for missing several stock-style provisions. The stock mounting points used for wiring and related hardware do not appear to be retained on the BD casting. There is also no evident provision for the temperature sensor used on earlier 6.7L applications, specifically 2007.5 through 2012 trucks. The presenter points to a plugged hole and says it is unclear where BD expects that sensor to be installed on those earlier engines.
He also notes that the BD part does not provide a place to mount the dipstick. These omissions matter because they affect installation completeness and compatibility, especially for earlier trucks or for users expecting the replacement elbow to preserve factory attachment points and sensor locations.
Although the BD elbow has a 4-inch inlet, the presenter says the wall thickness at the entrance is excessive. Banks addressed this issue on its own 4-inch version by CNC-machining the inlet, a change he says was worth 40 CFM by itself. That comment is used to illustrate how much flow can be lost even before air reaches the main body of the elbow.
However, he identifies the BD outlet as the larger problem. The passage transitions from round to an almost purely rectangular cross-section, and he argues that sharp-cornered rectangular geometry does not flow well. Even if the final outlet shape is rectangular, he says that is not the best way to make the transition. He also points out that the outlet appears heavily restricted despite post-casting cleanup with a ball end mill. In his view, that machining gives the impression of a larger passage but does little to improve actual flow.
The video also compares provisions for water-methanol injection. The presenter says this is an important feature because Banks offers its own system, Straight Shot, and nozzle targeting matters. The goal is to avoid spraying directly onto the opposite wall or onto the top or bottom of the elbow, which would reduce mixture quality and cooling effectiveness.
On the Banks design, the bungs are placed so the spray enters a larger cross-section of diffused airflow, promoting better mixing of water-meth and maximizing cooling effect. By contrast, the BD elbow includes two bungs that could be used for water-meth injection or a boost gauge, but their orientation appears to direct the spray into the opposite wall. The criticism is not that the bungs are absent, but that their placement is less effective for proper atomization and distribution.
By the end of the segment, the tested range is clearly bracketed. The stock 6.7L elbow flowed 432 CFM. The BD 4-inch elbow improved that to 697 CFM. The Banks Monster-Ram 3.5-inch version reached 906 CFM, and the Banks Monster-Ram 4-inch version achieved 1,008 CFM. The presenter highlights that the Banks 3.5-inch unit more than doubles the stock airflow, while the 4-inch version extends the lead even further.
He adds that Banks did not include a notch because the company uses a new fuel line, eliminating the need for that feature. Pricing is given as $339 for the natural-finish 3.5-inch Monster-Ram and $369 for the natural-finish 4-inch version. Viewers are invited to compare those prices against the BD part on their own. The segment closes with the conclusion that the major differences are driven by engineering of the internal path, especially from the hose joint to the outlet, rather than by nominal inlet diameter alone.