The 7.3L Power Stroke community has been debating the 6637 filter modification for decades. It’s an inexpensive, easy DIY intake that a lot of people swear by—but we’ve long suspected it does more harm than good. So, we decided to put it to the test on our dyno and settle the argument with hard data.
Before testing the 6637 filter, we first checked out another popular mod known as the “Zoodad mod.” This involves cutting a hole in the core support directly in front of the factory airbox inlet to give the engine a path to cooler outside air. On the dyno, we didn’t see any significant horsepower gains, but the mod made a huge difference in intake air temperatures (IATs). Charlie noted that the IATs “stayed cooler, they started cooler, and they ended cooler” compared to baseline runs. Because of this clear cooling benefit—especially for towing—we gave the Zoodad mod our KC Turbos approval.
Next, we swapped out the enclosed S&B intake and installed the popular 6637 open-element filter. The results were immediate—and not in a good way. The intake air temperatures were higher before the dyno pull even started, and the first run showed a noticeable loss in power and torque across the curve.“We lost 12 horsepower,” Charlie said, adding, “peak torque went down 38.” A second, more heat-soaked run showed even further losses.Even more concerning were the exhaust gas temperatures (EGTs), which spiked by around 200 degrees—a huge increase that can be dangerous under load. Charlie declared the myth of the 6637 as a performance upgrade “totally busted.”
This is exactly why we’ve always been cautious about this filter. Charlie explained, “It’s one of the first questions we ask: are you running a 6637 filter? I can’t tell you how many customers we’ve had where it fixed their truck.” We’ve traced problems like high EGTs, low power, excessive smoke, and even collapsed intake boots directly back to this filter. In reality, the 6637 performs no better than an old, dirty stock filter because it’s a hot-air intake—pulling superheated air from the engine bay.
The bottom line? On any 7.3L Power Stroke, pulling cool air from outside the engine bay is critical for safe, reliable power. Even if some 6637 filters flow well, they’ll always be a hot-air intake, and the data proves it.
Videos like this one from KC Turbos give the community real-world, data-driven insight, helping owners understand why their trucks may be underperforming and finally putting old myths to rest.