7.3 Power Stroke 6637 Filter Mod (BHAF): More Airflow, More Filter Area, Simple Install

Jan 20th 2026

The 6637 filter modification (often called the BHAF – Big Honkin’ Air Filter) is the classic 7.3 Power Stroke intake upgrade: replace the restrictive stock airbox with a large, heavy-duty style cylindrical filter. It's one of the more common filter styles in existence, with the majority of applications in the heavy-duty world (think semi-trucks and agricultural equipment). Because of the large application list the 6637 has, just about everyone has copied it in one way or another. WIX, NAPA, Donaldson, Fram, AFE, everyone has their version, including the best of them all, the Riffraff Diesel version (we're not biased at all...).

Why people do the 6637 mod

1. Less restriction

WIX lists their 46637 at 425 CFM, which is plenty of airflow capacity for a stock or mildly upgraded 7.3, and is a big reason it’s popular.

2. More filtration media (more dirt-catching area).

The BHAF-style filters are physically huge compared to typical panel filters; they have roughly 3x more surface area than a stock panel filter, and the size helps keep airflow high, whether slightly dirty or perfectly clean.

3. Heavy-duty roots.

As we said earlier, the 6637 was originally and most commonly sold as an AG/HD truck air filter, which is basically the point: big filter, built for work.

The clean way to do it: Riffraff Diesel’s 6637 kit

If you want the filter to look and fit like it belongs there, Riffraff Diesel’s 6637 Filter Kit bundles the filter, adapter tube, T-bolt clamp, and optional pre-filter—plus it calls out the correct tube style: 45° for 94–early ’99, straight for late ’99–’03. Add the Donaldson 6637 Mounting Bracket for an even cleaner install.

Install notes that actually save time

Riffraff’s instructions include the real-world stuff forums forget (link to the instructions is HERE):

Some trucks have a fusebox under the airbox; you may need to trim mounting tabs or “nestle” the new filter between them for hood clearance and stability.
Clean the oily residue in the intake pieces while you’re in there.
Adapter tube insertion depth is about 2.5 inches, and the “plastic bag trick” helps slide the cover on without snagging.
Tighten everything like you mean it—loose joints can let the turbo ingest unfiltered air and cause damage.