​The Ultimate Guide to the 6637 Air Filter: Performance, Protection, and Value​

The 6637 air filter is a high-performance, direct replacement air filter designed primarily for Ford Power Stroke 7.3L diesel engines, renowned for its exceptional filtration efficiency, increased airflow, and long service life. Offering a superior alternative to the stock factory air box, this filter provides a significant upgrade for truck owners seeking to improve engine performance and protection without the high cost of a complete cold air intake system. Its simple design, centered on a massive, pleated paper filter element, delivers proven results in dust filtration and horsepower gains, making it one of the most practical and effective modifications for the 7.3L Power Stroke community.

​What is a 6637 Air Filter?​​

The 6637 is not a model number from a major brand like K&N or S&B; rather, it is a widely recognized industrial part number, specifically from the Donaldson Company, a global leader in filtration systems. The full part number is typically ​Donaldson P6637. This filter was originally designed for industrial and heavy-duty applications but was discovered by the automotive aftermarket community to be an almost perfect fit for the 7.3L Power Stroke engine with a simple, inexpensive adapter. The "6637" designation has since become the generic name for this style of filter, and several companies now manufacture their own versions, often with minor improvements or different color housings, but all following the same core design principles. It is characterized by its large cylindrical shape, measuring approximately 6.75 inches in diameter and 7 inches in length, and features extensive pleating that provides a substantial surface area for air to pass through.

​Primary Applications: Which Vehicles Use the 6637 Filter?​​

The 6637 air filter is most famously associated with Ford trucks equipped with the 7.3L Power Stroke diesel engine. This includes:

• ​Ford F-Series Super Duty:​​ Models from 1994 (the introduction of the Power Stroke) through 2003, including the F-250, F-350, and F-450.
• ​Ford E-Series Vans:​​ E-350 and E-450 vans with the 7.3L Power Stroke engine.
• ​Ford Excursion:​​ The SUV version, from 2000 to 2003, which used the same powertrain.

It is crucial to confirm your engine type. The 6637 is specifically for the 7.3L Power Stroke, which is a direct injection, turbocharged diesel engine. It is not designed for the 7.3L IDI (Indirect Injection) engine that preceded it, nor for the 6.0L or 6.4L Power Stroke engines that followed. While its popularity is rooted in the Ford community, the filter's versatility means it can be adapted to other vehicles with custom intake setups, but its primary and most documented use remains with the 7.3L Power Stroke.

​Key Benefits of Upgrading to a 6637 Air Filter​

Truck owners choose the 6637 filter over the stock system for several compelling reasons that directly impact performance, engine health, and cost of ownership.

​1. Superior Filtration Efficiency​
The primary job of any air filter is to protect the engine from harmful contaminants like dust, dirt, and soot. The 6637 filter excels in this area. Donaldson's ​Advanced Pleat Technology​ and high-quality filter media are engineered to capture a very high percentage of particulate matter. Independent tests and widespread user reports consistently show that the 6637 provides excellent filtration, often outperforming the stock panel filter. For a diesel engine whose turbocharger and cylinders are highly sensitive to abrasive particles, this level of protection is paramount for long-term reliability.

​2. Significantly Increased Airflow​
The stock air intake system on the 7.3L Power Stroke is often criticized for being restrictive. The factory air box and small panel filter can limit the amount of air the engine can breathe, especially under heavy load or when performance modifications have been made. The 6637 filter solves this problem. Its large physical size and the immense surface area created by its deep pleats allow for a much greater volume of air to flow into the engine with less restriction. This reduced restriction is often measured as a lower pressure drop across the filter.

​3. Noticeable Performance Improvements​
The increased airflow translates directly into tangible performance gains. Owners typically report a more responsive throttle, a reduction in turbo lag, and a slight increase in horsepower and torque. The engine can breathe easier, which allows it to run more efficiently. The improved airflow is particularly beneficial for trucks with other modifications, such as upgraded turbochargers, tuners, or exhaust systems, as these modifications increase the engine's demand for air. The 6637 filter effectively "unleashes" the potential of these other upgrades by ensuring the engine is not starved for air.

​4. Long Service Life and Cost-Effectiveness​
A standard paper filter in the stock air box may need replacement every 15,000 to 30,000 miles, depending on driving conditions. The 6637 filter, due to its larger size and greater capacity for holding dirt, can often last between 50,000 and 100,000 miles before requiring service. This extended service interval means fewer purchases and less maintenance over the life of the truck. Furthermore, the initial cost of a 6637 filter and the necessary adapter is significantly lower than a branded cold air intake kit, while providing similar, if not better, filtration performance. This combination of low initial cost and long service life represents exceptional value.

​5. Simple and Reliable Design​
The 6637 setup is remarkably simple. It eliminates the complex and restrictive stock air box, replacing it with a single, robust filter element. There are no complex housings, multiple clamps, or fragile components. This simplicity translates to reliability. There are fewer parts that can fail or develop leaks. The installation process is straightforward, making it an accessible project for DIY enthusiasts.

​Potential Drawbacks and Considerations​

While the benefits are significant, it is important to consider the potential drawbacks to make an informed decision.

​1. Exposure to Engine Bay Heat​
A common concern with any "open" air filter element is the potential to draw in hot air from the engine bay, known as heat soak. Since the 6637 filter is not enclosed in a sealed box that draws air from outside the engine compartment, there is a possibility that under-hood temperatures could rise, especially during slow-speed or idle conditions. Hotter air is less dense, which can marginally reduce performance. However, most users find that while moving, there is sufficient airflow through the engine bay to mitigate this issue. Many owners construct simple heat shields or "isolation boxes" to direct cooler air from the fender well directly to the filter, optimizing intake air temperatures.

​2. Maintenance and Cleaning​
Unlike a disposable paper filter, the 6637 is a serviceable element. When it becomes dirty, it must be cleaned and re-oiled, not simply thrown away. This requires a specific cleaning kit and a little more time and effort than a standard filter replacement. Improper cleaning or over-oiling can harm the filter's performance or, in worst-case scenarios, damage the Mass Air Flow (MAF) sensor. Following the manufacturer's cleaning instructions precisely is critical.

​3. Fitment and Adapter Requirements​
The 6637 filter does not bolt directly onto the factory air intake tube. It requires an adapter, typically a simple silicone or metal coupler that connects the filter to the turbo inlet pipe. While these adapters are inexpensive and readily available, it is an additional component to source. Ensuring a tight, secure connection with proper clamps is essential to prevent unfiltered air from entering the engine.

​Comparing the 6637 to Other Air Intake Options​

When considering an air intake upgrade for a 7.3L Power Stroke, the 6637 filter exists in a category between the stock system and full cold air intake (CAI) kits.

• ​Vs. Stock Air Box:​​ The 6637 is a definitive upgrade in every performance aspect. It offers better airflow and filtration than the restrictive factory system at a fraction of the cost of a CAI kit. The stock system is adequate for a completely stock truck but becomes a bottleneck with any performance modifications.
• ​Vs. Branded Cold Air Intake Kits (e.g., S&B, AFE):​​ Branded CAI kits are more expensive but offer a complete, engineered solution. They usually include a custom air box designed to isolate the filter from engine heat and draw air from a specific cold source. They are often easier to install with precise fitment and may have a more polished, "finished" appearance. However, performance gains between a well-set-up 6637 and a CAI kit are often comparable. The 6637 is valued for its cost-effectiveness and proven performance.

​Step-by-Step Installation Guide​

Installing a 6637 air filter is a straightforward mechanical task that can be completed in about an hour with basic tools.

​Tools and Parts Needed:​​

• Donaldson P6637 or equivalent filter
• 6637 intake adapter (4-inch diameter to 3.5-inch or 3-inch, depending on your turbo inlet pipe)
• Two hose clamps (typically 4-inch and 3.5-inch or 3-inch)
• Socket set and screwdrivers
• Pliers

​Installation Steps:​​

1. ​Disconnect the Battery:​​ For safety, disconnect the negative terminal of the battery to prevent any electrical issues.
2. ​Remove the Stock Air Box:​​ Open the hood and locate the stock air box on the driver's side of the engine bay. Use a socket to remove the bolts or screws holding the lid in place. Disconnect the intake hose from the lid and remove the lid and the old filter. Then, unbolt the entire air box assembly from its mounting location and remove it from the vehicle. You may also need to disconnect a sensor or a resonator hose.
3. ​Prepare the Adapter and Filter:​​ Slide the appropriate-sized hose clamp onto the intake adapter. Then, attach the adapter to the open end of the 6637 filter and tighten the clamp securely. Do not overtighten, as this can damage the filter's neck.
4. ​Connect to the Turbo Inlet Pipe:​​ Locate the turbo inlet pipe, which is the tube that previously connected to the stock air box. Slide the second hose clamp onto this pipe. Carefully push the adapter end of the filter assembly onto the turbo inlet pipe. Ensure it is seated fully and evenly.
5. ​Secure the Connection:​​ Tighten the hose clamp on the turbo inlet pipe connection securely. This connection is critical; a leak here can allow dirt to bypass the filter and destroy the turbo and engine.
6. ​Secure the Filter:​​ The filter assembly will now be sitting in the engine bay. Many adapters have a bracket that can be bolted to the existing hole where the stock air box was mounted. If not, the filter is heavy enough to stay in place, but securing it with a bracket or zip tie is recommended to prevent excessive movement.
7. ​Reconnect the Battery and Inspect:​​ Reconnect the negative battery terminal. Before starting the engine, perform a visual inspection. Double-check that all connections are tight and that there are no obvious obstructions or loose wires near the filter.
8. ​Start the Engine:​​ Start the truck and let it idle. Listen for any hissing sounds that would indicate an air leak. You will likely hear a more pronounced turbo whistle, which is normal and desired.

​Maintenance and Cleaning Procedures​

Proper maintenance is key to the filter's longevity and performance.

​When to Clean:​​ A good rule of thumb is to inspect the filter every 10,000-15,000 miles. The need for cleaning depends entirely on driving conditions. If you drive in extremely dusty environments, you will need to clean it more frequently. A visual inspection is the best guide; if the pleats are caked with dirt and you cannot see the white filter media, it is time for a cleaning.

​Cleaning Process:​​

1. ​Remove the Filter:​​ Carefully loosen the hose clamp and remove the filter from the adapter.
2. ​Initial Cleaning (Dry Phase):​​ Gently tap the filter on a hard surface to dislodge loose, heavy dirt. You can also use compressed air from the inside out to blow out debris. Hold the nozzle a few inches away and use low pressure to avoid damaging the pleats.
3. ​Washing (Wet Phase):​​ Use a dedicated ​air filter cleaning solution. Spray the cleaner generously over the entire outer surface of the filter and let it soak for the recommended time (usually 10-15 minutes). This solution breaks down the oil and embedded dirt.
4. ​Rinsing:​​ Rinse the filter from the inside out with low-pressure water. Rinsing from the inside helps push the dirt out the way it came in, preventing it from being forced deeper into the media. Continue rinsing until the water runs clear.
5. ​Drying:​​ Allow the filter to air dry completely. This is the most important step. Never apply oil to a damp filter, and never use compressed air or heat to speed up drying, as this can damage the media. Let it sit overnight in a warm, dry area.
6. ​Oiling:​​ Once the filter is 100% dry, apply ​specific air filter oil. Spray or pour the oil evenly over the outside of the filter. The oil is what actually traps the fine dust particles. Do not over-oil; a light, even coating is sufficient. The media should be tacky to the touch, not dripping.
7. ​Reinstallation:​​ Once the oil has been applied, the filter is ready to be reinstalled on the vehicle.

​Troubleshooting Common Issues​

• ​Excessive Black Smoke Under Acceleration:​​ This is a classic sign of a restriction or a leak. If the filter is dirty and clogged, it can cause a rich air-fuel mixture, leading to black smoke. Conversely, if there is a leak between the filter and the turbo, unmetered air enters the engine, also upsetting the air-fuel ratio. Check for tight connections first.
• ​Loss of Power or Boost:​​ A severely dirty filter will act as a restriction, not allowing the turbo to spool properly, resulting in a loss of boost and power. Clean or replace the filter.
• ​MAF Sensor Contamination:​​ This is almost always caused by over-oiling the filter. Excess oil can be drawn off the filter and coat the sensitive MAF sensor, causing erratic readings, poor performance, and check engine lights. If this happens, the MAF sensor must be cleaned with a specialized electronics cleaner.

​Conclusion: Is the 6637 Air Filter the Right Choice for You?​​

The 6637 air filter has earned its legendary status among 7.3L Power Stroke owners for a simple reason: it works. It is a cost-effective, high-performance upgrade that delivers measurable improvements in engine airflow, throttle response, and filtration protection. While it requires a simple adapter and proper maintenance, its benefits far outweigh these minor considerations. For any owner of a 7.3L Power Stroke looking to enhance their truck's performance and longevity without a major financial investment, the ​6637 air filter​ remains one of the most highly recommended and practical modifications available. Its proven track record, backed by decades of real-world use, makes it a smart and reliable choice for protecting your engine's vital components.