Understanding Oil Separation and the Catch Can System

To maximize oil separation efficiency in your Nashville catch can, you first need a solid grasp of how the system works. The positive crankcase ventilation (PCV) system is designed to vent combustion gases that escape past the piston rings—a phenomenon known as blow-by. These gases contain unburned fuel, water vapor, and a fine oil mist. Without a catch can, this oily mixture is routed back into the intake manifold, where it deposits carbon on valves, intake ports, and combustion chambers. Over time, this buildup reduces engine efficiency, increases knock risk, and can lead to costly repairs.

A catch can intercepts this flow. It forces the blow-by gases through a chamber where abrupt changes in direction, cooling, and internal obstructions cause the oil droplets to coalesce and fall out of suspension. The liquid oil collects in the bottom of the can, while the cleaned gases continue to the intake. The efficiency of this separation depends on several design and installation factors, which we will examine in depth.

Key Factors That Influence Separation Efficiency

Installation Location and Routing

Where you place the catch can in the PCV circuit has a direct impact on how well it separates oil. The ideal location is as close to the valve cover outlet as possible. This minimizes the distance the oil vapor travels before it is captured. Long or heated hoses can allow some of the lighter oil fractions to remain in vapor form, reducing separation. Use the shortest, most direct route with minimal bends. Avoid routes that dip or create low spots where oil can pool, as that can lead to clogging and reduced airflow.

Also consider the orientation of the can itself. Most catch cans are designed to be mounted vertically with the inlet and outlet on top. Tilting the can can disrupt the internal baffle action and cause liquid oil to be pulled back into the intake under high vacuum. Secure the can reliably using a bracket that isolates it from excessive engine vibration, which can shake collected oil back into a mist.

Baffle and Media Quality

The internal geometry of the catch can is the heart of its separation ability. Simple empty cans rely solely on gravity, which is inefficient for the small oil droplets found in blow-by. Quality baffles, mesh screens, or packed media increase the surface area that the vapor must contact. As the gas swirls and passes through these obstacles, oil droplets collide, coalesce, and drain down. The higher the surface area and the more torturous the path, the better the separation.

Look for designs using stainless steel wool, brass wool, or stacked perforated plates. Avoid packing materials that can degrade or shed particles into the engine. Some high-end catch cans use a two-stage process: first a baffle to knock out large droplets, then a fine mesh or coalescing filter to capture the smallest oil mist. Check the manufacturer’s specifications for the micron rating of the media—lower microns mean better filtration of fine mist, but also require more frequent cleaning.

Can Size and Volume

Larger catch cans have more residence time for the vapor to slow down and separate. A can that is too small may not allow enough dwell time, especially at high engine speeds or under heavy load where blow-by volume increases. For a high-performance or heavily modified engine, a can with at least 500 ml to 1 liter of internal volume is recommended. However, larger isn’t always better if the can is poorly baffled. Excess empty space can allow the vapor to bypass the separation zones. Choose a can where the internal volume is matched with effective baffling.

Also consider the drain capacity. A can that fills quickly needs to be drained frequently. Some designs have a sight glass or dipstick to check the level without opening the can. For engines that see track days or heavy towing, consider a catch can with a dedicated drain valve or an optional electric drain pump.

Proper Ventilation and Check Valves

The PCV system relies on a delicate balance of vacuum and pressure. A catch can that introduces excessive restriction can cause crankcase pressure to rise, leading to oil leaks and seal damage. Conversely, too little restriction can allow unfiltered blow-by to re-enter the intake. Ensure the catch can’s inlet and outlet ports match the size of the existing PCV hoses (typically 5/8” or 3/4” ID). Avoid using restrictive fittings or reducers.

Some engines benefit from adding a one-way check valve in the PCV line to prevent backflow under boost conditions. If your Nashville catch can is installed on a forced induction engine, confirm that the can is rated for the pressure and that any check valves are located between the can and the intake manifold. Without a check valve, boost pressure can push oil back into the crankcase through the catch can, defeating its purpose.

Step-by-Step Guide to Optimizing Your Nashville Catch Can

Selecting the Right Catch Can for Your Engine

Before you even begin installation, take stock of your engine’s specific needs. Naturally aspirated engines have lower crankcase pressure and lower blow-by volumes than turbocharged or supercharged engines. A universal catch can may suffice for a mild street car, but a performance application demands a purpose-built unit with features like dual inlets (one from each valve cover), a large internal volume, and replaceable coalescing media.

Research what other owners of your vehicle model are using. Look for catch cans from reputable manufacturers such as Mishimoto, Radium Engineering, or Saikou Michi. These brands publish test data and provide application-specific kits that include brackets, hoses, and fittings. Avoid generic “ebay” catch cans that are often little more than empty aluminum containers with poor sealing.

Installing for Maximum Efficiency

  1. Locate the PCV valve and fresh air intake line. On most engines, the PCV valve is on the top or side of the valve cover. The fresh air intake line usually connects from the air cleaner box to the other valve cover or to the crankcase. Identify both lines.
  2. Disconnect the battery negative terminal. Safety first—avoid short circuits and prevent electrical issues.
  3. Remove the PCV hose from the intake manifold to the valve cover. This hose carries the crankcase vapors to the intake. Install the catch can in this line. If your catch can is a “closed” system, you will also install a second can (or a dual-port can) in the fresh air line to prevent oil from being pulled into the air intake under part-throttle conditions.
  4. Mount the catch can securely. Use the included bracket or fabricate one. Choose a location that allows easy access for draining and inspection. Avoid areas near exhaust manifolds or other high-heat sources; excessive heat can cause the collected oil to vaporize again.
  5. Cut the PCV hose and attach the catch can. Use barbed fittings and high-quality oil-resistant hose. Clamp the connections tightly with constant-torque clamps. Ensure the hoses do not kink or sag.
  6. Reconnect the battery and start the engine. Check for leaks at all fittings. Let the engine reach operating temperature and inspect the can for proper function. You may notice the can getting warm—that’s normal as it absorbs heat from the blow-by gas.

Maintenance Schedule and Drainage

Regular draining is the single most important maintenance task. The collected oil is a mixture of oil, fuel, water, and acids. If left to sit, it can become corrosive and damage the can’s interior. Drain the can every 1,000 to 3,000 miles depending on driving conditions. Short trips and cold weather produce more condensation and fuel dilution, requiring more frequent draining. After draining, wipe the inside with a clean rag to remove any sludge. Inspect the baffle or media for clogging; if the media is saturated, rinse it with a solvent like brake cleaner and let it dry before reinstallation.

Check the hoses and fittings for cracking or looseness each time you drain the can. A loose hose can introduce a vacuum leak, disturbing the air-fuel mixture. Replace hoses every two years or sooner if they show signs of oil softening. Some manufacturers recommend rebuilding the catch can at regular intervals, replacing seals and media packs. Follow the manual for your specific model.

Common Mistakes and How to Avoid Them

  • Installing the catch can in the wrong line. A common error is placing the can on the fresh air line instead of the PCV line. The fresh air line is under vacuum only at certain conditions; the PCV line sees the most consistent flow of blow-by. Route the can in the line that goes from the valve cover to the intake manifold.
  • Using undersized hoses. Restricting the PCV system increases crankcase pressure. Always use hoses with an inner diameter equal to or larger than the original PCV hose.
  • Neglecting to vent the can. Some catch cans have a built-in vent or filter. If you have a closed system (can vents back to intake), ensure the return line is free and the intake connection is after the mass airflow sensor to avoid unmetered air. If you have an open system (can vents to atmosphere), check local emissions laws; many areas require a closed system for street use.
  • Forgetting to drain during winter. In cold climates, water condensation inside the can can freeze and block the vapor path. Use a catch can with a heated option or increase the draining frequency to once a week during sub-zero temperatures.
  • Relying solely on a catch can for oil control. A catch cannot fix worn piston rings or a damaged PCV valve. If you see excessive blow-by (more than a few ounces every 500 miles), diagnose the engine’s health first.

Advanced Upgrades for Better Separation

For those who track their cars or run high-boost setups, standard catch cans may not be sufficient. Consider these upgrades:

  • Dual catch cans. Separate cans for the PCV and fresh air lines ensure each circuit is independently filtered. The fresh air line can be fitted with a small, low-restriction can to catch oil that would otherwise be sucked into the turbo inlet.
  • Electric vacuum pumps. Connecting a vacuum pump to the crankcase can dramatically reduce blow-by and improve ring seal. A catch can is still needed to protect the pump from oil contamination. This is common in drag racing and endurance applications.
  • Coalescing filter upgrades. Replace the standard foam or steel wool with a coalescing filter element rated for sub-micron particle capture. These filters can remove up to 99% of oil mist but require more frequent servicing.
  • Temperature-controlled drain valves. Some aftermarket catch cans feature a thermostatic valve that opens only when the can is warm, allowing condensed water to drain off before it can mix with oil. This helps separate water and reduces sludge formation.

Before upgrading, research compatibility with your specific vehicle. An article on EngineLabs provides a good technical overview of PCV systems and advanced catch can setups.

Conclusion

Maximizing oil separation efficiency in your Nashville catch can is not a one-time setup—it requires careful selection, precise installation, and consistent maintenance. By understanding the physics of oil separation, choosing a well-baffled can, placing it correctly in the PCV circuit, and staying on top of drainage, you can significantly reduce carbon deposits, lower oil consumption, and extend the life of your engine. Whether you drive a daily commuter or a weekend track weapon, a properly optimized catch can is one of the most cost-effective performance and reliability investments you can make. Start by auditing your current setup against the points above, and make incremental improvements where needed. Your engine will thank you with cleaner valves and consistent performance.