Small Engine Crankcase Venting and PCV Systems Explained

Posted by Mark on 6/26/2025 to How-tos

Crankcase Venting and PCV Systems Explained

Understanding Proper Ventilation for High-Performance Small Engines

When building or tuning high-performance engines—whether for racing go-karts, mini bikes, or performance mowers—crankcase venting is often one of the most misunderstood and overlooked systems. But it plays a critical role in maintaining engine efficiency, durability, and reliability. This article will explain how crankcase ventilation works, the purpose of a PCV (Positive Crankcase Ventilation) valve, and how to properly set up your system for your engine build.

What Is Crankcase Pressure?

During engine operation, combustion gases can leak past the piston rings into the crankcase—a phenomenon known as blow-by. These gases increase internal pressure inside the engine block. If this pressure isn’t properly controlled, it can cause:

Oil leaks from seals and gaskets
Aerated or foaming oil, reducing lubrication quality
Reduced horsepower due to internal drag
Oil contamination from fuel and combustion gases

Crankcase Venting: Why It's Important

The solution is to vent the crankcase, allowing pressure to escape and preventing buildup. But it’s not enough to just vent—it needs to be done properly and directionally.

Basic Vent (Open Venting)

An open vent—like a hose or fitting—is the simplest form of crankcase ventilation. But these are bi-directional. That means while pressure can escape, air can also be sucked back in when the piston moves down. This reintroduces moisture and contaminants, destabilizing crankcase pressure.

PCV Systems: The Smart Way to Control Pressure

A Positive Crankcase Ventilation (PCV) valve is a one-way check valve that only allows gases to flow out of the crankcase. It helps maintain a controlled vacuum or positive pressure, depending on how it's routed.

Benefits of a Proper PCV System:

Keeps air moving one way only
Reduces oil contamination
Prevents pressure spikes
Maintains ring seal under load
Controls oil mist and blow-by

Race Engine Application: More Than Just Emissions

In automotive engines, PCV systems were introduced as part of the emissions control system. In racing engines, however, a proper PCV setup is crucial for both performance and longevity.

For example:

Methanol engines create more moisture in the crankcase, which must be vented effectively.
High-RPM engines build pressure fast, so directional flow control is essential to avoid oil push-out.

How to Set Up a Proper Venting System

1. Use a One-Way PCV Valve

Avoid using open fittings. Install a billet PCV valve (like EC’s 280, 281 & 282) in a high location—like a valve cover or side cover—with a check valve built in.

2. Route to a Catch Can (Optional but Recommended)

Instead of venting to the atmosphere, route the PCV hose to a catch can to trap oil mist and vapor. Race tracks and organizations usually require catch cans.

3. Avoid Cross Venting

If you're using multiple vents, make sure they don’t cancel each other out. For example, two open vents can allow air to flow in and out simultaneously—defeating the purpose.

4. Use Proper Hose Diameter

A vent hose that’s too small will restrict airflow. Use at least 1/2” ID hose when using EC’s PCV.

Common Mistakes

Using an open barb fitting instead of a check valve
Blocking vents entirely
Routing PCV below oil level, which can suck oil into the valve or catch can
Over-venting: more holes, better airflow, is useless if the direction isn't controlled