Shop for crankcase ventilation parts and you'll see the terms "oil catch can" and "air/oil separator" (AOS) used almost interchangeably. They solve the same underlying problem, but the terms describe two different approaches to solving it — and the difference matters when you're deciding what to put on your engine.
What a basic oil catch can does
A traditional catch can is, at its simplest, an empty vessel plumbed into the PCV (positive crankcase ventilation) or CCV (crankcase ventilation) system. Oil-laden vapor and blow-by gases pass through it, the can's baffling knocks some of the oil out of the vapor stream, and the liquid oil collects in the bottom for you to drain periodically. Airflow continues on to the intake. A basic can traps some contaminants, but a lot of the finer oil mist and fuel vapor rides straight through with the air.
What an air/oil separator does differently
An air/oil separator is built specifically to separate the liquid oil from the air/vapor stream more thoroughly — using additional stages like cyclonic separation, multi-baffle chambers, or filtration media — before returning cleaner air to the intake. The goal isn't just to catch some oil in a jar; it's to actually separate the air from the oil as completely as practical.
Where the Tracy Lewis system fits
The Tracy Lewis Catch Can is built as a true air/oil separating crankcase evacuation system, not a simple catch jar. It runs full-time vacuum on the crankcase (rather than the part-time vacuum most catch cans rely on), and most configurations pair it with our Clean Side Separator to address ingestion upstream of the throttle body as well — something a basic "catch can" by itself doesn't do. See the full breakdown of how the system works on A Closer Look at the Tracy Lewis Performance Catch Can System.
Does the label matter when you're buying one?
Less than the design does. Whatever a product is called, ask two questions: does it run full-time vacuum or only part-time, and does it separate oil on both the dirty (crankcase) side and the clean (intake) side, or just one? Those design details predict real-world performance far better than the name on the box. For a full buying checklist, see Best Oil Catch Can: What to Look For Before You Buy.
Frequently Asked Questions
Is an air/oil separator better than a catch can?
It depends on the specific design, not the label. A well-designed catch can with multi-stage baffling and full-time vacuum can outperform a poorly designed "AOS," and vice versa. Compare the actual separation design, not the marketing term.
Can I use the terms interchangeably?
In casual conversation, yes — most builders do. Just don't assume two products are equivalent because they're both called a "catch can" or both called an "AOS." Compare the internals.
Does the Tracy Lewis Catch Can separate oil on both sides of the intake?
Yes. Most configurations include the Clean Side Separator, which addresses oil vapor ingestion upstream of the throttle body in addition to the crankcase-side separation the main can provides.
Shop the full Tracy Lewis Catch Can lineup, built for Camaro, Cadillac, Corvette, Mustang, F-150, and more.