Supporting Systems

Catch Cans and PCV: What Oil in the Intake Is Doing

July 29, 2026 · 7 min read · GZ Performance, Tampa FL

Every engine pushes some combustion gas past the rings into the crankcase. That blow-by carries oil mist, fuel vapour and water, and it has to go somewhere. The factory answer is to route it back into the intake and burn it. On a boosted engine that route sends oil through the intercooler and into the combustion chamber, and it matters more than it does on a stock car.

What PCV does

Positive crankcase ventilation draws crankcase vapour into the intake manifold under vacuum, through a valve that limits flow, and admits fresh air into the crankcase from the intake tract to replace it. It keeps crankcase pressure slightly negative, which helps the rings seal and keeps oil from being pushed out past seals and gaskets. It was designed for a naturally aspirated engine with manifold vacuum available most of the time.

What boost changes

• Under boost the manifold is above atmospheric pressure, so the PCV valve closes and the manifold route stops working. Crankcase vapour then has to leave through the fresh-air side, back into the intake before the compressor.
• A boosted engine makes more blow-by than a stock one — higher cylinder pressure pushes more gas past the rings — so there is more vapour to deal with.
• Oil carried into the compressor inlet passes through the turbo and the intercooler, where it coats the internal surfaces and reduces cooling efficiency.
• Oil that reaches the combustion chamber lowers the effective octane of the mixture, and on a knock-limited engine that is a direct increase in detonation risk. We described the consequences in knock.
• On direct-injection engines, oil vapour deposits on the back of the intake valves with no fuel wash to clean them, and the build-up restricts flow over time.
• A PCV valve that fails or leaks under boost pressurises the crankcase, which pushes oil past seals and back up the turbo drain.

What a catch can does

A catch can sits in the crankcase vent line and separates oil from the vapour before it reaches the intake. A good one has baffles or media that give the vapour a tortuous path and a surface to condense on; a poor one is an empty container that catches very little. The vapour continues to the intake; the oil collects in the can and is drained periodically.

On a boosted engine there are two vent paths — the PCV side to the manifold, and the fresh-air side to the pre-compressor intake — and both carry oil under different conditions. A single can on one side catches oil under one condition and misses it under the other. A proper installation deals with both, either with two cans or a can plumbed to see both paths with check valves to keep boost out of the crankcase.

Venting to atmosphere

Removing the PCV system and venting the crankcase to a breather is common on race engines and problematic on a street car. Without vacuum, the crankcase runs at or above atmospheric pressure, which reduces ring seal and encourages oil leaks. It also smells, and on a daily driver in traffic that gets old quickly. A closed system with proper separation keeps the benefits of PCV and removes the oil. An open breather setup belongs on a car that is no longer a street car.

What a good setup looks like

1. A baffled catch can or air-oil separator with real separation, not an empty cylinder.
2. Both vent paths handled, with check valves so boost cannot enter the crankcase through the PCV line.
3. Hose sized for the flow, because a restrictive vent line is a pressurised crankcase.
4. A drain routine — the can is emptied at every oil change at least, and more often on a car that makes a lot of blow-by.
5. A check of the PCV valve itself, because a stuck or leaking valve defeats everything downstream.

It is a modest cost, it keeps the intercooler clean, it protects the couplers, and it removes a knock contributor from an engine that in Florida heat has enough of them already. It goes on most of the boosted builds we do for cars from Lutz, Wesley Chapel and across Tampa.

Questions we get asked

Do I need a catch can on a turbo car?

We recommend one on any boosted engine. Blow-by carries oil into the intercooler and the combustion chamber, where it reduces cooling efficiency and lowers effective octane. A baffled can removes most of that oil before it gets there.

Why is there oil in my intercooler?

Usually from crankcase ventilation carrying oil vapour into the intake before the compressor. A catch can on the fresh-air vent side reduces it substantially. If the amount is large, check the turbo drain and crankcase pressure as well.

Can I just vent the crankcase to atmosphere?

On a street car we would not recommend it. Without vacuum the crankcase sits at or above atmospheric pressure, ring seal suffers and oil leaks are more likely, and it smells. A closed system with proper separation is the better answer for a car that is driven daily.

How often should I empty a catch can?

At least every oil change, and more often on an engine that produces a lot of blow-by or in a humid climate where the can collects water as well as oil. A can that overflows sends everything it collected into the intake at once.

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