Car Advice

What is a catch can and why do you need one?

You’ve heard that a catch can is a good idea, but why? To explain, we fitted one to our Ford Ranger…

THE STORY starts with the crankcase. This is the part of the engine block that houses the pistons, which go up and down within the block.  There’s also the crankshaft, and connecting rods (conrods) to link the pistons to the crankshaft.  Like any moving metal parts, crankshafts require lubrication, and that’s done by drawing oil from a storage area called the sump. The actual lubrication is done via a fairly complex design of small holes through which oil is pumped, creating a thin film of oil. A byproduct of this process means the air in the crankcase is full of oil.

When the pistons go up and down as part of the normal four-stroke cycle a lot of gas pressure is created. Ideally, that gas cannot escape past the piston seals down into the crankcase, but nothing is perfect so inevitably some gas manages to get past – this is called ‘blow by’, as it “blows by” the piston seals into the crankcase. This gas needs to be expelled as its not helping with lubrication, and with lots of gas in the crankcase you get a positive crankcase pressure, which means its harder for the piston to move up and down. Blow-by tends to get worse with an engine’s age.

The easy solution is to vent the crankcase to the outside air with a small pipe out in the vehicle’s exterior airflow called a road draft tube. As speed increases, the vehicle’s slipstream creates a low pressure area around the end of the pipe which sucked air from the crankcase. To equalise, another vent brought in fresh air. But the road draft tube isn’t perfect. It relies on significant vehicle speed to operate effectively, otherwise the suction effect would not be created, and it was a way for water to enter the crankcase too.  But there was a bigger problem – pollution.

The road draft tube basically emitted to the outside air crankshaft gases, a noxious mix of air that had been in the crankcase, unburned fuel, and burnt gas from the cylinders. That was pretty bad news for emissions, so a new solution was needed.

Enter the PCV, or positive crankcase ventilation system. This works pretty simply; air is piped to the crankcase via the engine’s existing air filter, run through the crankcase, and then through an oil/air seperator (PCV valve) back into the engine’s air intake.  

The problem with the PCV system is that some of that nasty, oily air can find its way back into the engine’s cylinders, which work best with clean fresh air that can be mixed with the right amount of fresh fuel. Using the ex-crankcase air is inefficient, and tends to build up sludge over time.

This is where an oil/air seperator comes in, otherwise known as a catch can. That’s because it’s literally a can that catches, and what it catches is oil. It works in two ways; by gravity, relying on the fact that oil is heavier than air to separate the two, and more importantly by a filter.

Here’s a dirty EGC (exhaust gas recirculation) valve in the Flashlube workshop, showing the sort of gunk that a…catch can can catch!

So, basically a catch can ensures the air which is vented from the crankcase is clean before it is fed back into the engine’s air intake. Here’s a schematic of how it works:

Image courtesy Flashlube (

So for all those reasons we’ve fitted a Catch Can Pro to our Ford Ranger PX and will let you know how it goes over time. Here’s how it looks under the bonnet, not getting in the way, just replacing one of the factory hoses with two shorter hoses. The bracket mounts to existing holes. Not a complicated install, hardest part was removing the factory hose clamps from the original hose.

The reason that catch cans are now rather popular of late is because of ever-higher engine compression ratios increasing the chances of blow-by, and because many people modify their engines with chips which increase compression still further. 

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Ben Tate
Ben Tate
3 years ago

A work mate used to tip a point of engine oil into the petrol tank of his HT GTS 253 Monaro. It was meant as an upper cylinder lube on long trips. They were the days before engine computers.

That’s a BAD idea these days. Engine oil in the petrol reduces octane rating resulting in knock sensors possibly retarding spark timing and costing fuel economy and power.

Those who drive a high power high boost car are more likely to produce blowby. A catch can might reduce blowby and that might reduce oil feeding into the engine causing a loss of power.

For the average car and driver I doubt it would be noticed.

Ben Tate
Ben Tate
2 months ago

Oops. Silly me. Sorry Sergeant!

There is a real need for catch cans on some cars including … MINE. It has Direct Injection!

Port Injection sprays onto the stems of inlet valves washing them. Because Direct Injection is downstream, the inlet valves don’t get washed by fuel injectors.

And the muck that builds up on inlet valves comes through the PCV hose. That’s what the catch can stops. I should have picked that up from your article.

Ben Tate.

Robert Pepper

Robert Pepper