I was reading a blog by an autocross racer and he lost an engine due to a block crack, which apparently was caused by broken ring lands. In the blog he says that the problem could have been avoided by having a "catch can". What is that?

  • The only "catch can" I'm aware of is for catching gunk and oil as it is drawn through so it doesn't get drawn into the intake via the PCV. To me, it sounds like someone doesn't really know what they are talking about or getting their terms mixed up. A catch can would not prevent ring land failure. Commented Dec 29, 2022 at 3:13
  • @Pᴀᴜʟsᴛᴇʀ2 The damage was caused by fragments of the lands getting somewhere bad apparently. The blog is here. Decide for yourself whether the author "knows what he is talking about". lotustalk.com/threads/… Commented Dec 29, 2022 at 3:21
  • I think he is suggesting that oil has been pulled through the PCV into the cylinder and has damaged the piston on the compression stroke. It also looks like there was no other damage, all the pieces look like they stayed in place. I didn’t read all of it however.
    – HandyHowie
    Commented Dec 29, 2022 at 11:50

1 Answer 1


A catch can is a device which sits in the path between the PCV valve and the intake. It's job is to catch any liquids coming out of the crankcase and give them a place to settle so they don't get into the intake tract. Many people use them on Direct Injected (DI) engines. Without them, a lot of the oil/gunk gets deposited on the back of the intake valves. With port injected engines, the fuel injectors spray the back side of the valve and help keep them clean.

The following is a diagram of how a catch can works (pulled from Autodesk Instructables):

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As for the claim of the catch can preventing ring land damage, I'd suggest it's highly unlikely. The author of the thread stated they do road racing in their Lotus and the hard lateral G's causes oil to get into the PCV and then into the cylinder which causes the ring lands to fail. The only way this could happen is if there was enough oil present to cause hydrolock, which could potentially cause ring land damage, however there are two issues. First, the amount of oil it'd take to cause hydrolock would have left evidence, which from the writ doesn't look possible. Second, a hydrolock situation would have caused greater damage ... a lot more than just busting ring lands on one piston.

After reading the thread, it is my opinion the ring lands failed on their own. If there was an issue a catch can could have solved here, there would most likely be damage to all or at least more than one piston. This is not to say a catch can wouldn't have other benefits, it's just it wouldn't have prevented a ring land failure.

The other thing of note is, if there is that much oil escaping through the PCV system, either more/better baffling around the PCV valve is needed or it should be located to a better position where less oil could be picked up.

  • Oil separators or filters are factory equipped in vehicles with particulate filters, either gas or diesel. The issue is the oil burns and the anti-wear metals in the oil become ash in the exhaust. The ash permanently clogs the filter because metals can't be evaporated/oxidized like soot can, leading to its eventual replacement.
    – user71659
    Commented Dec 29, 2022 at 20:40
  • @user71659 - That's good information, but has nothing to do with this question. Commented Dec 29, 2022 at 22:32
  • "Without them, a lot of the oil/gunk gets deposited on the back of the intake valves." That's not the reason, the reason is DPF/GPF ash and catalytic converter poisoning.
    – user71659
    Commented Dec 29, 2022 at 23:10
  • @user71659 - I'm sorry, I thought the question had to do with a catch can and busted ring lands? What does DPF/OPF have to do with anything here? Commented Dec 30, 2022 at 2:43

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