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There was a massive, terribly heated conversation about catch cans on a Mazda forum I followed. After over a year's worth of arguing back and forth the bottom line was:

- Catch cans work in principle but they need to be ventilated to the atmosphere to be highly effective (which is illegal due to auto air pollution standards).
- The legal, non-ventilated catch cans used for cars are nowhere near as effective because they have to return vapors back to the crankcase.
- The best in class catch cans installed on Mazdas didn't capture any significant oil/gas vapors after a year's worth of use. Just a few drops or nothing at all.
- Some catch can manufacturers use scare tactics and internet articles to sell them, some of which they may have authored. They often post on car forums spreading fear about DI engines.
- No one using catch cans on their Mazda was ever able to factually state that they solved any problem, yet they did cause problems.
- On older DI engine designs which had significant carbon issues, catch cans were able to only slightly delay carbon accumulation, not resolve it.
- Most modern DI engines now have very effective methods of controlling carbon deposits, far more so than catch cans.
- Mazda's latest DI engine design never developed any significant carbon deposit issues after seven years on the market and hundreds of millions of miles of accumulated use proving that modern DI engine designs may no longer be susceptible to excess carbon deposits.
- Catch cans could possibly void engine warranties.
- Catch cans can blow out engine seals if not properly maintained or if clogged (one of the reasons most manufacturers do not use them).

I've never installed a catch can on a car, but I have on motorcycles and boats. On poor engine designs, they may help a bit, on good engines they do virtually nothing. The one I have on my newest boat with an excellent DI engine design does absolutely nothing, it's never captured a drop of oil or gas vapor, only some insignificant water vapor and very little of that. I should have never installed it, it was a waste of money and could void my engine warranty.

As long as you don't overfill your oil and use oils which are ASTM D5800 certified for evaporation loss, which most modern synthetic oils exceed, then you should have no need for a catch can on the Ascent. Do not install a catch can only on speculation or fear, use one only if it's proven to solve a problem on a specific engine which is known to have carbon deposit issues.
 

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My catch cans are NOT ventilated to the atmosphere, and they definitely remove oil. I know because I have to empty them.

Look at this diagram from Radium Engineering.



I use a dual catch can set up plumbed in to work on both side of the PCV. I used 2 universals and it works great.

Perrin has a closed system that drains back into the crankcase and uses a coolant loop to aide in the oil separation. Crawford also makes a similar set up. Both the Perrin and the Crawford could be adapted to fit the 2.4DIT DIY style easily.

 

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There was a massive, terribly heated conversation about catch cans on a Mazda forum I followed. After over a year's worth of arguing back and forth the bottom line was:

- Catch cans work in principle but they need to be ventilated to the atmosphere to be highly effective (which is illegal due to auto air pollution standards).
- The legal, non-ventilated catch cans used for cars are nowhere near as effective because they have to return vapors back to the crankcase.
- The best in class catch cans installed on Mazdas didn't capture any significant oil/gas vapors after a year's worth of use. Just a few drops or nothing at all.
- Some catch can manufacturers use scare tactics and internet articles to sell them, some of which they may have authored. They often post on car forums spreading fear about DI engines.
- No one using catch cans on their Mazda was ever able to factually state that they solved any problem, yet they did cause problems.
- On older DI engine designs which had significant carbon issues, catch cans were able to only slightly delay carbon accumulation, not resolve it.
- Most modern DI engines now have very effective methods of controlling carbon deposits, far more so than catch cans.
- Mazda's latest DI engine design never developed any significant carbon deposit issues after seven years on the market and hundreds of millions of miles of accumulated use proving that modern DI engine designs may no longer be susceptible to excess carbon deposits.
- Catch cans could possibly void engine warranties.
- Catch cans can blow out engine seals if not properly maintained or if clogged (one of the reasons most manufacturers do not use them).

I've never installed a catch can on a car, but I have on motorcycles and boats. On poor engine designs, they may help a bit, on good engines they do virtually nothing. The one I have on my newest boat with an excellent DI engine design does absolutely nothing, it's never captured a drop of oil or gas vapor, only some insignificant water vapor and very little of that. I should have never installed it, it was a waste of money and could void my engine warranty.

As long as you don't overfill your oil and use oils which are ASTM D5800 certified for evaporation loss, which most modern synthetic oils exceed, then you should have no need for a catch can on the Ascent. Do not install a catch can only on speculation or fear, use one only if it's proven to solve a problem on a specific engine which is known to have carbon deposit issues.
Great info, thanks!
 

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Discussion Starter · #7 ·
Why not just vent to atmosphere and just reconnect for emissions test>?
 
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Why not just vent to atmosphere and just reconnect for emissions test>?
Maybe because if people cheat on air pollution emissions standards that's a bad thing for breathing clean air. People's lungs are worth more than any car engine.
 

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I wonder why FA20D requires extra injector(s) to clean the intake valves while all other FA2# variants don’t include them. I’m aware of the involvement of Toyota’s D4-S technology.

Make you wonder right? Porsche is the closest apple-to-apple comparison when it comes to engine design and they use AOS
 

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I wonder why FA20D requires extra injector(s) to clean the intake valves while all other FA2# variants don’t include them. I’m aware of the involvement of Toyota’s D4-S technology.

Make you wonder right? Porsche is the closest apple-to-apple comparison when it comes to engine design and they use AOS
There are many effective ways to combat excess carbon build-up. Mazda diverts hot exhaust gasses to the valves to mitigate carbon much the same way as the cleaning cycle of an oven removes carbon. Mazda's Skyactiv DI engines have an excellent reliability record and do not have carbon build up issues.
 

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There are many effective ways to combat excess carbon build-up. Mazda diverts hot exhaust gasses to the valves to mitigate carbon much the same way as the cleaning cycle of an oven removes carbon. Mazda's Skyactiv DI engines have an excellent reliability record and do not have carbon build up issues.
Yes, I believe there are numbers of ways to combat excess carbon build-up. In order for this conversation to be practical, we need to know the air/vapor quality between the two engines as it's being transported from the crankcase to the head (and intake plenum).

Do you have any data to show that the air/vapor quality between the two engines are the same?
 

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Yes, I believe there are numbers of ways to combat excess carbon build-up. In order for this conversation to be practical, we need to know the air/vapor quality between the two engines as it's being transported from the crankcase to the head (and intake plenum).

Do you have any data to show that the air/vapor quality between the two engines are the same?
I doubt if anyone outside of Subaru and Mazda engineering has that specific, very esoteric information.
 

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Discussion Starter · #16 ·
I was planning to keep this car over 80k or so i might install one regardless. Can only help i would think
 
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I was planning to keep this car over 80k or so i might install one regardless. Can only help i would think
If you do install one, please always make certain you empty it regularly and ensure that it never clogs. A full or clogged catch-can can blow out engine seals. If this happened, it would not be covered under warranty.
 

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I doubt if anyone outside of Subaru and Mazda engineering has that specific, very esoteric information.
Exactly, we should only navigate thru the known lane (and not the unknowns). If we lay out the features of the FA2# variants, the FA20D is significantly different than other FA20 or FA24. The difference is the Toyota's D-4S technology. Based on that general and apparent observation, the oil catch can (or AOS) can be a feasible alternative to the D-4S feature.

Bringing Mazda's technology in just complicate the conversation, unless you have a feasible solution to implement on the FA24.
 

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Discussion Starter · #19 ·
If you do install one, please always make certain you empty it regularly and ensure that it never clogs. A full or clogged catch-can can blow out engine seals. If this happened, it would not be covered under warranty.
I guess that brings up another point as to always check your PVC valve every 15k or so. If that clogs or gets stuck i'm assuming it would cause the same type of issue.
 

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Exactly, we should only navigate thru the known lane (and not the unknowns). If we lay out the features of the FA2# variants, the FA20D is significantly different than other FA20 or FA24. The difference is the Toyota's D-4S technology. Based on that general and apparent observation, the oil catch can (or AOS) can be a feasible alternative to the D-4S feature.

Bringing Mazda's technology in just complicate the conversation, unless you have a feasible solution to implement on the FA24.
No, it doesn't complicate the conversation. My point is that many modern DI engines have successfully resolved the issue of carbon build up and that simply because an engine uses a DI design, it can no longer be assumed that it will have such an issue. Few manufacturers would design a new engine without addressing a known serious problem with the primary technology used on that engine. These days no auto manufacturer would purposely set themselves up for failure by foolishly ignoring past problems, especially an excellent manufacturer such as Subaru.

If we should only navigate thru the known lane (and not the unknowns) as you say, then we should also never assume that any new DI engine requires a catch can until it has proven to have a known issue that a catch can could resolve. Installing potentially useless components onto an engine not known to have any such issues, based only upon gross speculation, is illogical. If we follow that logic, we would all be installing all kinds of gadgets on our engines to try to ward off all kinds of other unproven problems. That simply makes no sense. Or to put it more succinctly using the wise adage - "If it's not broke, don't fix it." Words to live by.

So I'm not sure where you or the others are coming from. If you're trying to say we should all install a catch can on our Ascents simply because it's a DI engine and some prior DI engines had issues, then that is unproven speculation based upon outdated beliefs and I definitely don't agree, and no one else should either. If however, you're saying, let's explore the possibility that there may potentially be an issue, then I don't have a problem with that, although I believe, based upon modern DI engine design, that you'll ultimately find that such an issue does not exist. This is exactly what happened on the Mazda forum and why I mentioned it.

Anyone advocating for others to use catch cans needs to first prove that there is an actual problem rather than offering a solution to a problem which does not yet exist and probably will not ever exist.
 
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