Catch can restrictions
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From: Inland NW
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Joined: Sep 2008
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From: San Diego, CA
I think the problem started with the EVO engines, it's an EPA thing.... The crankcase vent became a closed system... it could no longer vent to the atmosphere..
So the MoCo started recirculating the crankcase vents to the air cleaners.. This "allows" the CCV gases to be ingested by the intake and burned by the engine...
The problem is, who wants the nasty blow-by gases ingested by their engine...??? It can really gunk up the pistons with carbon deposits...
The breather mod changes the path of the breather system from being ingested by the intake, to a hose that runs down to the frame somewhere... venting the CCV gases away from the intake..
These hoses MAY drip a little oil, and definitely release vapors into the atmosphere...
Some people like to put something at the end of the hose that's redirecting CCV gases. Some put a small filter of some type to catch any oil/gunk and some like to use a proper catch can... the problem with both is that they require some kind of time/maintenance to ensure the modified breather system remains open and free to vent the gases... Many just let the hose terminate at a low spot on the frame and let it vent to the atmosphere unimpeded....
Here's a couple pics of a breather mod I did to one of my Twin Cam bikes.....
Road Captain
Joined: Sep 2014
Posts: 629
Likes: 446
From: Oklahoma
I installed a catch can on a 2014 Mustang GT I had and it did accumulate some oil over time that I would dump periodically. So there is truth to the principal behind it.
Cars have used positive crankcase ventilation (a pcv valve) for many years using manifold vacuum regulated by the pcv valve to vent the crankcase pressure that occurs when the engine is running.
Before the pcv valve came along it was just a big breather tube somewhere on the engine with a cap on it with a filter media in the cap that would get oily and dirty over time and need to be cleaned. This is pretty much what the above posts vent systems is doing. There is no manifold vacuum aiding the process of reducing crankcase pressure but you are not running the vapor back thru the engine I agree.
The old breather tube system on cars used a pretty good size tube. Because the pcv valve uses manifold vacuum the diameter of the hose can be much smaller and still capable of relieving crankcase pressure. If you pull the pcv valve out on a car and put your finger over the end while the engine is running there is strong vacuum there.
My question is by eliminating the vacuum source in the Harley system and using the same size orfices in the breather bolts and same size hoses to vent to the atmosphere is that big enough to adequately relieve crankcase pressure?
I am not a engineer but I know engineers put a lot of math into designing these things.
And yes it was the EPA that caused the manufacturers to change from the old breather tube systems to the pcv valve systems. Even though crankcase vapors are being routed back thru the engine with a pcv valve cars run hundreds of thousands of miles before the engine needs to be rebuilt.
I think a catch can inline to manifold vacuum works as advertised and doesn't reduce the efficiency of the system reducing crankcase pressure. I am not sure just venting thru the breather bolts to the atmosphere is enough...
Cars have used positive crankcase ventilation (a pcv valve) for many years using manifold vacuum regulated by the pcv valve to vent the crankcase pressure that occurs when the engine is running.
Before the pcv valve came along it was just a big breather tube somewhere on the engine with a cap on it with a filter media in the cap that would get oily and dirty over time and need to be cleaned. This is pretty much what the above posts vent systems is doing. There is no manifold vacuum aiding the process of reducing crankcase pressure but you are not running the vapor back thru the engine I agree.
The old breather tube system on cars used a pretty good size tube. Because the pcv valve uses manifold vacuum the diameter of the hose can be much smaller and still capable of relieving crankcase pressure. If you pull the pcv valve out on a car and put your finger over the end while the engine is running there is strong vacuum there.
My question is by eliminating the vacuum source in the Harley system and using the same size orfices in the breather bolts and same size hoses to vent to the atmosphere is that big enough to adequately relieve crankcase pressure?
I am not a engineer but I know engineers put a lot of math into designing these things.
And yes it was the EPA that caused the manufacturers to change from the old breather tube systems to the pcv valve systems. Even though crankcase vapors are being routed back thru the engine with a pcv valve cars run hundreds of thousands of miles before the engine needs to be rebuilt.
I think a catch can inline to manifold vacuum works as advertised and doesn't reduce the efficiency of the system reducing crankcase pressure. I am not sure just venting thru the breather bolts to the atmosphere is enough...
Seasoned HDF Member
Joined: Sep 2008
Posts: 14,158
Likes: 11,247
From: San Diego, CA
The vacuum for a PCV valve, if I remember correctly, comes forom the intake manifold or the throat of the carb. That introduces true vacuum directly to that valve...
The Harley breather system terminates in the air cleaner, behind the filter... On my Twin Cams, that vent in the back plate (which needed to be plugged), was plugged with a 4 or 6mm set screw (I forget which). It is a pair of vary small vents in an area of low vacuum (if you have a proper filter)... It appears to be designed to allow the CCV gases some where to go. It doesn't seem to need vacuum to help vent the CCV cases... that job seems to be handled by the Breather assemblies in the heads, via the umbrella valve, to ensure "exit only" flow...
The mod I've always used has hose with 3/8" ID all along the length, which is less restrictive to the flow than the back plate 4/6mm vents. I additionally always make sure the hose is always routed downward, with no low spots, so gravity will aid in the evacuation of any oil/gunk in addition to the vapors..
So far, I have not noticed any issues with the use of my external breathers on 4 Twin Cam motors including a Stage I 88" w/carb, 88" with a 95" big bore kit/EFI, Stage II 103" EFI, and finally a 124" S&S crate engine @ 135HP & 151 TQ in a 2016 bagger...
I know my experience is not scientific, but I also haven't read of any issues form an external system...
While that doesn't mean they don't exist, it does make me feel comfortable with using them...
Road Captain
Joined: Sep 2014
Posts: 629
Likes: 446
From: Oklahoma
I have modded 4 Twin Cam engines with an external breather... I'm certainly no engineer, but I believe the external breathers do as good a job as when directed to the intake for the following reasons...
The vacuum for a PCV valve, if I remember correctly, comes forom the intake manifold or the throat of the carb. That introduces true vacuum directly to that valve...
The Harley breather system terminates in the air cleaner, behind the filter... On my Twin Cams, that vent in the back plate (which needed to be plugged), was plugged with a 4 or 6mm set screw (I forget which). It is a pair of vary small vents in an area of low vacuum (if you have a proper filter)... It appears to be designed to allow the CCV gases some where to go. It doesn't seem to need vacuum to help vent the CCV cases... that job seems to be handled by the Breather assemblies in the heads, via the umbrella valve, to ensure "exit only" flow...
The mod I've always used has hose with 3/8" ID all along the length, which is less restrictive to the flow than the back plate 4/6mm vents. I additionally always make sure the hose is always routed downward, with no low spots, so gravity will aid in the evacuation of any oil/gunk in addition to the vapors..
So far, I have not noticed any issues with the use of my external breathers on 4 Twin Cam motors including a Stage I 88" w/carb, 88" with a 95" big bore kit/EFI, Stage II 103" EFI, and finally a 124" S&S crate engine @ 135HP & 151 TQ in a 2016 bagger...
I know my experience is not scientific, but I also haven't read of any issues form an external system...
While that doesn't mean they don't exist, it does make me feel comfortable with using them...
The vacuum for a PCV valve, if I remember correctly, comes forom the intake manifold or the throat of the carb. That introduces true vacuum directly to that valve...
The Harley breather system terminates in the air cleaner, behind the filter... On my Twin Cams, that vent in the back plate (which needed to be plugged), was plugged with a 4 or 6mm set screw (I forget which). It is a pair of vary small vents in an area of low vacuum (if you have a proper filter)... It appears to be designed to allow the CCV gases some where to go. It doesn't seem to need vacuum to help vent the CCV cases... that job seems to be handled by the Breather assemblies in the heads, via the umbrella valve, to ensure "exit only" flow...
The mod I've always used has hose with 3/8" ID all along the length, which is less restrictive to the flow than the back plate 4/6mm vents. I additionally always make sure the hose is always routed downward, with no low spots, so gravity will aid in the evacuation of any oil/gunk in addition to the vapors..
So far, I have not noticed any issues with the use of my external breathers on 4 Twin Cam motors including a Stage I 88" w/carb, 88" with a 95" big bore kit/EFI, Stage II 103" EFI, and finally a 124" S&S crate engine @ 135HP & 151 TQ in a 2016 bagger...
I know my experience is not scientific, but I also haven't read of any issues form an external system...
While that doesn't mean they don't exist, it does make me feel comfortable with using them...
I know whenever I go look at a used car I pull the pcv valve out while the motor is running. If air is blowing out the hole I know the motor is worn out and has a lot of blow by and I pass. I don't care how low miles they say the engine is.
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From: Big Sky Country
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