External Breather Question
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Road Warrior
Joined: Jul 2011
Posts: 1,326
Likes: 331
From: Central TX
External Breather Question
Fairly new to the M8s and trying to learn here. I installed fittings and hoses on my TC intake to vent oils from heads to atmosphere instead of dumping dirty air back into intake. I have been looking to do the same on my 128 M8. I was also looking at the transmission cover vent with check valve to relieve extra crank case pressure.
Here is my question: I am seeing several videos and posts stating the external breather at the intake is not good, as it does not allow the engine to build vacuum. I have seen several of these videos, but they dont explain this, they just state it. Not an engine expert by any means, but I dont really see a big difference in how the external breather prevents a vacuum. I thought the umbrella valves is what let pressure out thru and prevented air from entering back into the breather bolts? Stock routing of hoses dumps right into entrance of TB, but are not sealed in any way. I am just not seeing a difference between dumping into TB and dumping outside air cleaner. Can someone please explain how stock breather hoses allows vacuum and external breathers dont?
Here is my question: I am seeing several videos and posts stating the external breather at the intake is not good, as it does not allow the engine to build vacuum. I have seen several of these videos, but they dont explain this, they just state it. Not an engine expert by any means, but I dont really see a big difference in how the external breather prevents a vacuum. I thought the umbrella valves is what let pressure out thru and prevented air from entering back into the breather bolts? Stock routing of hoses dumps right into entrance of TB, but are not sealed in any way. I am just not seeing a difference between dumping into TB and dumping outside air cleaner. Can someone please explain how stock breather hoses allows vacuum and external breathers dont?
HDF Community Team
Joined: Sep 2013
Posts: 2,351
Likes: 2,310
From: Wisconsin
This can not be explained because it is incorrect. External breather routing is ok with the stock set-up. Don't bother adding any extra vents to the engine.
Thread Starter
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Road Warrior
Joined: Jul 2011
Posts: 1,326
Likes: 331
From: Central TX
Last edited by txphatboy; Dec 5, 2021 at 03:54 PM.
HDF Community Team
Joined: Oct 2018
Posts: 4,344
Likes: 3,391
From: Seattle
You do not need any external venting(breather bolt to Filter/Catchcan) for the intake so long as you vent the transmission with something like a Trask Checkm8, or you have a Vented Dipstick with a 2020 M8 oil pump setup with the case seal(P/N 62400205).
The only thing you need once you have a proper oil pump is the vented trans or dipstick.
Nothing else is required for venting.
The best venting location is from the trans top cover.
Here's some video coverage of this by someone who went through the options:
The only thing you need once you have a proper oil pump is the vented trans or dipstick.
Nothing else is required for venting.
The best venting location is from the trans top cover.
Here's some video coverage of this by someone who went through the options:
Thread Starter
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Road Warrior
Joined: Jul 2011
Posts: 1,326
Likes: 331
From: Central TX
Yeah, these are some of the videos I was referencing. The videos are well done and very helpful, but I just dont get how rerouting breather lines from intake to outside the intake prevents a vacuum. The only way I can imagine this happening is if the suction on intake is greater than the vacuum on the breather hose, and does not allow any air to enter back into the breather hose. But they are basically open-ended hoses that can pull air in under vacuum. The umbrella valves should stop this from happening.
Another question - does the transmission cover vent work effectively if you still have the old oil pump with no seal? I am leaning towards the tranny cover mod, and will likely change out oil pump down the road, but am not having any sumping issues. If I am spending money and time on mods now, the Dark Horse Compensator will probably be next big money item.
Another question - does the transmission cover vent work effectively if you still have the old oil pump with no seal? I am leaning towards the tranny cover mod, and will likely change out oil pump down the road, but am not having any sumping issues. If I am spending money and time on mods now, the Dark Horse Compensator will probably be next big money item.
HDF Community Team
Joined: Oct 2018
Posts: 4,344
Likes: 3,391
From: Seattle
Yeah, these are some of the videos I was referencing. The videos are well done and very helpful, but I just dont get how rerouting breather lines from intake to outside the intake prevents a vacuum. The only way I can imagine this happening is if the suction on intake is greater than the vacuum on the breather hose, and does not allow any air to enter back into the breather hose. But they are basically open-ended hoses that can pull air in under vacuum. The umbrella valves should stop this from happening.
Another question - does the transmission cover vent work effectively if you still have the old oil pump with no seal? I am leaning towards the tranny cover mod, and will likely change out oil pump down the road, but am not having any sumping issues. If I am spending money and time on mods now, the Dark Horse Compensator will probably be next big money item.
Another question - does the transmission cover vent work effectively if you still have the old oil pump with no seal? I am leaning towards the tranny cover mod, and will likely change out oil pump down the road, but am not having any sumping issues. If I am spending money and time on mods now, the Dark Horse Compensator will probably be next big money item.
It talks about how the engine can push oil up the breather tube if you don't have an updated pump with the seal.
Banned
Joined: Jan 2019
Posts: 3,036
Likes: 1,013
From: Nunya.
I vent to the atmosphere as the stock system creates a venturi and can suck oil/mist/vapor from the breathers umbrella system if the system is not 100% sealing. The venturi could be what they're talking about it might create a pulse that works with the motor but it's insignificant I think .I noticed no change to my idle quality or the bikes running characteristics when I vented the head breathers on any bike I've owned , none . Just a cooler, cleaner running engine .
I don't want hot oil vapor going into my engine that causes carbon build up on the piston crown and dilutes my air fuel mixture and makes my bike run hotter. Look at a bikes pistons that has the head breather vented to the atmosphere(or catch can) and look at a stock bikes pistons after a 10 or 20 thousand miles and you'll see what I mean .
A lot of those videos are horseshit and people who have no idea quote them as gospel .Go ask any mechanic/engine builder how good the internet has been for his business , the smile will tell you all you need to know. Those videos are the "a fool and his money " thing , they're about making money. There are some good videos those Moonshine Harley guys go OK but act like fkwits doing it but if your knowledge base is from what some guy/s said on the net you actually don't know **** , spending a lifetime riding and working on these bikes gets you a knowledge base. Ask those guys IRL.
Talk to an engine builder or an old Harley greybeard guy about it as they know , internet blow ins don't. .
I don't want hot oil vapor going into my engine that causes carbon build up on the piston crown and dilutes my air fuel mixture and makes my bike run hotter. Look at a bikes pistons that has the head breather vented to the atmosphere(or catch can) and look at a stock bikes pistons after a 10 or 20 thousand miles and you'll see what I mean .
A lot of those videos are horseshit and people who have no idea quote them as gospel .Go ask any mechanic/engine builder how good the internet has been for his business , the smile will tell you all you need to know. Those videos are the "a fool and his money " thing , they're about making money. There are some good videos those Moonshine Harley guys go OK but act like fkwits doing it but if your knowledge base is from what some guy/s said on the net you actually don't know **** , spending a lifetime riding and working on these bikes gets you a knowledge base. Ask those guys IRL.
Talk to an engine builder or an old Harley greybeard guy about it as they know , internet blow ins don't. .
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Stellar HDF Member
Joined: Mar 2017
Posts: 3,312
Likes: 1,878
From: Conn.
I inquired about the trans vent cover with a call to fuel moto. I asked specifically for my stage 2. The tech told me with stage 2 they run the oil level at half way. I said I run mine at a cold check at half way on the dipstick. He replied perfect. I have no signs of oil in filter or on ends of stock rubber hoses. Not going to worry about unless something changes.
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HDF Community Team
Joined: Sep 2013
Posts: 2,351
Likes: 2,310
From: Wisconsin
Yes, adding that 1/2 quart of compressible air volume to the oil tank reduces the variation in pressure with piston motion and seems to help out the breather system. I too run the stock breather set-up on the M8 with the occasional use of carbon removing fuel additives.