Crankcase ventilation pros and cons
Thread Starter
|
Cruiser
Joined: Mar 2025
Posts: 243
Likes: 87
From: SWFL
Crankcase ventilation pros and cons
My bike came to me with crankcase vented to atmosphere via a hose to bottom. My intake is oil free. Then I heard Kevin Baxter at Skunk Works explain that while doing that is great for the intake and pistons, it is also defeating the vacuum assisted crankcase ventilation that the motor was designed with, which can cause issues with the oiling system.
I am interested in knowing more about this, partly because I am having an odd oiling system thing going on with my bike as well. So I would like to hear some views on this.
Venting to atmosphere, good or bad?
I am interested in knowing more about this, partly because I am having an odd oiling system thing going on with my bike as well. So I would like to hear some views on this.
Venting to atmosphere, good or bad?
Thread Starter
|
Cruiser
Joined: Mar 2025
Posts: 243
Likes: 87
From: SWFL
The EPA didn't come along until the early 70's. And I don't think they told manufacturers how to build their engines, as in how they had to deal with crankcase fumes. But they did tell them here are the emissions numbers you need to hit from now on. The manufacturers did the rest. Policy aside, I'd like to know how/why venting to atmosphere effects other things in the motor.
Seasoned HDF Member
Joined: Jun 2012
Posts: 6,133
Likes: 2,526
From: CT/FL
I can't see how moving the hose out of the air filter housing and into the open atmosphere makes any difference at all to crankcase vacuum. The air filter is at atmospheric pressure or it would hurt performance.
ps, If you want to play around, add a one-way canister in the vent line to keep vacuum in the crankcase.
ps, If you want to play around, add a one-way canister in the vent line to keep vacuum in the crankcase.
Seasoned HDF Member
Joined: Nov 2006
Posts: 20,961
Likes: 19,055
From: The Wasteland
I cant see how venting to atmosphere would affect vacuum. The carb is still breathing through the air filter, and the small holes where the hoses go through the back of the carb plate are sealed off.
HDF Community Team
Joined: Nov 2012
Posts: 6,186
Likes: 2,412
From: USA
Positive crankcase ventilation actively draws blow by fumes out of the crankcase. An overboard discharge leaves the crankcase full of fumes, with basically only the excess being pushed out via the overboard discharge.
One can create a bit of a venturi, to help pull the fumes out while the vehicle is moving, by positioning the end of the discharge hose to a low vacuum area. The effect is small, but it is real.
The crankcase fumes contaminate the crankcase oil, and use up the additives in the oil that combat this contamination. You may even see the oil darkening earlier because of this.
The effect on oil was much worse long ago with carburetors and chokes. Today, with active fuel injection, the engines burn far more cleanly, and blow by contaminants are greatly reduced.
One can create a bit of a venturi, to help pull the fumes out while the vehicle is moving, by positioning the end of the discharge hose to a low vacuum area. The effect is small, but it is real.
The crankcase fumes contaminate the crankcase oil, and use up the additives in the oil that combat this contamination. You may even see the oil darkening earlier because of this.
The effect on oil was much worse long ago with carburetors and chokes. Today, with active fuel injection, the engines burn far more cleanly, and blow by contaminants are greatly reduced.
Trending Topics
Seasoned HDF Member
Joined: May 2008
Posts: 21,029
Likes: 12,550
From: South Carolina
HD Forum Stories
The Best of Harley-Davidson for Lifelong Riders
10 Worst Harley-Davidson Motorcycles Ever
Pouria Savadkouei
Killer Custom's Jail Break Is The Breakout That Refused to Blend In
Verdad Gallardo
Crazy Bunderbike Build Looks Amazing, But Is It Impossible to Ride?
Verdad Gallardo
Harley-Davidson Reveals Super Cool Cafe Racer Concept
Verdad Gallardo
Engraved Rebellion: Inside Bundnerbike's Glam Rock II
Verdad Gallardo
10 Motorcycles You Should Never Buy
Joe Kucinski
10 Things Harley-Davidson Needs to Fix in 2026
Verdad Gallardo
Southpaw Super Glide: A Left-Hand-Drive 1979 Harley FXE Built to Fit the Rider
Verdad Gallardo
The Best and Worst Harley-Davidson Moves of 2025
Verdad Gallardo
Thread Starter
|
Cruiser
Joined: Mar 2025
Posts: 243
Likes: 87
From: SWFL
There you have it already, hardline opinions on both sides of the discussion. They can't all be right. For those who say they can't see what difference it makes relative to vacuum, that's easy. Sure the carb is at atmosphere, but we are talking about intake vacuum here from cylinder suck. Hell, that is how many car dashboard vent functions worked, or at least used to. So, yeah, it makes a difference.
Oil consumption and new oil going black really fast are two of the issues I have on this bike with 135 psi compression. Basically wondering how and if this lack of vacuum crankcase situation is playing a part in that.
Oil consumption and new oil going black really fast are two of the issues I have on this bike with 135 psi compression. Basically wondering how and if this lack of vacuum crankcase situation is playing a part in that.
Seasoned HDF Member
Joined: Mar 2010
Posts: 21,006
Likes: 7,541
From: Poolville
There you have it already, hardline opinions on both sides of the discussion. They can't all be right. For those who say they can't see what difference it makes relative to vacuum, that's easy. Sure the carb is at atmosphere, but we are talking about intake vacuum here from cylinder suck. Hell, that is how many car dashboard vent functions worked, or at least used to. So, yeah, it makes a difference.
Oil consumption and new oil going black really fast are two of the issues I have on this bike with 135 psi compression. Basically wondering how and if this lack of vacuum crankcase situation is playing a part in that.
Oil consumption and new oil going black really fast are two of the issues I have on this bike with 135 psi compression. Basically wondering how and if this lack of vacuum crankcase situation is playing a part in that.