Crankcase ventilation pros and cons
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Joined: Jun 2012
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From: CT/FL
@foxtrapper PCV routes crankcase fumes into the intake manifold where there is actual vacuum to help draw them out. We are talking about a hose to the air filter here, right?
The timed breather and later umbrella valves in the head make this a one-way action driven by crankcase pressure when the pistons are dropping, and sort of keep a vacuum on the crankcase when the pistons are rising.
The timed breather and later umbrella valves in the head make this a one-way action driven by crankcase pressure when the pistons are dropping, and sort of keep a vacuum on the crankcase when the pistons are rising.
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From: Poolville
@foxtrapper PCV routes crankcase fumes into the intake manifold where there is actual vacuum to help draw them out. We are talking about a hose to the air filter here, right?
The timed breather and later umbrella valves in the head make this a one-way action driven by crankcase pressure when the pistons are dropping, and sort of keep a vacuum on the crankcase when the pistons are rising.
The timed breather and later umbrella valves in the head make this a one-way action driven by crankcase pressure when the pistons are dropping, and sort of keep a vacuum on the crankcase when the pistons are rising.
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From: South Carolina
An FYI for those unfamiliar with them, when the pistons are on the upstroke on an Evo or Shovelhead, the breather gear closes so air cannot be drawn back into the crankcases.
Last edited by Dan89FLSTC; Nov 4, 2025 at 10:10 AM.
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Tourer
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From: SWFL
"honestly with only 135 PSI cranking pressure I would say that you're getting way too much carbon down into your oil via the Rings, so don't be blaming it on engine venting.."
I'm not blaming anything just yet, it is just a question. My FSM states 90 psi or higher compression with less than a 10% difference between cylinders is considered normal. I'm at 135 psi so I don't follow that logic. But I do understand it could be the rings and blow by, just haven't seen the evidence yet.
Also, my bike is carbed, with a breather gear. I plan to go into the cam chest soon, so I will check the gear when I'm in there. Also, replacing the umbrella valves when the rocker boxes are off.
.
I'm not blaming anything just yet, it is just a question. My FSM states 90 psi or higher compression with less than a 10% difference between cylinders is considered normal. I'm at 135 psi so I don't follow that logic. But I do understand it could be the rings and blow by, just haven't seen the evidence yet.
Also, my bike is carbed, with a breather gear. I plan to go into the cam chest soon, so I will check the gear when I'm in there. Also, replacing the umbrella valves when the rocker boxes are off.
.
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From: Coromandel Peninsula New Zealand
The breather line on my shovel exits by the number plate, it spits out drops of milkshake on the first hard run after it has been sitting, but nothing is contaminating the oil.
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From: Cental, MI
I defy Anyone to Suck anything In from a Crankcase Vent line... Tho it makes Perfect sense by the "numbers" It Just don't happen.. Only Pressure can be Measured there...
The only Plus to routing the breather to the Intake would be to help evacuate Crankcase Pressure!! Now That can be Proven. It is also a proven Fact that any way that we can relieve Crankcase Pressure helps Performance!
In the old Days racers employed some very convoluted methods to help "draw" from the crankcase Vent.. Because until we could use Mechanical means (a Vacuum Pump) we were required to Only rely on Engines own functions..
ie the Intake worked well...Only If, the oil could be Separated.. I never did...as I could Not separate the Oil..
Now.. the Mechanical pump...alot like the New Racing Oil Evacuation Pumps..can Almost get Crankcase pressure to zero!! A certain Performance Enhancement..
The only Plus to routing the breather to the Intake would be to help evacuate Crankcase Pressure!! Now That can be Proven. It is also a proven Fact that any way that we can relieve Crankcase Pressure helps Performance!
In the old Days racers employed some very convoluted methods to help "draw" from the crankcase Vent.. Because until we could use Mechanical means (a Vacuum Pump) we were required to Only rely on Engines own functions..
ie the Intake worked well...Only If, the oil could be Separated.. I never did...as I could Not separate the Oil..
Now.. the Mechanical pump...alot like the New Racing Oil Evacuation Pumps..can Almost get Crankcase pressure to zero!! A certain Performance Enhancement..
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From: USA
Originally Posted by Joe12RK;[url=tel:22150342
22150342[/url]]@foxtrapper PCV routes crankcase fumes into the intake manifold where there is actual vacuum to help draw them out. We are talking about a hose to the air filter here, right?
Harley does use the hoses that run to the back of the air filter to vent the crankcase to the combustion chambers and burn off the fumes.
Engine design significantly alter the design needs in order to do this. Single cylinder engines can use the piston strokes to pump fumes out of the crankcase, very much akin to 2-stroke engines. A multi cylinder engine with an open crankcase will not generate that pumping action in the crankcase, so it must utilize intake vacuum more directly in order to achieve that effect. This is largely why automobiles use venting directly into the intake manifold. They cant make it work in the more passive manner that a single cylinder or V-twin engine can.
Seasoned HDF Member
Joined: Jul 2014
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From: Cental, MI
In broad terms, any pcv system is drawing fumes out of the crankcase, and running them through the combustion chamber(s) to burn them off. The exact means of doing this differ with the different manufacturers, engines and time era.
Harley does use the hoses that run to the back of the air filter to vent the crankcase to the combustion chambers and burn off the fumes.
Engine design significantly alter the design needs in order to do this. Single cylinder engines can use the piston strokes to pump fumes out of the crankcase, very much akin to 2-stroke engines. A multi cylinder engine with an open crankcase will not generate that pumping action in the crankcase, so it must utilize intake vacuum more directly in order to achieve that effect. This is largely why automobiles use venting directly into the intake manifold. They cant make it work in the more passive manner that a single cylinder or V-twin engine can.
Harley does use the hoses that run to the back of the air filter to vent the crankcase to the combustion chambers and burn off the fumes.
Engine design significantly alter the design needs in order to do this. Single cylinder engines can use the piston strokes to pump fumes out of the crankcase, very much akin to 2-stroke engines. A multi cylinder engine with an open crankcase will not generate that pumping action in the crankcase, so it must utilize intake vacuum more directly in order to achieve that effect. This is largely why automobiles use venting directly into the intake manifold. They cant make it work in the more passive manner that a single cylinder or V-twin engine can.
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