Proper Venting

Leviathon
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Posts: 117
Joined: Mon Jul 14, 2014 10:04 am

Proper Venting

Postby Leviathon » Mon Feb 19, 2018 8:28 am

Out of curiosity - what is proper venting? PCV to Carb out of 1 VC, then just a breather on the other? Does it matter what side?
Steve and Mary Levi
1973 32' Marinette Fisherman
Twin Chrysler 360's
El Ohssa
St. Joseph, Michigan

EWRice
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Posts: 237
Joined: Thu Feb 04, 2016 12:37 pm

Re: Proper Venting

Postby EWRice » Mon Feb 19, 2018 10:11 am

Depends on your engine and wether or not it is stock or cranked up. Chryslers used a pcv in one valve cover and either a breather or vent hose to the spark arrestor on the other. Doesn't matter which valve cover is used for what. Mercruiser, Crusader, OMC, PCM and others only used a vent tube from the valve cover to spark arrestor, sometimes two. My graymarine is also this way.

The problem with pcv systems is that they were designed for cars and really only work under part throttle cruising conditions or slower. An engine develops more crank case gas the faster and harder you push it. Especially in a marine application. So it really is useless beyond part throttle and that is why it was abandoned in later years.

Some high performance engines use a crankcase evac system that uses exhaust flow to create a vacuum. That pulls harder the harder you run the engine regardless of throttle setting.
Muskegon Lake
1972 32' Express flybridge
"AL13"
Twin 318s
On board air & prime 920
1963 Thompson Super Sea Lancer
Graymarine 327
1961 Alumacraft 12'
'55 10hp Johnson

Leviathon
Aluminum Star
Posts: 117
Joined: Mon Jul 14, 2014 10:04 am

Re: Proper Venting

Postby Leviathon » Mon Feb 19, 2018 1:23 pm

Thanks - that helps. I will be running twin small block crusaders now set up with a PCV that goes to the base of the carb and the other VC is just capped off. That is the way it was set up before I rebuilt, so I guess I will go with it. (Rochester Carbs)
Steve and Mary Levi
1973 32' Marinette Fisherman
Twin Chrysler 360's
El Ohssa
St. Joseph, Michigan

EWRice
Royal Aluminum Star
Posts: 237
Joined: Thu Feb 04, 2016 12:37 pm

Re: Proper Venting

Postby EWRice » Mon Feb 19, 2018 1:35 pm

When I build engines for boats, I always set them up so the dipsticks and oil fill is easy to get to. On most twin engine boats that means the inside valve cover gets the oil cap. Makes life so much easier.
Muskegon Lake
1972 32' Express flybridge
"AL13"
Twin 318s
On board air & prime 920
1963 Thompson Super Sea Lancer
Graymarine 327
1961 Alumacraft 12'
'55 10hp Johnson