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bcclew

Regular Contributor
i used to have crusaders now i have 5.7 chev long blocks edelbrock intake and edelbrock carbs i am wondering why these engs never use pcv system? inoticed a lot of soot on choke plate. comming from valve cover vent hose too flame arester.so again i am wondering why not a pcv?thanks for any help
 
Wikipedia has a good article on PCVs and crankcase ventilation in general. With this knowledge you can figure out how your system is intended to work. You can bet there is some kind of ventilation scheme in place.

CaboJohn
 
I doubt that it is soot on the choke plate...

Remember PCV systems were driven by EPA for automobiles...like John infers, turns out to have other benefits, too.

The hoses to the air cleaner provide crankcase venting, it just isn't "positive". An educated guess says two big drivers: 1) cost associated with changing parts - impacts lots of things, and 2) simplicity - the hose to the flame arrester system has no moving parts...
 
Rereading your post about your Edelbrock setup, you say "these engines never use pvc system". I could be incorrect, but I think that is dead wrong and that if you don't have a pcv valve you likely have an incorrect setup done by someone that thought he was making an improvement but was not. Perhaps it was intended for race cars that change oil every few passes. All engines have some blow by at the rings, particularly at high rpm or after some wear. This means small amounts of moisture, raw gas and combustion gas are entering the crankcase. For long oil/engine life these gases need to be vented out. In the old days there was a vent tube, but there are engine life advantages (as well as pollution advantages) to "sucking" out these gases (i.e. positive crankcase ventilation). If you only have one hose venting (as I think you are saying), you will keep from pressurizing the crankcase (which would be bad for the seals) but you will not evacuate these harmful gasses.

The basic pvc system is to have a hose to let fresh air into the crankcase, usually running from a point above the carb at the air filter (or spark arrestor) to a rocker arm cover and then have a second hose (for suction) connected from the crankcase (via a rocker arm cover) to some point that sees suction, usually directly into the intake manifold or to a plate below the carb. The problem with this basic system is that when the engine needs venting the least (at idle), it has the highest vacuum and sucks the hardest. This is also is the time when the extra air coming though the crankcase to the manifold (and bypassing the carb) is most likely to have a bad effect on the fuel/air ratio and the idle performance. The PCV valve resides in the suction hose and closes in times of high vacuum (at idle) to limit the amount of extra air the engine can get. So unless someone has done something very clever to provide crankcase suction, you need two hoses and a pcv valve.
CaboJohn
 
Certainly the older big block Crusaders did not use PCV. Two hoses up to the outside of the flame arrestor is it. One engineering issue is the high power setting, for hours at a time, that the marine gas engine endures. High power settings are low vacuum, meaning a low capacity to scavenge the crankcase, just when it needs it the most. Overwhelming a PCV valve from blowby will cause crankcase positive pressure, and can cause seal failures. The operating risk is low in using vent hoses to the flame arrestor. This may be one reason for recommending 50 hour oil changes.
 
Just for sh!ts and giggles, do a search re; PCV valves used in Marine applications. You'll be surprised to see how many people just don't understand the PCV valve's role in crankcase ventilation..... let alone where and how to plumb it in.
Marine loads are achieved while under wider throttle positions, which reflects lower manifold pressures (i.e., vacuum).

PCV valves became a component of emission control systems in automobiles that do not necessarily apply to Marine Engines. Some used a valve with mild spring tension, some were not spring loaded.
I don't think that you'll see these used on many early Marine engines.
Some of the later engines and the MPI engines brought this back.

Some PCV valves have a gravity specific value, and you'll see that most are mounted rather vertically. These also require X amount of manifold pressure to lift the valve and cause crankcase gasses to flow.

While we do add a clean air source to the crankcase (to sweep the gasses away), crankcase gasses alone may create more volume than what the incoming air breather provides.
IOW, it's not XX CFM in, and the same XX CFM out via the breather source. The gasses escaping past the piston rings and valve guides offset an otherwise in/out balance.

Speaking of crankcase pressure, can you imagine the pressure needed to actually blow a main seal lip?
To a degree, I'd have to consider this a myth, unless the engine offered zero means of ventilation!

Our marine carburetors are not ported for a PCV valve.
The port in the intake manifold is not good option either, in that this is AFTER any fuel metering, and we could risk leaning out a cylinder or two, if under the right conditions.
A lean F/A may lead to detonation and eventual piston damage while under Marine engine loads.
Automotive engine loads -vs- Marine engine loads = Apples/Artichokes.

If you are Hell Bent on installing a PCV valve system, use caution with regard to where and how you supply vacuum for it.

.
 
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As I've said before, you can learn something new every day. That's why I enjoy getting you guys that have done this for years to weigh in. Clearly the lack of good ventilation on many marine engines would help explain why traditionally the oil is changed much more often than on car engines. I have read that modern oils really do not "break down under high loads", and the reason for changing them is contamination so this fits. My marine engines do have PCV valves, but because they are MPI style, the fuel is added just before the valves and leaning is not an issue. The fuel is metered based upon manifold vacuum (not throttle position), and the computer does not care what the source of the air might be.
CaboJohn
 
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