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It seems that at some level of increased performance, the original PCV valve spec does not flow enough air to relieve the crankcase of pressure.
I have an "automatic" pressure relief valve. If I exceed 6,000 rpm or so, the pressure pops my dipstick up about 2 inches.
 

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1965 Mustang GT. 11.898 @ 113.646, all motor, three pedals
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Discussion Starter · #22 ·
Whereas I exceed 7000 rpm every time out of the garage 😝 thus the need for more efficient crankcase evac. Primarily for high rpm ring seal, but also for oil leaks that I’m getting real tired of fixing.
 

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I should mention that I've had some good luck with the PCV valve for a 2003 Mustang Cobra that was equipped with a large positive displacement supercharger from the factory. The valve is identical dimensionally to the stock late model 5.0L PCV valve, but flows significantly more air. Motorcraft EV153.
 

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Ok, I'm stupid. Why won't a road tube flow enough pressure?
Road tubes do a good job while the vehicle is in motion. The air flowing across the opening causes a vacuum and sucks the gases and such out of the crankcase. When the car is idling at a stop, then no air is moving across the opening of the road tube.
 

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Ok, I'm stupid. Why won't a road tube flow enough pressure?
Well, I never said that, and a draft tube will flow as much as much as atmospheric pressure will allow to be spit out a 5/8 or so inch ID pipe... The benefit of a PCV system is two-fold.... one, the gases are evacuated from the crankcase under suction, for 99% of the time when vacuum is present and, two, redirected into the combustion chamber to be burned vs. puked out into the atmosphere as a pollutant or puked out onto the ground.
 

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1965 Mustang GT. 11.898 @ 113.646, all motor, three pedals
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Discussion Starter · #29 ·
I’m reading the tech info. Tuning recommendations are (essentially) based on greater than/less than 10” idle vacuum. My deal makes 11” Hg, so I’m right on the in-between edge Lol 🤣

I haven’t been this excited to play with new/shiny stuff for a while. This is gonna be fun 👍
 

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A properly functioning PCV is a joyous achievement - mainly due to the lack of oil leaks and how much crisper and cleaner the engine performs throughout the operating range.
 

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A properly functioning PCV is a joyous achievement - mainly due to the lack of oil leaks and how much crisper and cleaner the engine performs throughout the operating range.
Ok, but why no leaks? Is it because of trapped pressure? Why is the pressure trapped if you have an "open" system with a breather cap on one side and a tube without a PVC on the other?
 

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1965 Mustang GT. 11.898 @ 113.646, all motor, three pedals
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Discussion Starter · #32 ·
Ok, but why no leaks? Is it because of trapped pressure? Why is the pressure trapped if you have an "open" system with a breather cap on one side and a tube without a PVC on the other?
Without vacuum evacuation, the flow capacity is limited to the ID of the holes/grommets in the valve covers (and the breathers themselves), and it’ll build crankcase pressure. All that pressure has to go somewhere, often past the crankshaft seals, causing leaks. This usually isn’t a problem with typical street engines operating at typical street rpm as PCV systems were designed for that (relatively low cylinder and crankcase pressure)

I lost the ‘typical’ part long ago 😜
 

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Of course, during "non-typical" driving, e.g. WOT (=typical for ArizonaGT as I understand it :)), there's no vacuum evacuation, since there's (almost) no vacuum at the source (the intake manifold plenum) and the ventilation just bleeds of the pressure.
 

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1965 Mustang GT. 11.898 @ 113.646, all motor, three pedals
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Discussion Starter · #34 ·
Of course, during "non-typical" driving, e.g. WOT (=typical for ArizonaGT as I understand it :)), there's no vacuum evacuation, since there's (almost) no vacuum at the source (the intake manifold plenum) and the ventilation just bleeds of the pressure.
And that^
My current deal is fairly high strung, but my soon-to-be Dart 363 will embarrass this little Mexican 333 that I’m running now. The 363 will get an appropriate evac system for near zero manifold vacuum at WOT. It’ll need it 😉
 

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Of course, during "non-typical" driving, e.g. WOT (=typical for ArizonaGT as I understand it :)), there's no vacuum evacuation, since there's (almost) no vacuum at the source (the intake manifold plenum) and the ventilation just bleeds of the pressure.
Well, actually there is. At WOT vacuum is low, but not absent. Bernoulli says that a low vacuum velocity is high and, at low vacuum, the PCV valve is wide open. Therefore you're going to move a fair amount (in volume) of crankcase vapors as opposed to when the throttle (and PCV) are near closed.
 

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Well, actually there is. At WOT vacuum is low, but not absent. Bernoulli says that a low vacuum velocity is high and, at low vacuum, the PCV valve is wide open. Therefore you're going to move a fair amount (in volume) of crankcase vapors as opposed to when the throttle (and PCV) are near closed.
Sure, there is some vacuum, but very little, unless the carb is too small. And yes, the PCV valve opens up to compensate. But the discussion was about a reduction in leaks if the PCV system keeps the crankcase pressure below atmospheric. My comment to that was that at WOT there will be very little vacuum in the manifold and even less in the crankcase and thus the influence on leakage will also be very small. Especially considering the blowby will be higher due to the high cylinder pressures (even though the relative loss of the charge doesn't necessarily have to be worse)

BTW: Bernoulli says that pressure is reduced when velocity goes up.
 

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Sure, there is some vacuum, but very little, unless the carb is too small. And yes, the PCV valve opens up to compensate. But the discussion was about a reduction in leaks if the PCV system keeps the crankcase pressure below atmospheric. My comment to that was that at WOT there will be very little vacuum in the manifold and even less in the crankcase and thus the influence on leakage will also be very small. Especially considering the blowby will be higher due to the high cylinder pressures (even though the relative loss of the charge doesn't necessarily have to be worse)

BTW: Bernoulli says that pressure is reduced when velocity goes up.
Bernoulli's Law applies whichever way you want to state that vacuum is inversely proportional to velocity. Air has mass so if you're moving 100 cfm of air at a pressure differential of 2"hg that's moving more air than if you moved 50 cfm at 4"hg. Also bear in mind how much pressure you need to build in the crankcase to cause oil, which isn't under any other pressure, to leak.

As far as the aspect of the PCV system managing crankcase pressure, the system only needs to flow as much air as blow-by generates, and only needs a small pressure differential to do so. If the size of the PCV valve orifice, hose, tube, etc., can't do that then no magical PCV valve can overcome those restrictions.
 

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Bernoulli's Law applies whichever way you want to state that vacuum is inversely proportional to velocity.
Nope. Pressure is inversely proportional to velocity. Lower pressure means more vacuum. Like in the throat of a carburetor. The flow area is reduced, the air velocity increases and pressure in the throat decreases relative to surroundings (=vacuum). The higher the velocity, the more vacuum.
 
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