[Woodchuck]

Get (or modify yours) collectors with evac bungs, pipe and a couple Thermactor anti-backfire valves and have at it....

 

[Woodchuck]

What I really need to do is a belt drive vacuum pump

Why? Go electric if you have the need for power brake booster supply.

 

[Woodchuck]

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.

 

[Woodchuck]

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.

 

[Woodchuck]

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.

 

[Woodchuck]

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.

 

[Woodchuck]

Nope. Pressure is inversely proportional to velocity.

Exactly, and VACUUM is simply NEGATIVE pressure. LOW vacuum, HIGH velocity.

 

[Woodchuck]

Pressure is anything above zero. Vacuum is anything below zero. Zero is the absence of pressure or vacuum.

One inch of mercury equals 0.491154 pounds per square inch whether you're measuring vacuum or pressure.

 

[Woodchuck]

I think this is more an "argument" about definition than about physics. Out of curiousity, when I would install an annular booster in a carburator that increases the air flow velocity through it, would you say that the vacuum at the suction hole(s) to the jets has gone up or down (i.e. more or less vacuum)

The annular discharge booster doesn't increase airflow velocity any more than a straight-leg or down-leg booster of the same dimensions. The difference is in the multiple, small discharge holes around the inner part of the venturi that produces better fuel atomization than the other types. As far as vacuum at the "suction holes"... Don't confuse manifold vacuum, created by the movement of the pistons within the cylinders, with the venturi effect. They are two different "animals". One is responsible for drawing a fuel/air mixture into the cylinders, through the intake valves. The other is the pressure differential created by slowing down airflow using a restriction and using that differential to siphon fuel from a reservoir (float bowl). They are, for all intents and purposes, independent of one another as a large volume of air passing through the intake manifold and into the cylinders may result in a low manifold vacuum because there is low restriction (throttle plates open) that same flow of air passing through the restriction of the venturi will cause a large pressure differential from atmospheric pressure. Remember that the air does not remain restricted...after passing through the venturi the space widens and the air, once again, slows down.

 

[Woodchuck]

OK, the annular booster was not the best example. How about an airplane wing: speed up the air on the top side, pressure on the top goes down, wing goes up.

About the rest, I know all that. But the discussion was about vacuum in the crankcase and how that helped with reducing oil leaks. I then said that effect will be very low at WOT, because there's very little manifold vacuum at WOT. Then you brought in the Bernoulli effect but in my opinion you mixed up high and low vacuum and how it correlates to air velocity.

But let's not dillute this topic any more with this side discussion

LOL. One last shot....

At WOT (low vacuum), air flow, in terms of the volume of air moved and its velocity, is at its highest.

At IDLE (high vacuum), air flow, in terms of the volume of air moved and its velocity, is at its lowest.

VELOCITY is only possible when there is a pressure differential, otherwise the air mass is "stagnant".

VACUUM is created by placing a RESTRICTION in the path of the mass, SLOWING its VELOCITY. A perfect example of this is a carburetors throttle plates. When closed, vacuum above the throttle plates is nil, and there is not enough vacuum to pull fuel through the booster. Below the throttle plates the vacuum is high, and the pressure differential pulls fuel from the idle discharge holes. When the throttle plates are opened and the restriction removed, the pressure differential between above and below the throttle plates disappears and now its a differential between the intake vacuum and atmospheric pressure, pulling large amounts of air through the carburetor.

"Manifold Vacuum" will vary, depending on where it is measured. It will be highest directly below the throttle plates and lowest as it exits the manifolds intake ports. This pressure differential is responsible for forcing air into the cylinders when the intake valve opens. In fact, when the intake valve is closed, the velocity of the air mass will actually build positive pressure on the back side of the intake valve under large throttle openings as air piles up waiting to get in.

At WOT, and with the PCV valve open wide, you're going to get the maximum amount of crankcase blow-by evacuated due to velocity, not vacuum.

 

[Woodchuck]

OK, I will respect the topic starter's wishes and will reluctantly resist the urge to fire back

One last question though, just because I wonder what you mean:

What velocity do you mean, of the air flow through the throttle bores?

Yes. The speed of the air, in feet per minute, times the volume of the air, in cubic feet, is going to equal "CFM" or Cubic Feet per Minute.

FWIW, not "mad" at all... maybe CRAZY but not mad. lol. Enjoy a theoretical discussion about hot air, when it doesn't involve politicians.

 

[Woodchuck]

Come on guys, we all know the only really important question is, "What is the air speed velocity of an unladen swallow?"

If their wings ain't flappin' it's 32 feet per second, per second....

But how much does a hen way ?

"About two and a half pounds".

 

[Woodchuck]

Well then, since Woodchuck enjoys it so much

Strictly speaking, speed of air [ft/min] x volume of air [cubic feet] = something in [ft^4/min] but I'm sure you meant speed of air [ft/min] x flow area [sq ft] = volume flow [CFM]

The flow through the PCV valve is a function of the flow area and the pressure difference over the valve. A high air flow velocity through the carb and manifold does not necessarily create a low pressure at the PCV valve exit. Unless it is positioned in a low pressure wake area, which is possible.




I know, hence the "wise ass" comment

Yes, you are absolutely correct! I should have said the flow area, in square feet. On the other hand, any air passing over an orifice is going to induce a low pressure area. Think sandblaster gun or siphon-cup paint sprayer.

 

[Woodchuck]

My (better) response to "smart ass" ---- It better be ,it went to school every day I did.

I can tell you that a couple times I was a "wise ass" and my Dad made my ass smart.

 

[Woodchuck]

Just a couple of times?

I ain't no dummy.... After that, I busted my ass to make him happy... now I have to live with a crack right up the center.....

 

[Woodchuck]

Ok so I guess we have the info we need?? So what catch can will be a good one??

Whichever one drains back into the engine. My brain wanders..... when it comes to oil, why don't folks talk more about accumulators?

 

[Woodchuck]

‘Drain back’ catch can… Ive never used (or seen) that 🤷

Amazon.com: Oil Reservoir Tank with Breather Filter + Drain Valve 2 Port Plug (1 oulet + 1 intlet) Oil Catch Can,Compact Baffled Engine : Everything Else

Amazon.com: Oil Reservoir Tank with Breather Filter + Drain Valve 2 Port Plug (1 oulet + 1 intlet) Oil Catch Can,Compact Baffled Engine : Everything Else

www.amazon.com

 

[Woodchuck]

Not what i really want as that one has a air breather so its going to sucking air threw that filter and not total vacuum threw the PCV..

Then you leave the filter nipple "capped".