Need to get rid of my road draft tube on the Chevy 261, and I wonder, if s there are any cons going to ported vacuum at the carb with the PCV hose? Usually it should be connected to crankcase vacuum, but for several reasons I want to avoid that if I can. So I came up with the idea to use the free (now blocked with a rubber cap) ported vacuum port on my carb. I think I could even skip the use of a PCV valve at all, and just use a hose in this configuration. Your opinions please gentlemen. Frank.
Did you ever consider why the size of the vacuum line to a PCV valve is about 3/8" ID, while that to a ported vacuum connection is about 1/8" ID?
They are not designed for that. A 3/8” line to a V100 valve screwed to a plate under the carb or the manifold works fine with inlines. The hose can come from the rocker cover or the side breather tube. Make sure there is a deflector if using the rocker cover. Up on the old draft tube is far enough away from the crankcase for oil splash and already has them in the top area of the tube.
Opera Snapshot_2022-10-27_160141_i.pinimg.com by FishFry posted Oct 27, 2022 at 4:09 PM I cut away the actual road draft tube that goes down to the street, but keep the rest and feed the hose from the top, where the filler and baffle is (about a foot over the crank case), so I guess I'm safe regarding any oil splash.
Just to avoid some possible confusion, you'll need to connect the hose to a source of manifold vacuum. To visualize your plumbing requirements it will help if you think of the PCV system as two seperate, but ultimately connected components. One side of the system needs to connect to a vacuum source, like manifold vacuum. The other side of the system needs to vent the crankcase to atmospheric pressure so it can supply clean air to flow thru the crankcase. The need for clean air is why you see a filter of some sort used in the valve cover breathers or in a vented oil filler cap. In a factory PCV system you may also see a breather filter installed in the air cleaner housing.
No offense, but I know how it is done from the factory (or aftermarket) and how it all should work, like filtered air in/gases sucked out etc. - but as I said, I want to avoid using manifold vacuum for several reasons, that's why I asked if I could use the ported vacuum.
Here's a link to a Hamb discussion from 2019 on this subject with photos that may be helpful.... Technical - Discrete PCV system for a Chevy 235 | The H.A.M.B. (jalopyjournal.com)
Is there a reason why you can't run a 3/8 hose connected to manifold vacuum? will it mess with your Fish carb? as Squirrel said there is not enough flow from a 1/8 ported vac source. You may be better off leaving the road draft tube setup
What you are considering doing is not a PCV SYSTEM. It will not ventilate the crankcase properly. FORD nuts have tried the same with the FYB having the breather on the lower block skirt. It doesn't work. Why no manifold vacuum?
My manifold vacuum is already low (because of the cam) if I introduce a (though somewhat controlled) vacuum leak it will go down 2ish hg more. The ported vacuum port has pretty much the same vacuum numbers - actually it has way more at high rpm, when most of the blow-by gases are made, while the manifold vacuum would be almost zero - so at high rpm, that 3/8 hose does pretty much nothing anyway, and has no airflow at all, when it would be needed the most, while ported vacuum would be at it's max with way more airflow.
That doesn't make sense. How did you discover this? Did you measure ported and manifold vacuum at the same time (under the same operating conditions)? There's no way that ported vacuum can be higher than manifold vacuum.
If you to spend $$$ on a M/E Wagner dual port PCV valve they are made to operate on a low vacuum engine, actually any engine. The crankcase side breather needs a grommet at the top, the open hole plugged and a 3/8” hose to the intake under the carb. Crankcase vapors mix well with the fuel/air mixture directly under the carb is best. You can always install a catch can (oil separator) between the PCV valve and the intake as another measure of keeping oil out of the intake system. They work great and I have them on 2 OT cars.
Manifold vacuum is load dependent (aka throttle position) - so yeah at idle, manifold is much higher (also at cruising speed, when the throttle is almost closed) - but anything else than this, they are pretty much the same. Here is a test:
That's not ported vacuum, that's venturi vacuum. Works opposite of ported/manifold vacuum. Ford used that on the old 'Loadamatic' ignition systems, more recently it's been used for emissions purposes.
I don't want to come across as an axx, but I would just try what I wanted to do! Either works or not. Simple as that. NO conflicting ideas. My .02. Ben
Of course I try it, just wanted to make sure I don't miss anything important, or - best case - find somebody with experience cause he has already tried that. Can't imagine after 70 years of PCV that I'm the first one, coming up with that idea.
Unless you are building a land speed car you will spend more time at idle or part throttle than at wide open throttle, so you want the PCV to be plumbed where you will get vacuum most of the time.
It's a vacuum source that's ported into the carb venturi above the throttle blades. When the throttle is opened, velocity through the venturi creates a vacuum that increases with that velocity. Exactly opposite of 'normal' ported or manifold vacuum.
Manifold vacuum is under the throttle plate, ported is above. That venturi thing is just a fancy name for ported vacuum.
DAMN! Is this a street or strip engine? You have little choice with that low of a manifold vacuum value (what is the vacuum @ idle exactly - need to know this). You would have to plumb in an electric vacuum pump in some fashion. You may need to go here - https://www.summitracing.com/parts/sum-120108
Seems to me carb venturi vacuum with enough volume would likely mess with signal, an exhaust venturi would likely be better. Grant
No, they're three separate functions. Manifold vacuum gets its inlet source from below the throttle blades. Highest vacuum under closed throttle conditions, vacuum drops as the throttle is opened to near zero vacuum at wide open throttle. Ported vacuum has its inlet located just above the throttle blade in such a way that as the throttle is opened the inlet position relative to the throttle blade moves from above the blade to below, effectively becoming manifold vacuum and acting the same. This is commonly used to pull vacuum advance timing out at idle but can lead to overheating in extended idling conditions due to the 'late' timing. Venturi vacuum has its inlet located above the throttle blade and stays there. Works exactly opposite from manifold vacuum. Zero or near-zero vacuum with closed throttle blades, increasing with more opening with maximum vacuum at WOT. And moving enough volume of air to effectively vent the crankcase will be tough with a small hose. While it can easily pull a vacuum in an advance canister with its 2 or 3 cubic inch volume, the crankcase is hundreds of times larger. A 3/8" hose is none too large.
PCV = Positive Crankcase Ventilation. "Positive" is short for pressure...... in the crankcase...... that builds with RPM and/or load and needs to be evacuated. You want as close to 0-pressure in the crankcase as you can get. (Negative pressure, vacuum would be better) A PCV system needs a PCV valve to function correctly. And it needs sealed valve covers, no breathers. The small amount of 'vacuum leak' through a PCV valve at idle is not enough to have any effect on the engine operation to cause a bog or stumble. But having just the hose with no valve might.
No it's not, totally different. Manifold vaccum and ported vaccum are the vaccum that is created when the movement of the pistons down the bores causes a pressure differential in the space above the piston, which is transfered to the manifold through the intake ports, and into the carburetor via the throttle. Venturi vacuum is the vacuum that is created due to the effect of the air flowing through the carburetor venturi, as the air moves past an opening it draws air out of that opening. So at idle and low throttle opening there is a lot of manifold vacuum, but very little venturi vacuum because the air flow through the carburetor is limited. At WOT there is no manifold vacuum (or ported vacuum), but the airflow through the carburetor creates maximum venturi vacuum. As @Crazy Steve said, they work the opposite of each other. Whether the video was showing venturi vacuum or not I don't know, I'm just saying venturi vacuum is not just a fancy word for ported vacuum, they are two distinctly different things.
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