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post #1 of 13 (permalink) Old 02-02-2011, 01:00 PM Thread Starter
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vaccuum

Does anyone feel like a brief disertation on engine vaccuum?
I know the trans ues it as does the carb&brake booster.
(am also baffled by the PCV valve going to a vaccum source(?) that makes NO sence)
Is there somewhere i can find out exactly where sall my vac. lines need to be on my 67 Goat?
thanks as always
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post #2 of 13 (permalink) Old 02-02-2011, 02:16 PM
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PCV = Positive Crankcase Ventilation
When under heavy throttle it is very possible to get compression blow by past the rings. The PCV valve directs this pressure to the intake manifold where it is burned rather then, like push out a rear or front main seal, or blow excess oil out the valve covers.
If your car is an A/C car, it may have a vacuum line going to the interior A/C controls also.
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post #3 of 13 (permalink) Old 02-02-2011, 02:19 PM
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Originally Posted by Topkat View Post
Does anyone feel like a brief disertation on engine vaccuum?
I know the trans ues it as does the carb&brake booster.
(am also baffled by the PCV valve going to a vaccum source(?) that makes NO sence)
Is there somewhere i can find out exactly where sall my vac. lines need to be on my 67 Goat?
thanks as always
Hey Top,
I'll take a shot at it. Engine vacuum / manifold vacuum is generated by the pistons of a running engine "sucking" against a restriction, in this case, the throttle plates of a closed/partially closed carburetor. That's why at "full throttle" when the restriction of the throttle plates is removed, vacuum drops to zero (or very close to it) - the engine is being allowed to "eat" all the air it wants. Vacuum is important for proper operation of the carburetor idle and part throttle circuits. When air-flow through the main carb venturis is very low (idle or part throttle) the carb allows engine vacuum to "suck" fuel through these passages. At wide open throttle (WOT) when the vacuum goes away, the idle and part throttle circuits in the carb stop working becuase there's not enough vacuum to activate them. Instead, now that the air flow through the carb is high, the main venturis come "on line" and begin operating to supply fuel (metered by the primary, secondary, and power circuits). The purpose of the accelerator pump in the carb is to supply a shot of fuel to help carry the engine through that transition point, when the idle/part throttle circuits are shutting down and the "main" circuits are coming on.
There are usually two types of manifold vacuum "connections" available on a carb, ported and non-ported. A non-ported connection "sees" real manifold vacuum from below the throttle plates. A ported source is located above the throttle plates in the carb, so it won't "see" full manifold vacuum until the throttle plates are open somewhat. Typically, the vacuum advance on the distributor is connected to a "ported" source and everything else uses a non-ported source. "Lots of stuff" has used engine vacuum as a source of "free" energy over the years. Back in the days of 6-volt electrical systems it was common to use it to power windshield wipers. That's why they'd stop working whenever you stepped on the throttle.

PCV, "Positive Crankcase Ventilation" is one of the earliest forms of "pollution control". Before PCV engines generally just used an open tube that vented the crankcase to the atmosphere. Problem was, on an engine with very worn rings/cylinders that generated lots of "blow by", all that smoke just vented. Crankcases do need to be vented, otherwise the "back sides" of the pistons are going to build pressure and tend to do things like pushing out gaskets and creating leaks, not to mention working against the power the engine is trying to make. Along comes PCV. The reason it's connected to engine vacuum is so that the vacuum will "suck" this pressure/gasses out of the crank case and feed it (along with fuel/air) into the cylinders to be "burned" and passed out with the rest of the exhaust. The reason you need a valve is to prevent air/fuel mixture from flowing the other way INTO the crank case at WOT where the heat and pressure can ignite it, causing a "crankcase backfire" which -really- does a number on gaskets and seals, besides being dangerous. The PCV valve acts as a one-way valve that allows crank case pressure and gasses out without allowing air/fuel in.

As far as how to route/connect all the vacuum hoses, from about 67 and up it got more and more complicated as emissions controls were introduced. I know on my 69 there was a vacuum valve mounted to the water crossover on the intake that "mixed" a combination of vacuum advance, vacuum retard, ported and non-ported vacuum all depending on engine temp. The best reference for your car would probably be a copy of the factory shop manual.

Hope that helped...

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post #4 of 13 (permalink) Old 02-02-2011, 07:00 PM
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Bear, you could be a technical professor at any college. (Heck, perhaps you ARE!). I've been an automotive tech for over 30 years, and have yet to read such a clear, easy to comprehend explanation of the engine as an "air pump". Excellent. The older non PCV cars of the '50's and back had the "road draft tube" that was cut at an angle on the end. An open crankcase breather was in line with the fan and radiator. The idea was, as the car went down the road, air would be drawn into the breather by engine vacuum and scavange the unburned gases and blowby out of the tube. Air passing past the tip of the tube sucked out the fumes by venturi action: the faster the car went, the better this system worked This worked ok, but not well at idle, and not nearly as well as a PCV system. Also, as stated, when the throttle is closed, the engine is "sealed" and will pull about 20 inches of mercury (HG) or "vacuum". When the throttle is snapped wide open, a huge air leak is created, letting atmospheric pressure inside the engine, dropping the vacuum to 0 inches. Venturi effect does the rest to fill the cylinders.
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post #5 of 13 (permalink) Old 02-02-2011, 07:08 PM
 
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Here is a diagram of a 67 PCV hose set up.



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post #6 of 13 (permalink) Old 02-02-2011, 07:23 PM
 
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Here is a pic of the vacuum line FROM the brake booster TO the front of the carb (below the throttle plates)............sorry it is a little blurry (all the drugs, pills, and inhalers that keep me breathing also make my hands shake)



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post #7 of 13 (permalink) Old 02-02-2011, 08:39 PM
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Wow, thanks for the kind words GeeTee...

I'm not a prof, I'm just an old fart professional computer nerd (mainframes) who's cursed with a simple mind, an incurable case of "stubborn", and who's too dang cheap to pay anyone to "fix" anything I own or build anything for me no matter what it is - so either I "outlast" it and figure it out, or I completely destroy it in the effort and have to replace it. My sweet bride thinks I can fix anything - she just doesn't realize I'm too dumb not to try anything and to mule-headed to quit.

Things have to be in simple terms for me to understand them, and the stubborn in me refuses to "just accept" something until it makes sense to me.

That's why to this day I still don't "get" quantum theory despite multiple attempts at reading about it. That stuff is just plain weird....

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post #8 of 13 (permalink) Old 02-02-2011, 09:43 PM
 
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You do mainframe operations or coding? I too date back to paper card and a world of 80 columns

I just finished an eBook about auto evolution. Just the stuff they went through to reach what we have for cabin heat is amazing.
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post #9 of 13 (permalink) Old 02-02-2011, 09:59 PM
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Originally Posted by Mike_V View Post
You do mainframe operations or coding? I too date back to paper card and a world of 80 columns

I just finished an eBook about auto evolution. Just the stuff they went through to reach what we have for cabin heat is amazing.
Howdy Mike, I'm one of those dinosaurs they call a "systems programmer". At my current job, I work with a team of people who are responsible for the care and feeding of CICS, running on a pretty large configuration of muliple parallel Sysplex's running z/OS (I still call it MVS most of the time) - if all that means anything to you.

When I write code, it's usually in Assembler.

(And I still haven't broken myself from the habit of calling them "columns". )

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post #10 of 13 (permalink) Old 02-02-2011, 10:13 PM
 
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Oh yeah - been there done that. I was part of the group getting TCP/IP working on the mainframe in the 90s, and writing all the interfaces for CA tools. I don't know if they still have it, but I used a z/OS (z390) emulator for a lot of testing. In no way was any of that fun - not even a little.
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