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Author
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Topic: Vacuum tree in thermo housing?
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Fastback68
Gearhead Posts: 2089
From: Sucat, Paranaque, Philippines
Registered: Jul 99
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posted 04-27-2003 11:19 PM
   
I just started studying vacuum, and I've got a couple of Grade 1 questions!
On a 68 302 4V, the thermostat housing also houses the vacuum tree for the distributor advance. According to Osborn's, the distributor connects by one hose to the tree, and two hoses lead from there, one to the carb and one to another vacuum tree at the rear of the intake manifold.
1) Since the thermostat housing is full of hot water, doesn't this mean there will be a supply of water vapor to the carb?
2) Why does the tree need two hoses going out (to the carb and the other tree) just to advance the distributor? Is this to maximize vacuum, or just to keep it more steady?
Thanks  So much to learn ...
Simon
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H8CHEVY
Gearhead Posts: 278
From: Southern Cali
Registered: Jun 2000
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posted 04-27-2003 11:48 PM
If your not running your smog equipment, just disconnect the tree and run the vac advance off the dist. straight to the carb. ------------------
"In the wise words I was once told bye Jiggy"
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KULTULZ
Gearhead Posts: 470
From: Rockville, MD
Registered: Oct 2002
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posted 04-27-2003 11:59 PM
quote:
1) Since the thermostat housing is full of hot water, doesn't this mean there will be a supply of water vapor to the carb?
No. It (vacuum control) uses the heat of the coolant to signal when vacuum will be available to whatever the vacuum will be operating. You most likely have a twin diaphram distributor vacuum advance and the additional vacuum signal will be used to either retard the vacuum advance (for emission reasons) or speed up the engine if it is operating too hot (putting manifold vacuum signal on the advance rather than ported vacuum signal). quote:
2) Why does the tree need two hoses going out (to the carb and the other tree) just to advance the distributor? Is this to maximize vacuum, or just to keep it more steady?
As stated before, emission control. It might be best to record how the vacuum lines are routed and then defeat this system by running a ported vacuum signal straight to the distributor advance. Later models will use a like system to control the EGR valve, heat riser, etc.
[This message has been edited by KULTULZ (edited 04-28-2003).]
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Fastback68
Gearhead Posts: 2089
From: Sucat, Paranaque, Philippines
Registered: Jul 99
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posted 04-28-2003 12:27 AM
Thanks loads! This is fascinating, and I need time to digest it. Just one thing though (I don't know how important it is):
quote:
You most likely have a twin diaphram distributor vacuum advance
The 302 4V auto came with a single diaphragm.
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horseballz
Gearhead Posts: 234
From: LasVegas,NV
Registered: Dec 99
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posted 04-28-2003 10:55 AM
Simon,
First question is: does or did your car have a smog/air pump?
Second question: are you sure that you have the original distributor? I've always been under the impression that all (small blocks at least) had the dual diaphram distributor in '68?
As far as the "vaccuum tree" it is a temperature activated vaccuum switch that has vaccuum supplied to it from 2 different sources, one from the carb and one from the manifold. When engine is cold, it supplies vaccuum to your distributor advance from one source (manifold I believe) and after the engine warms up it supplies from the other (carb). I might be exactly backwards on where it comes from when, but you get the idea. Also tied into this system in some way that I don't remember are feeds to the smog/air pump and the flapper in the original air cleaner. Like H8CHEVY said earlier, if you have neither the pump or stock air cleaner, you can ignore/delete/remove this switch, unless you have some other need for for it. 
Gene------------------
'68 coupe
289/3speed(soon to
be 5.0/5speed)
'85 F150 Beater
300/Overdrive Stick
Pre-dented&scratched
'02 Ranger 4x4 Edge
'02 Miata
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Hell_Fish
Gearhead Posts: 712
From: Austin, TX.
Registered: May 2002
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posted 04-28-2003 11:43 AM
I had the same question about my 351. Thanks guys.------------------
Dustin C
"The Mopar Guy!"
56 F-100
Mech. for wifes 69 Mustang coupe
65 Plymouth Barracuda
70 Dodge Swinger
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KULTULZ
Gearhead Posts: 470
From: Rockville, MD
Registered: Oct 2002
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posted 04-28-2003 01:40 PM
If you don't want to remove the installation because say you are restoring the vehicle, you can insert input shaft needle bearings into the appropriate vacuum line to defeat the system.
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