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Vacuum Canister

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e_thomp View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote e_thomp Quote  Post ReplyReply Direct Link To This Post Topic: Vacuum Canister
    Posted: Sep/29/2014 at 9:16am
I have a 69 Javelin with 390 engine and Borg warner M12 Auto Transmission. Since rebuilding the engine and adding a slightly oversized cam, I am having a problem with the transmission giving me a hard downshift. I was told that I needed to add a vacuum canister. I was wondering if I could use the factory canister that came with the AMC Air Conditioned cars. My car is not A/C but I have access to a good canister. I don't want to waste my time if (for some reason) it will not work for the transmission.
Thanks for your help.
Eric
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uncljohn View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote uncljohn Quote  Post ReplyReply Direct Link To This Post Posted: Sep/29/2014 at 10:21am
An AC canister generally used with the later model AMC cars (and others) has a one way check valve in it that holds the vacuum at it's highest level to insure that the controls on the dash board which are vacuum operated, do not change position when the engine looses vacuum such as it would when passing some one or climbing hills.  If the transmission uses vacuum to sense a need for a shift and the higher the vacuum the firmer the shift than something like that would work.  There is a down side to consider, the vacuum tap for the transmission does change as engine loading changes.  I guess I don't know how important it is that the vacuum level match engine loading is, but it is a factor anyway and by adding a cam shaft to the engine you have in general reduced the amount of working vacuum at any given engine loading condition. 
So try it, don't know if it would hurt anything but can not see a reason not to try it. 
The transmission is indeed an old school design so just be aware of what is going on so you can deal with it if you have to. 

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e_thomp View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote e_thomp Quote  Post ReplyReply Direct Link To This Post Posted: Sep/29/2014 at 10:54am
Thanks John. I appreciate the information. I have seen canisters from JEGS and other places but I can get the factory one a little cheaper and most of all, the factory one looks like original equipment and I am trying to keep the car looking as original as possible.
Thanks Again,
Eric
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uncljohn View Drop Down
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Post Options Post Options   Thanks (1) Thanks(1)   Quote uncljohn Quote  Post ReplyReply Direct Link To This Post Posted: Sep/29/2014 at 11:19am
The Pacer used one that looked like a coffee can painted black, other AMC models used a plastic one shaped about the size of a base ball or a little bigger..  GM used the same one.  They were used in the later AMC cars with vacuum controlled rather than cable controlled heater and A/C systems. I do not think earlier AMC cars used them at all. But I do not see many of the pre-1970 models any more and everything I own or have owned is 1970 or newer.  Just install one so that when the engine draws a vacuum, (some times it is easier to think of that function as negative pressure) that the vacuum gets held in the cannister if the other end is plugged or hooked up to something. I do not remember if anything is marked on them at the moment. 



70 390 5spd Donohue
74 Hornet In restoration
76 Hornet, 5.7L Mercury Marine Power
80 Fuel Injected I6 Spirit
74 232 I-6, 4bbl, 270HL Isky Cam
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e_thomp View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote e_thomp Quote  Post ReplyReply Direct Link To This Post Posted: Sep/29/2014 at 12:04pm
I guess my next question is, Do I need one with the check valve or not? I see them both ways.  Personally, I do not see how they do anything without a check valve but I could be wrong.
Eric
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Post Options Post Options   Thanks (1) Thanks(1)   Quote uncljohn Quote  Post ReplyReply Direct Link To This Post Posted: Sep/29/2014 at 12:18pm
The only ones I have seen that AMC has used has a check valve in them as the purpose was to control the vacuum operated heat/AC controls in the later model AMC cars and of course because I was looking for them, I found them in the same application in GM vehicles too. 
I see no value in them for what you want to use them for unless there is a check valve in them or if not a check valve a really really ---- really tiny tit connected to the engine so that it lags the engine intake manifold vacuum changes. 
What you are trying to do is see to it that a larger value of vacuum is available at the transmission and if the canister does no more than follow the lack of pressure in the intake manifold, it is no better than having a piece of rubber tubing connected. 
Like I say I am not sure it will do what you want simply because I use the later Torqueflyts which are not controlled that way.  If the check valve is too much of a restriction and you won't know with out trying one,  a large canister with an adjustable bleed might work too.  If you are not familiar with them they might be hard to find, but as the 70's progressed into the 80's pre computer control used a lot of  these things  in controlling smog during that era where there was high flow in and low flow out and things of that nature. 
Also hard to find as late model AMC's are primarily worth $300.00 a ton when flattened. 


70 390 5spd Donohue
74 Hornet In restoration
76 Hornet, 5.7L Mercury Marine Power
80 Fuel Injected I6 Spirit
74 232 I-6, 4bbl, 270HL Isky Cam
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Post Options Post Options   Thanks (1) Thanks(1)   Quote Photon440 Quote  Post ReplyReply Direct Link To This Post Posted: Sep/29/2014 at 12:42pm
Forgive me if I'm wrong, it's been a long time since I had a BW auto apart, and that was for a six cylinder so may be different.  But doesn't your modulator have an adjustment screw inside for the purpose of varying the shift time or firmness?  I found this example from an International site:


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e_thomp View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote e_thomp Quote  Post ReplyReply Direct Link To This Post Posted: Sep/29/2014 at 1:52pm
I was leaning towards the modulator also, but my transmission guy stated that he had adjusted it and still was not able to get it right. He felt as if the modulator was working correct it just was not capable of doing the job with the cam that was in the engine.
Eric
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Post Options Post Options   Thanks (0) Thanks(0)   Quote purple72Gremlin Quote  Post ReplyReply Direct Link To This Post Posted: Sep/29/2014 at 6:07pm
For the modulator to work right, you need 11 or more all the time at. the modulator.   
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Post Options Post Options   Thanks (1) Thanks(1)   Quote purple72Gremlin Quote  Post ReplyReply Direct Link To This Post Posted: Sep/29/2014 at 6:10pm
Hard downshift?    It does or doesnt downshift?    A mild cam shouldnt cause problems unless you got one of those "thumper" cams?
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