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bradself
Samba Member
Joined: January 13, 2013
Posts: 167
Location: SF, CA
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 Posted: Tue Dec 23, 2014 12:10 pm Post subject: Auto trans modulator vacuum signal?? |
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| Anybody know why the vacuum modulator signal comes from both in front of the throttle butterfly and on the plenum side? --brad |
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raygreenwood
Samba Member
Joined: November 24, 2008
Posts: 21562
Location: Oklahoma City
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| Posted: Tue Dec 23, 2014 1:22 pm Post subject: |
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Because the vacuum at just behind the throttle plate with somtimes be erratic and lower than it needs to be depending on throttle position. The manifold vacuum is more constant and higher at times.
I have had conversations a few years back with some of the early users of megasquirt with regard to manifold vacuum and using a MAP sensor. The vacuum varies widely in the manifold and runner system. Vw appeared to have done quite a bit of research regarding vacuum signature and placement of various hoses especially the MPS. Ray |
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Lars S
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Joined: October 04, 2007
Posts: 788
Location: Sweden
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| Posted: Wed Dec 24, 2014 11:48 pm Post subject: |
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In principle plenum vaccum is highest at closed throttle and lowest at VOT.
Upstream the throttle (in front of the butterfly) it is the opposite, lowest vaccum at closed throttle and highest at VOT.
At VOT both are wery much the same, at closed throttle they differs the most plenum vac beeing much higher.
Hope i got this right(?) 
/Lars S
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raygreenwood
Samba Member
Joined: November 24, 2008
Posts: 21562
Location: Oklahoma City
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| Posted: Wed Dec 24, 2014 11:59 pm Post subject: |
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Yes...quite well. The thing thtas most important to remember is that not all areas of the plenum and runner system are of the same vacuum level at the same time at anycgiven throttle position.
Its highly variable. The vacuum connection positions for various components...work quite sell...whereas c connecting, something like the MPS or the vacuum throttle dashpot to other places in the plenum. ..cause. ery different results.....not alwayscbad....but rarely the same.
It leads one to think....and this comes from a lot of personal experience as well.....that VW researched their plenum designs pretty well.
By the way....Merry Christmas to all. If you dont celebrate christmas.......just have a great evening all around! 
Ray |
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