[Diy_efi] MAP vacuum signal isolation?

[Tom Visel]

Your current source of vacuum shouldn't give you any problems with your 
MAP signal.  If you have a lumpy cam you might want to take measures to 
smooth the MAP vacuum signal.  MS folk are just bubbling over with 
application-specific info like this.

In general, the MAP should be on its own line, whether short, long, 
orificed or not, small or large diameter, and whether or not it has a 
plenum in-line.  If it has to share a line with something that uses 
vacuum, there may be surges of pressure as the other device turns on and 
off.  Since the advance can is a dead end, there is no MAP-based reason 
not to connect it to the same line.  It should just act as a 
buffer/plenum.  However...

Are you sure you want to connect the advance can to manifold vacuum?   
The sudden loss of advance as the throttle is opened can lead to a big 
stumble.  Ported vacuum - just above the throttle plates - is the 
"inside the box," conventional way of setting up vacuum advance.  When 
the timing is advanced at part throttle, cylinder pressures are allowed 
to rise (more torque) when they would otherwise be low, giving greater 
fuel economy.  Without timing control in your ECM, it might be best to 
get your engine management dialed in without the vacuum advance and then 
see what changes (if any) need to be made when the vacuum advance is 
connected.

Hope this helps,
TomV



pparaska at comcast.net wrote:

>I'm running a Megasquirt-I with a Holley Pro-Jection 4bbl TBI.
>
>I've been grabbng the vacuum signal off of a line from a full
>time vacuum source on the front bottom of the TBI (below the
>throttle plates.  I share that source with a vacuum advance can
>on the HEI distributor, through a T in the hose coming from the
>port on the TBI that splits off to the MegaSquirt's MAP sensor
>and also to the HEI vacuum advance can.
>
>Is there a problem with this kind of setup?  Could I be
>getting strange signals to the MAP sensor while the
>vacuum advance can is changing it's volume (diaphram
>moving)?
>
>In general, is it better to feed the MAP a signal straight
>from a manifold plenum that is removed in distance from
>the power brake booster and other vacuum ports (Vac
>advance, etc.)?
>
>TIA,
>Pete
>http://AlteredZ.com
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>
>  
>
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