Here's a nice visual of ported vs. manifold vacuum, from a Ford 5.0
site:
http://www.carbdford.com/tech/portedvsmanifoldvacuum.htm
Vacuum retard is independent of the source of the vacuum, although as
Bob says it's usually used with a ported source. The vacuum diaphram on
the dizzy is placed such that the points plate (or the equivalent in the
45DE4 dizzy) is rotated in the opposite direction as a vacuum advance,
thus retarding the timing. This is why the Bently manual states that the
dynamic timing spec for most stock US 1500's is 2 degrees ATDC with the
vaccuum hooked up.
-=Chris
<-----Original Message----->
From Robert E. Shlafer
Sent: 5/5/2004 10:37:47 AM
To: james.f.juhas@snet.net
Cc:
pasgeirsson@worldnet.att.net;antony@antgel.co.uk;spridgets@autox.team.ne
t
Subject: Re: vacumn advance
Yep...
There's ported vacuum "before" (the butterfly) and there's ported vacuum
"after" the butterfly (intake manifold).
You see the former on pre-emission
Spridgets and the latter on the later emissions engines, usually.
And of course, they work differently,
the latter being more of a vacuum
retard rather than advance I think but,
I'm not positive on this 'cause I always
got rid of the emissions stuff and used
a mechanical dizzy (#40819 - Cooper S
mechanical (only) curve) in place of
whatever was in there originally.
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