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Interesting
https://www.slantsix.org/forum/viewtopic.php?t=31071		 Page 1 of 1
Author:  68barracuda [ Sat Sep 13, 2008 8:03 am ]
Post subject:  Interesting
http://www.gofastnews.com/board/technic ... ified.html
Author:  otherroutes [ Mon Sep 15, 2008 5:37 am ]
Post subject: 
Thanks for posting that, I found it really interesting. In particular, the explanation of why vacuum advance is required is especially clear.

I'd be interested to hear /6 forum views on Steve Davis' view as to why the vacuum can should be, contrary to established wisdom, connected directly to manifold vacuum rather than ported.

If you've not read the full article (which you probably should to get this in context), he says:

"Ported vacuum is sourced just above the carb butterflies. At idle there is no vacuum signal to the distributor so no vacuum advance is pulled in. This means at idle the timing is way retarded over what is otherwise needed. It is only when the throttle is opened slightly that the butterfly goes past the vacuum port. At this point the port is now connected to manifold vacuum and advance is pulled in.

Manifold vacuum taps into the intake manifold below the carb throttle plates so actually sees manifold vacuum. If you are looking for the best idle it’s manifold vacuum all the way. The ported vacuum was a means toward cleaning up emissions at idle. By having the engine demand more air (and consequently more fuel) at idle the hydrocarbon parts per million could be reduced. To get the idle speed down the timing used was retarded in as much as no idle vacuum advance was present."
Author:  SlantSixDan [ Mon Sep 15, 2008 10:25 am ]
Post subject: 
That's about six different kinds of wrong. Ported vacuum has been in use since long before "emission control" was a known phrase or phenomenon. Some emission-controlled Chrysler vehicles of the mid-late '70s use manifold vacuum spark advance from the factory, but for the most part everything works a lot better on our cars when ported spark advance is used. There is no running benefit to vacuum advance at idle, which in fact causes driveability problems and throws off the positional calibration of the throttle plate(s) with respect to idle transfer ports, etc.

This thread reminds me I need to work on getting some of the old Slant-6 News tech articles digitised and posted on here...Doug D's article on recurving slant-6 distributors, for instance. I'm sure this Steve Davis guy knows how to build a distributor, and maybe his advice is less wrong with respect to Shabby and Fraud engines, but I kind of doubt it. :roll:
Author:  68barracuda [ Mon Sep 15, 2008 10:55 am ]
Post subject: 
I was hoping you would comment :twisted:
Author:  Doc [ Mon Sep 15, 2008 11:09 am ]
Post subject: 
A ported vacuum signal works better for an engine that sees a lot of variable loads, like a vehicle driven on the street.
If this were not true, the carburetor manufacturers would have saved themselves all the trouble and expense of putting a ported vacuum source onto carburators.

For engines that run in a steady state, ported vacuum (vacuum advance) is not needed.
DD
Author:  bwhitejr [ Mon Sep 15, 2008 1:10 pm ]
Post subject:  Interesting
Doc,

Where is that article on re-curving posted?

bwhitejr
Author:  lincparts [ Mon Sep 15, 2008 1:42 pm ]
Post subject:  good article
not the one you want but a good recurving post..
http://www.slantsix.org/forum/viewtopic.php?t=13264
Author:  Doc [ Mon Sep 15, 2008 1:44 pm ]
Post subject:  Re: Interesting
Quote:
Doc,

Where is that article on re-curving posted?

bwhitejr
See This Link
Author:  otherroutes [ Tue Sep 16, 2008 12:33 am ]
Post subject: 
Thanks for posting that link Doc. It's interesting that step 22 on page 4 says to "... connect the adjustable vacuum advance unit to the manifold vacuum source." [ducks] ;)
Author:  Eric W [ Tue Sep 16, 2008 1:26 pm ]
Post subject: 
That statement might just be there for testing purpuses.

Is the spring kit from Crane available anymore?

Dan, if Dave Vizard says this Steve Davis guy is good, then he is. I believe when he says to use manifold vacuum for a vacuum advance can, he is talking about engines running large cam's that have low vacuum at idle. I can understand where this would help idle quality and fuel economey at crusing (freeway) speeds.
Author:  Aggressive Ted [ Tue Sep 16, 2008 1:51 pm ]
Post subject: 
Eric,

No, I tried to order it a couple of years ago...need to buy old distributors to rob the springs. My favorite ones came from a built big block 400. 340 HPO distributors are good too.
Author:  Eric W [ Tue Sep 16, 2008 2:07 pm ]
Post subject: 
Thats what I thought. I've got my distributer dialed in pretty good now, just wanted to try the spring for medium to light vehicle's. Thanks Ted.
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