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Re: [GMCnet] Vacuum Advance [message #65891 is a reply to message #65881] Sat, 28 November 2009 20:50 Go to previous messageGo to previous message
Gary Casey is currently offline  Gary Casey   United States
Messages: 448
Registered: September 2009
Karma:
Senior Member
Wow - I'm amazed that an issue like this would be so controversial.  I think Bob has the grasp of the subject, but here are a few clarifications:  The retarded spark at idle was primarily to reduce idle hydrocarbon emissions, not NOX, although it probably does that as well.  There were no NOX standards at the time.  There is one benefit of a retarded spark at idle - it is easier to get a stable idle at a lower rpm.  It is probably better to get rid of it, but I guess I wouldn't call it a "very" detrimental feature, just a detrimental feature.  Yes, because more idle fuel flow is required the engine will run hotter at idle, but I don't think a little extra heating at idle is a big deal - we have a big radiator and lots of underhood space for ventilation at idle.  There is a possibility - a slight one - that the idle ports in the carburetor will be better matched to the "no-vacuum" condition, so it might be more difficult to get a really good idle
adjustment.  Oh, and I think having full vacuum at idle will reduce the tendency for dieseling.  But at cruise or acceleration conditions there will be no difference.
Gary



Hi Jeff

Here is quote from Bob de Kruyff on April 3 of this year.

"Start Quote"
This is a volatile subject, but here is the way an engineer of the times
sees it. The engines of this vintage used a retarded timing scheme at idle
in order to control NOx emissions. That was accomplished with a fancy name
called ported vacuum which meant very little if any vacuum to the
distributer advance at idle. This caused several problems such as poor cold
start performance and hot engine temps while idling. The tvs will sense hot
engine temperatures and allow full manifold vacuum to the distributer
advance allowing not only a higher idle speed, but more importantly greater
efficiency and much less heat rejection. Other than complying with
emissions, it is a very detrimental feature of the times. Get rid of it and
go to manifold vacuum to the distributer. This has become such a contentious
issue on this forum that even the engineer who developed the system and
promoted manifold vacuum was chased away and resigned from the net because
of the nay sayers.

-------------------------------------------------------------------------
Bob de Kruyff
"End Quote"

I suspect Bob knows more about this than anyone else on the GMCnet. I accept
his word as gospel.  My GMC runs manifold vacuum just like all cars before
the pollution 'fixes' of the seventies.  If your engine is properly tuned it
should catch on the first revolution, long before there is any vacuum built
up in the intake.

Dave



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