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Re: [GMCnet] Vacuum Advance [message #65881 is a reply to message #65879] Sat, 28 November 2009 18:22 Go to previous messageGo to previous message
dmumert is currently offline  dmumert   United States
Messages: 46
Registered: December 2006
Location: Canada
Karma:
Member
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


> Subject: Re: [GMCnet] Vacuum Advance
>
> Very good explanation. I no longer have the TVS switch on mine and
> didn't know if it had been connected wrong some time ago. There is no
> vacuum at idle on the carb port, which was why I thought it seemed
> wrong.
>
> Thanks for all the info
> --
> Jeff Willard

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Dave Mumert Olds, AB

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