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Ignition timing (again)
#1334216 Mon Nov 18 2019 04:53 PM
Joined: Oct 2019
Posts: 35
Wrench Fetcher
Guys, bear with this rookie - trying to understand vacuum and centrifugal advance. Is there anytime when the truck is on the road that vacuum advance AND centrifugal advance are not engaged at all? At idle, vacuum advance is engaged, correct?

Thank you

Re: Ignition timing (again)
WarEagle1 #1334219 Mon Nov 18 2019 05:02 PM
Joined: Feb 2004
Posts: 20,344
No, that's wrong. At idle neither vacuum nor centrifugal advance is a factor- - - - -"initial" timing only. Just off-idle the vacuum starts to come in, based on throttle opening and engine load. Centrifugal advance is engine speed related only- - - -the higher the RPM, the more advance comes in until the advance weights reach the limit of their travel (around 2500 RPM or so). The only real purpose for vacuum advance is fuel economy. At high speed, light throttle cruise, the ported vacuum adds around 8-10 degrees of advance above centrifugal. Anytime a load goes on the engine that drops manifold vacuum, the vacuum advance goes away and you're left with initial + centrifugal for load pulling or passing power. Back off the throttle, and the vacuum advance returns.

Industrial and heavy truck engines usually don't have vacuum advance at all, since they're run at or near full load most of the time and fuel economy isn't a factor.

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