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Distributor advance

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Red Devil View Drop Down
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Post Options Post Options   Thanks (1) Thanks(1)   Quote Red Devil Quote  Post ReplyReply Direct Link To This Post Posted: Jan/17/2020 at 11:47am
Originally posted by 69 ambassador 390 69 ambassador 390 wrote:

Phat is 100% correct. AMC did not use manifold vac to the distributor at idle EVER. The later cars had a TCVS that connected the distributor to ported vac under normal conditions and would switch to manifold vac if coolant temps got above a set high limit. This was done to reduce further heating by advancing the timing, thus raising the idle and also reducing EGTs to help cool the heads. Running manifold vac under light load and lean cruise can actually raise NOX emissions by increasing combustion pressures and temps in the chamber. In short, You would normally run ported, just like AMC did.

... on my '73 and '74 AMCs, it used Manifold vacuum at idle when cold (to improve driveability when cold, according to the TSM) and ported when warm, switched by CTO valve. Also had TCS that limited vacuum advance in high gear / above certain rpm. All emissions related.


Some engines like vacuum advance connected to manifold vacuum to get advance at idle ... but can cause timing instability without the right vacuum advance unit. Once throttle blades are opened enough (just off idle), manifold and ported vacuum is the same. Use what the engine likes.

Hope this helps,RD
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Heavy 488 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Heavy 488 Quote  Post ReplyReply Direct Link To This Post Posted: Jan/17/2020 at 11:50am
I always felt that vacuum advance connected directly to manifold is more of a bandaid around other issues.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote PHAT69AMX Quote  Post ReplyReply Direct Link To This Post Posted: Jan/17/2020 at 1:46pm
One 'reason' or 'scenario' that I have run manifold vacuum to vacuum advance
that I have used and it worked well is with a big cammed 'hot rod' engine
to increase idle vacuum, minimize primary throttle blade idle opening,
and reduce static initial spark advance to make hot starts easier.
But this is a somewhat 'far to one end' situation that did not seem to fit the original posters situation.
Ah, and thank you for posting about the dual-vac-adv exception on some cars
and on later cars in some case with TCS etc that it calls for it.
I always get stuck thinking of only a narrow window of application.
Thanks everyone for their opinions, I like seeing all sides of a situation,
and seeing most thing are not necessarily a single simple straightforward answer.
Keeps up thinking and learning and that is good.

gnrand - LOL - yes, you can increase your timing at idle and it may run better,
just be careful not to end up with to much when the mechanical advance comes in.
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