idle sounds different with ported advance - Hot Rod Forum : Hotrodders Bulletin Board
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Old 01-25-2011, 08:43 PM
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idle sounds different with ported advance

When i run ported advance the idle sounds much different than when i use manifold vacuum advance. the idle sounds much better when i use ported, it sounds more lopey i guess, but the off idle performance isnt very good. when i use manifold it helps off idle, but even if the idle rpms are the same, the sound is very bland. has anyone else noticed this or am i crazy?

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Old 01-25-2011, 09:10 PM
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You aren't crazy.

Remember that "lope" is the result of the engines' inefficiency at low speed. The cam will cause the engine to lope due to a large amount of overlap. That causes low vacuum, making for leanness from a weak carb signal, and reversion at low speeds. Think of it as an uncontrolled EGR valve, of sorts.

By using manifold vacuum for your vacuum advance, the vacuum advance is working to supply more advance at idle. This lessens the propensity for the cam to have a lope, but at the same time that's also telling you the engine is now running more efficiently.
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Old 01-26-2011, 06:04 AM
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What COBALT327 said...
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Old 01-26-2011, 08:55 AM
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Quote:
Originally Posted by stroker444
When i run ported advance the idle sounds much different than when i use manifold vacuum advance. the idle sounds much better when i use ported, it sounds more lopey i guess, but the off idle performance isnt very good. when i use manifold it helps off idle, but even if the idle rpms are the same, the sound is very bland. has anyone else noticed this or am i crazy?
Ported (or timed) vacuum source simply uses the throttle blades to obstruct the vacuum source so that the vacuum advance is not functional till the throttle is opened, usually coming off idle and going to the transition feeds.

So if you connect the vacuum advance to the timed port when the engine is idling there no more advance than the base setting. If the connection is made to a manifold port, then as soon as the engine is started full manifold vacuum is applied to the vacuum advance mechanism so that what ever amount of vacuum advance is built into the unit, usually around 10 to 14 degrees, is added to the base timing setting. This will affect the sound of the idle and the idle speed. More advance will make the exhaust sound crisper and the idle speed will come up. On a gas meter the NOx will be low and the HCs high when idling on ported vacuum. The reverse will be shown when idling on manifold vacuum where the NOx is higher and the HCs lower. If you have to pass emissions using the timed port works better as excess HCs are easier to clean up than excess NOx. From an idle quality stand point a large cammed engine will want more initial advance than a smaller cammed engine simply because the effects of large amounts of overlap and late closing intakes tends to dilute the fresh mixture with exhaust and pumps some back into the intake. This lowers the number of fresh mixture molecules in the cylinder which slows the burn rate (i.e. makes the engine act as if the timing was retarded) so the additional idle vacuum advance provides more time for the combustion reactions to complete before the exhaust valve opens, while, also, allowing the cranking advance to be kept moderately enough that the starter isn't being back loaded by early firing cylinders coming on the compression stroke.

Bogie
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Old 01-26-2011, 11:30 AM
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Lets hope he took some notes. Church.
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Old 01-26-2011, 12:44 PM
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The cliff note version:

manifold vac has increased your idle timing whereas ported had no timing effect at idle
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Old 02-05-2011, 02:19 PM
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Not to step on any highly qualified toes here, but IMO the ported vacuum means you only have intial timing at idle, say 12 degrees or so, instead of around 28 degrees you would have with full vacuum. This will definitely change the sound because with the full vacuum combustion is completed before the exhaust valve opens whereas with the significantly later timing of the ported vacuum the combustion is actually still underway when the exhaust opens, you actually can hear its still burning, giving that more aggressive sound and lope. At least thats what I thought causes the difference, as I noted this too. At shows I switch the vacuum source from full to ported just for the more aggressive sound
But normally I use full vacuum as it gives better throttle response and fuel economy.
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