Vacuum advance hosen

[Mark --]

I grew up plumbing the vacuum advance to venture vacuum. I understood that as the flow in the carb increase so did the resultant vacuum in the upper part of the carb. This indicated that the engine was higher in RPM and additional ign. advance would be proper. Later as EPA messed around you got different laypouts which utilized manifold vacuum to advance a hot idleing engine or even retard it in certain conditions which was usually conttrolled by a temp acuated valve system.

I've read in different posts to just put the hose on the manifold directly and your all set.

Wouldn't you loose your advance just as you stomped on the gas? :?:

And when would you get the vacuum advance back - Really high RPM as manifold vacuum returned ?? (Some anyway) :?:

I'm I ahead or behind the curve??..... :?

Mark --

[73 Century]

Yes you will lose your VAC advance when you stomp the pedal but that is what you want. Then your centrifigal advance takes over and does it's job. Manifold vac will come back when you let off.
Under full throttle you want the cent. to do the job and under cruise you want the vac to do it.

[pageboy]

Check out this link, and scroll down to the post titled "pageboy on the ignition curve". It's a bit lengthy so I won't copy and paste it here.

http://www.cowboyseven.us/forum/index.php?topic=6.45

[Clay]

Mark,
Manifold vacuum will make your engine run cooler at idle. The reason for this is that the mixture is fired sooner in the cycle and the resulting heat goes out the exhaust instead of soaking into the cylinder walls like a later fireing cycle.
Tighter emission laws have engines running on ported vacuum. This means that the Dist. has no vacuum at idle or on some engines no vacuum in 1st. and 2nd. gear only high and overdrive where max performance is needed.
Engines tend to overheat while idling in traffic on a hot day so a bypass is used and the Dist. is routed to manifold vacuum to help the engine to keep cooler.
When you stomp on the gas there is no vacuum anywhere untill engine RPM builds because the engine "sees" only atmospheric pressure.
Your best milage will be seen at manifold vacuum but the engine likely will not pass emissions testing if needed.
Vacuum advance has little or nothing to do with performance because when your foot is in it for power there is no vacuum. When you lift the foot to stop the power the vacuum comes but the engine is no longer in power mode but economy cruise mode.
Tuning your mechanical advance by weights and springs( to vary the timing of advance as well as speed of advance) it the way to better performance.
Most engines don't like over 35 degrees total (initial+mechenical) advance. Where you place the initial can be different with each engine but 12-16 degrees before is ballpark for most engines.
Vacuum cans add between 10 to over 20 degrees to initial at idle but this starts to come out as throttle is applied.
If you get ping at higher RPM (enough RPM to bring in mechanical advance) with a little load and more vacuum, a vac can with less advance in it or one that is actuated by higher vacuum is needed. (Adjustable type).
Clay

[STDog]

Clay, sounds like you just described my 70 deville. And better than the shop manual.

It has carb vacuum in high gear, thanks to the TCS switch. But that goes to a temperatue switch that give manifiold vacuum when the temperature goes up.

I had that know at low rpm with a load thing too. puklling the TCS out, running straight from the carb, and a new vacuum advance unit got rid of most of it.

Miss. has no emmisions test, and besides, it's 35 years old, and now catilytic convertor. It probably couldn't pass a modern emmisons test with all the emmissons stuff working :)

[dave_brode]

Mark,

You loose the vac when you stomp it, no matter where you source it.
Dave




--"]I grew up plumbing the vacuum advance to venture vacuum. I understood that as the flow in the carb increase so did the resultant vacuum in the upper part of the carb. This indicated that the engine was higher in RPM and additional ign. advance would be proper. Later as EPA messed around you got different laypouts which utilized manifold vacuum to advance a hot idleing engine or even retard it in certain conditions which was usually conttrolled by a temp acuated valve system.

I've read in different posts to just put the hose on the manifold directly and your all set.

Wouldn't you loose your advance just as you stomped on the gas? :?:

And when would you get the vacuum advance back - Really high RPM as manifold vacuum returned ?? (Some anyway) :?:

I'm I ahead or behind the curve??..... :?

Mark --[/quote]