Vacuum advance ?????

[Len]

Question ???? should , at idle should there be any vacuum advance at the distributor ?
If not why ?
If so why?

[spencerforhire]

At idle, you have full vacuum and therefore full advance. I've always found that most engines like full advance at idle. The "why" is little harder to explain; think of the amount of timing at idle as a baseline that's needed for the motor to operate efficiently with no load, and that it needs to be less under acceleration at low rpm(full throttle= 0 vac) and more at higher rpm(where the centrifugal advance comes in).
Some carbs have both ported(no vac. at idle) and unported(full) connections. I've seen cars that didn't run properly after a carb swap because someone didn't check which spigot the vacuum advance was hooked to.

[marco_1978_spyder]

Question ???? should , at idle should there be any vacuum advance at the distributor ?
If not why ?
If so why?

I learned what I know about vac advance from Lar's Grimsrud's "Timing and Vac advance 101" which can be found around the net:


http://www.stevesnovasite.com/forums/sh ... hp?t=11689

I'll reiterate here.

To use it or not depends on the engines intended use... Race car only? nope, no need for it. Many race engines dont even use centrifical advance. Just one locked in setting is all thats needed for an enge that just fires up, races, and shuts down again.

On the street...people may want to keep your plugs clean for many miles, and squeeze the most economy out of the motor as possible while operating it for long periods of time and varying loads.

To have the timing instantly adjusted based on engine load and or vacuum, can help the above cause.

During high vacuum, low load situations, the engine can take advantage of lots of spark advance. This helps idle cooling and keeps the plugs from fouling under the usually richer idle mixture. So running a vac advance can to provide advance at idle and low load situations, is a benefite to a street machine on a budget, that is expected to be fast, yet cruise the local cruise night without overheating, eating gas, and fouling plugs.

Following this advice, I run mine connected to manifold vacuum to get some extra timing at idle. They say most street smallblocks can handle as much as 50* at idle. -no load-

If your engine see's alot of idle time. run a vac advance.

[cosvega76]

If you have a mild cam, smooth-idling engine, use the manifold vacuum port for the vacuum advance. If you have a lumpy-idling cam, use the "ported" vacuum port so it eliminates the advance at idle, it'll reduce the fluctuations of the idle caused by the timing change with the vacuum.

Chuck

[Len]

Wow . I guess if you believe what you read . And i have read it Thanks Marco for posting it. it does make sense . Yup laid timing does make heat
I grew up on base timing +vac advance +centrifugal advance = optimal spark timing . Street driven
Based on the thought that as eng rpm just off idle needs some spark advance thus ported at the vacuum advance . as the centrifugal is not rotating fast enough to advance the timing yet . As rpm picks up the centrifugal starts to take over at a giving rpm. Based on the weights and springs and rpm.s .
There is no doubt that spark timing is all about getting the spark there on time and that is what the advance is for along with base timing .
Base plus what you get when and where will give what you want along with the carb settings .To fast on the timing will create a stumble off idle
Len

[73astregt]

len if you look in the best of section on the site here there is a good article on vacume advance posted by buscop. it is right at the bottom of the list.

[Rickracer]

If you have a mild cam, smooth-idling engine, use the manifold vacuum port for the vacuum advance. If you have a lumpy-idling cam, use the "ported" vacuum port so it eliminates the advance at idle, it'll reduce the fluctuations of the idle caused by the timing change with the vacuum.

Chuck

I think you got that a little skewed, the more radical the cam, the more initial timing it will require to idle decent and clean. For instance, on my flat top .060 over 350, with gasket matched 882s, a Strip Dominator, 700 DP, and a Summit 1105 cam (224/234@.050, .465/.488, 112 LSA), it wanted 18° initial. With that much initial, and a 2800 stall converter, it would idle at 500 rpm in gear, going boo-gity....boo-gity....boo-gity,, and would idle like that all day long, if necessary. It responded much better from a bit higher idle, more like 650~700, but the point is that it would happily idle at even 400 rpm out of gear. Back it off to even 10° of initial, it would barely idle at 800 rpm. I never used vacuum advance on that motor because I didn't feel like fooling with it, 18° initial and 36° total was perfect, although it might have gotten a little better fuel mileage with some additional vacuum advance at light loads. If I'd spent the time and money to add and dial in an adjustable vacuum advance, my V8 S10 that got 18 mpg average, 20 mpg highway, might have been able to get 22+.

[Roccosvega]

I know if you run a cam say 2* advanced your engine will even want more timing advance yet.Compression and duration have a lot to do with
how much more advance your going to need at idle.

Robert

[cjbiagi]

My advice would be to try both and see what your engine combo likes. My understanding is ported advance came to be to lower emissions as many older cars used manifold vacuum.

[chevy art]

rickracer and roccosvega are 100% right. with big cams, especially with big block, the engines will not idle properly unless you have between - 18 -20 degrees initial advance. you also have to set the total advance, usaually between 34 and 36 degrees total, and still have the high initial and adjust the distributor inside so you can have the bottom and top of the advance curve where you want it. i thin k you all know what i mean. hope i explained it right. bottom line is you want say 18 degrees when you check timing at idle and 36 degrees total when you rev it up art

[cosvega76]

That's true, you want a lot of initial static timing. In my post, I suggested to use the "ported vacuum" port for the vacuum advance. If the manifold vacuum port is used the timing will advance when the vacuum is up, and will drop when the vacuum drops. This will make the idle RPM fluctuations greater, which will be even more apparent if there isn't enough converter for the cam. The ported vacuum port will not receive vacuum until the throttle is opened enough to expose airflow through the venturi, which is above idle concerns.

Chuck

[marco_1978_spyder]

Ultimately, Vac advance is a device for economy and driveability. It has Zero to do with performance.
It has nothing to do with the distributors mechanical curve, and likewise shouldn't interfere with it.

Thats why I recommend* Manifold vac:

1. Take advantage of a cooler engine at idle

2. Cleaner plugs are always a plus.

3. Doesn't interfere with the mechanical curve, so you can get that dialed in perfectly.

* Read up on it, and do what you think is best. I'm not saying this is how it has to be. Just what I've deducted. and I read alot about it at one time.

[Len]

Ok I am really lost now .
I hear the 1/4 milers just use a base timing no curve .
I heard that vacume advance dosnt have anything to do with the dist curve.
I heard try it all to see what works for me .
But what I have not heard anything about is primary saturation and collapse time then to the secondary induction and travel time to the plug and the ark time . Vrs pistion speed and valve timming. Always thought that the faster the piston speed the sooner the spark has to start. To arrive on time and the best time to arrive is up to the eng design. IE say 10 deg btdc and it is the advances job to accomplish maintaining 10 deg throughout the min and max rpm of the eng. (Timing vrs piston speed )To soon = rattling pistons , to late= lack of power.
Just a theory if you use ported vac on the vac advance you could run lighter weights in the dist thus less wear on the dist in the long run. Using ported vac for the fist part of you advance curve then the weights take over to finish the curve to max rpm.
Just in theory if you run manifold vac to you advance you are illuminating part of you spark curve, from off idle to where the weights pick up you are also running base timing IE 10 deg bdtc plus the vac advance say 10 deg = 20 deg of advance at and idle of say 800 rmp . So just off idle you may incur spark before the intake vale is totally closed causing a small or even minuet back pressure in the intake that could create a stumble just of idle until the base +manifold vac advance = piston and valve timming.
Just thinking
Len

[marco_1978_spyder]

Len, everything your mentioning is what you attempt to obtain from the Mechanical curve.

You get the mech curve set to achieve that, Then...if you want street economy, you add on some extra timing from a Vac advance unit at Idle and Cruise, where the engine needs it.

A properly adjusted vac advance unit shouldn't cause any stumble or hiccups. If it does, don't run it.

Adjustable units from Crane and the like are the way to go. You can dial in exactly what the engine will tolerate.
Unlike a factory unit where your just hoping it's what you need.

Running from the can from the ported side was done by manufacturers in the 70's as a band aid fix for emission controlled vehicles. They ran extreemly retarded idle settings to get the exhaust temps way up so as to burn off more hydrocarbons or something along those lines.. So no vac advance was used at idle.

[Len]

Ok I sort of got it advance timing for performance and retard for economy ? I am still worried About heat generation seems to me advance generates heat in the eng and retard generates heat in the ex to light the cat for emissions sake and thus the backed down timing at idle . But yet the curve for some performance is there in carb cars.
Just thinking
Len