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Old 04-15-2008, 06:42 PM
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Vacuum Advance Canister question

Whats up guys.

I'm still tuning my engine.

Question about the vacuum advance canister.

I have a distributor in my vehicle very similar to this.

http://cgi.ebay.com/ebaymotors/CHEVY...QQcmdZViewItem

I'm assuming the vacuum canister is very much a typical stock configured part, nothing fancy hear.

At what hg" level would a cannister of this nature start adding advance to the curve.

Is it correct that as vacuum drops the cannister adds less advance to the curve?

weathers gettin nice on the east coast help

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Old 04-15-2008, 06:54 PM
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I'm not sure what a stock vacuum advance canisters curve is but I am almost positive that it adds advance as the vacuum decreases. This only makes sense as the motor needs more advance as the RPMs are increased. Increased RPMs means that the throttle is open more and therefore has less vacuum. It would be counter productive to retard timing with the canister when the mechanical advance is trying to add it in!
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Old 04-15-2008, 07:21 PM
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Im a rookie gear head here but I was under the notion that the canister reduced its advance as vacuum reduced because mechanical advance would be coming in at this point (rpm's rising).
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Old 04-15-2008, 07:23 PM
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Vacuum cannister for your distributor advance? You sure your not talking a vacuum reserve cannister for your power brakes?
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Old 04-15-2008, 07:27 PM
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Yes... the distributor.

The day they start making power brake vacuum canisters that influence your timing curve let me know.
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Old 04-15-2008, 07:35 PM
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I asked a simple question, referring to a vacuum reserve cannister, your smart remarks aren't necessary or appreciated. I did miss the vacuum 'advance' part of your post though.
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Old 04-15-2008, 07:44 PM
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the vacuum advance is independent of centrifigal advance and RPM. it advances solely based on engine vacuum and the more vacuum-the more advance.

as to your question about what vacuum and how much advance there is no right answer. it depends on the part # of the particular canister, they are all over the place.
i was just reading about the different canisters on another forum yeasterday, if you want a link let me know.
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Old 04-15-2008, 08:02 PM
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Quote:
Originally Posted by strokey
Whats up guys.

I'm still tuning my engine.

Question about the vacuum advance canister.

I have a distributor in my vehicle very similar to this.

http://cgi.ebay.com/ebaymotors/CHEVY...QQcmdZViewItem

I'm assuming the vacuum canister is very much a typical stock configured part, nothing fancy hear.

At what hg" level would a cannister of this nature start adding advance to the curve.

Is it correct that as vacuum drops the cannister adds less advance to the curve?

weathers gettin nice on the east coast help
Go here and read all about it http://www.corvette-restoration.com/...c_Adv_Spec.pdf

Some things to contemplate:

- Vacuum advance is intended to provide additional advance under light load and cruise where vehicle speed can be high but engine RPM and throttle opening modest. Under these conditions the mixture in the engine has a low density. Remember air and vaporized gasoline are compressible fluids, therefore, their density varies with temperature and pressure. Vacuum is a measure of the lack of pressure, the gauge is read inverse to barometric pressure. In other words, if the atmosphere is at 29 inches of mercury and your engine is idleing at a vacuum of 20 inches of mercury, the "atmospheric" pressure inside the intake is ((29-20)/29))X100 or 31% of barometric pressure)

- Low density mixtures burn slowly, therefore, more advance is required. Cam timing and compression are also conditions that effect mixture density. The longer the cam timing or higher the lift; the lower the bottom end RPM mixture density so more advance is required. The higher the compression ratio, the higher the bottom end density and less advance is required, this also applies to the effects supercharging. Gets complicated quickly, doesn't it?

- The vacuum advance as you read the http attachment has a misnomer in it called "advance start", this is really the drop out point. Engine vacuum is applied against a spring loaded diaphragm. Under high vacuum, the diaphragm is pulled against the spring advancing the ignition timing to it's maximum point. As load or RPM is increased, the assumption is that the throttle blades are opened. this results in a reduction of manifold vacuum, the spring pushes the diaphragm closed reducing the advance. It is also assumed that RPMs increase such that the centrifugal advance picks up as the vacuum drops off. There's a hole in this design, which is RPMs are not necessarily related to engine load. But within this is a safety feature in that when the engine is forced to lug (low RPMs with lots of throttle opening) the lack of vacuum reduces the advance and the lack of RPMs keeps the centrifugal out. This reduces the tendency to detonate but it sure sucks gas.

- Accel, Crane and others sell adjustable vacuum advances which let you tailor the amount and rate to your needs.

Bogie
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Old 04-15-2008, 08:14 PM
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Techron - I'd love to have the link, Ive been reading old posts for hours trying to diagnose.

I think I've finally narrowed the problem.

I spent time blaming my sour performance on the stock stall converter and stock highway gears, then the swap meet 650 holley double pumper with a kit in it.

I bought a brand new box stock holley 670 and noticed much crisper throttle response in park, yet the same completely no idle/stall when in drive.

I'm running a 388 ci sbc with a comp xe284h. The car makes 6-7 hg" of vacuum in gear at 800 rpm's.

Ive been blaming the drive train/running gear for the high idle it needs in park in order to sustain a sufficient idle when placed in gear. Here is what I think my problem is.

The car has a real sweet attitude idling at 1300 in park. This is probably at the point where it is producing a good ammount of manifold vacuum.

When the car is placed in gear the rpm's drop and it immediately falls on its face and stalls.

I think this is because of the non adjustable factory style advance canister. It is providing no advance in drive ( at 600-800 rpm's, again making 6-7 hg" of vacuum).

In a nutshell to even have the car driveable I was running at 1300 rpm's in park. I have now realized that at this point (in park) the vacuum advance is running at full clip providing an additional 15 or so degrees of timing, thus a total of around 27. Then when you switch to drive and the rpm's drop (as well as manifold vacuum levels) you are back to the base timing of around 12 degrees (because the canister is providing little or no advance at this point with such a low ammount of vacuum).

Simply there is no happy medium with this cam and this advance canister with this configuration.

I need an adjustable can badly.

I may be preaching to the quire.

But someone please let me know if I am on the right page.

Last edited by strokey; 04-15-2008 at 08:53 PM.
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Old 04-15-2008, 09:00 PM
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good find bogie, lars emailed me that paper a few days ago, it's one of the references i was refering too, you beat me to it.

here is the other discussion on vacuum advances i was refering to:

http://forums.corvetteforum.com/show....php?t=1993068

enjoy reading....
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Old 04-15-2008, 09:21 PM
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Quote:
Originally Posted by strokey
Techron - I'd love to have the link, Ive been reading old posts for hours trying to diagnose.

I think I've finally narrowed the problem.

I spent time blaming my sour performance on the stock stall converter and stock highway gears, then the swap meet 650 holley double pumper with a kit in it.

I bought a brand new box stock holley 670 and noticed much crisper throttle response in park, yet the same completely no idle/stall when in drive.




---------------------------------------------------------------


this sounds about right to me, i think you are on the right track. you need a vacuum advance with a weak enough spring to advance at idle . 12 degrees at idle with no vac advance with that cam is going to make the engine idle like crap. as far as a specific part # i can't hepl you there--i think the link i made for you may have some talk about low vacuum at idle engines IIRC.

OOPS, got it in the wrong place, just imagine this is at the bottom, i don't want to type it all over...

----------------------------------------------------------------






I'm running a 388 ci sbc with a comp xe284h. The car makes 6-7 hg" of vacuum in gear at 800 rpm's.

Ive been blaming the drive train/running gear for the high idle it needs in park in order to sustain a sufficient idle when placed in gear. Here is what I think my problem is.

The car has a real sweet attitude idling at 1300 in park. This is probably at the point where it is producing a good ammount of manifold vacuum.

When the car is placed in gear the rpm's drop and it immediately falls on its face and stalls.

I think this is because of the non adjustable factory style advance canister. It is providing no advance in drive ( at 600-800 rpm's, again making 6-7 hg" of vacuum).

In a nutshell to even have the car driveable I was running at 1300 rpm's in park. I have now realized that at this point (in park) the vacuum advance is running at full clip providing an additional 15 or so degrees of timing, thus a total of around 27. Then when you switch to drive and the rpm's drop (as well as manifold vacuum levels) you are back to the base timing of around 12 degrees (because the canister is providing little or no advance at this point with such a low ammount of vacuum).

Simply there is no happy medium with this cam and this advance canister with this configuration.

I need an adjustable can badly.

I may be preaching to the quire.

But someone please let me know if I am on the right page.
now i get the lengthen my post to 10 characters message.
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Old 04-15-2008, 11:29 PM
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Quote:
Originally Posted by strokey
Techron - I'd love to have the link, Ive been reading old posts for hours trying to diagnose.

I think I've finally narrowed the problem.

I spent time blaming my sour performance on the stock stall converter and stock highway gears, then the swap meet 650 holley double pumper with a kit in it.

I bought a brand new box stock holley 670 and noticed much crisper throttle response in park, yet the same completely no idle/stall when in drive.

I'm running a 388 ci sbc with a comp xe284h. The car makes 6-7 hg" of vacuum in gear at 800 rpm's.

Ive been blaming the drive train/running gear for the high idle it needs in park in order to sustain a sufficient idle when placed in gear. Here is what I think my problem is.

The car has a real sweet attitude idling at 1300 in park. This is probably at the point where it is producing a good ammount of manifold vacuum.

When the car is placed in gear the rpm's drop and it immediately falls on its face and stalls.

I think this is because of the non adjustable factory style advance canister. It is providing no advance in drive ( at 600-800 rpm's, again making 6-7 hg" of vacuum).

In a nutshell to even have the car driveable I was running at 1300 rpm's in park. I have now realized that at this point (in park) the vacuum advance is running at full clip providing an additional 15 or so degrees of timing, thus a total of around 27. Then when you switch to drive and the rpm's drop (as well as manifold vacuum levels) you are back to the base timing of around 12 degrees (because the canister is providing little or no advance at this point with such a low ammount of vacuum).

Simply there is no happy medium with this cam and this advance canister with this configuration.

I need an adjustable can badly.

I may be preaching to the quire.

But someone please let me know if I am on the right page.
I'm going to quibble with your thinking a little. Although for an XE-284 1300 is a bit fast while 800 is probably too slow, so the engine is easy to stall when it's dropped into gear and wants to creep against the brakes when-ever stopped. You also probably are getting a hard engagement when dropping it into gear, these can and will bust inner tranny parts.

For a stock converter you're probably right up against the stall, although stall speeds are kind of mushy things to describe.

With a hot cam when driven on the street you're looking for something a little different in a high stall converter than a drag car. for the street you don't want such a hard engagement, but unless you're going on Pinks, you don't need a loose converter that doesn't hookup till the torque peak on the cam. All you need is something high enough to keep from knocking your teeth out when you put the tranny in gear or pulling you through an intersection with the brakes set.

My recommendation is that you talk to Comp and some converter folks, as well as lay your hands on a Summit catalog, they always include a little tech article regarding converter stall against cam timing. It's kind of generic, but it's a decent data point and you'll get plenty of those, decent and otherwise.

Bogie
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Old 04-16-2008, 10:03 AM
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Quote:
Originally Posted by strokey
Whats up guys.

I'm still tuning my engine.

Question about the vacuum advance canister.

I have a distributor in my vehicle very similar to this.

http://cgi.ebay.com/ebaymotors/CHEVY...QQcmdZViewItem

I'm assuming the vacuum canister is very much a typical stock configured part, nothing fancy hear.

At what hg" level would a cannister of this nature start adding advance to the curve.

Is it correct that as vacuum drops the cannister adds less advance to the curve?

weathers gettin nice on the east coast help
//////////////////////////////////////\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\

Something NEW: Canister for Dis. Adv,?

I never used a Tank for the Dis.Adv.
When the throttle is pressed Quickly, the Manifold & Carb Vac. Port, Experience a Vacuum Force. The Dis. Vac.Adv. Plate Turns to advance the Dis. For the Increase of Fuel & Air Mix.

The Canister I got goes to the Vac. Operated Equip.( Heater Doors,Etc.)
The Brakes: Get the Vac. From The Intake. ( EXCEPT: My Tunnel Ram.)
So I Disconnected my Brakes. Ya-Huuuuu.
I'm going to use an Elect. Vac. Pump.
&
Add a Electric Oil Pump Aux.
&
Elect. Water Pump.
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Old 04-16-2008, 10:22 AM
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Once again, speaking of the vacuum advance cannister on the distributor, not a reservoir can for the brakes...
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Old 04-16-2008, 10:32 AM
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http://cgi.ebay.com/ebaymotors/CHEV...5QQcmdZViewItem

Quote:
Originally Posted by strokey
Once again, speaking of the vacuum advance cannister on the distributor, not a reservoir can for the brakes...
================================================== ======

The Pic. in the Link: Shows only the Diaphragm that makes the lever move the advance base!

trying to fix the link

Last edited by Jmark; 04-16-2008 at 10:44 AM. Reason: Other.
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