Vacuum Advance Canister Compatibility

[gzig5]

I figured out that the diaphragm in the vacuum advance can will not hold vacuum. So I'm trying to find a suitable replacement and need some help deciding which replacement can to go with. The distributor is part number 3874298 which I believe is a 1973 small block, electronic ignition. The vac can is number 3656766 which has an 11 stamped on the arm and should give up to 22 crank degrees advance. Haven't been able to find any new ones of that PN nor have I found a cross reference list for the various Mopar cans. I did find the attached list of replacement cans on an old thread.

vacuum advance question

Does anyone have a part number or stock application for an adjustable small block vacuum advance pot? Thanks! Also, is there a way you can tell by looking at aftermarket replacements if they are adjustable? Like this one- BWD V194 - Vacuum Control | O'Reilly Auto Parts

www.forabodiesonly.com

I found one listing for a VC-260 replacement can which has a application sheet that says it is direct replacement for 3656766 among others. But, that model doesn't show up on the list so I don't know what the other vacuum specs are. This list is a little confusing in that when comparing specs from models I found on Summit, the max degrees on the list usually gives a range and may or may not match the degrees stamped on the arm. Not sure how that works.

I know that there were different cans for various emissions optimization on different engines, models, and locations, but I don't need to worry about that. So does it matter at how much vacuum the advance starts for a performance application? Engine is a stock stroke 340, 10.7:1 compression, with 242/247@.050" duration on 109 lobe separation. Dyno'd at 460hp. Builder said it wanted a lot of initial timing. I figure if nothing else I would get one that has at least 11 distributor degrees and I can modify to limit it if needed. Not sure that I'll be running the vac advance but would like the option figuring it should help efficiency at low load cruise for long road trips. Do the start and total vac specs really matter on a build like mine? Any recommendations for a replacement and source? I prefer to use this distributor as there is nothing else wrong with it.

 

[Mattax]

Since its not factory engine here's my two cents.
With that cam it will want a lot of initial, even with the relatively high compression.
The govorner in the advance will probably be stamped 12 or 15 degrees.
You can look up the distributor and its specs in the shop manual. They're listed by number in the back of th electrical section.

Weld up the inside of the slots and then turn the spring perch so there is a little less tension on the primary spring. The amount needed to be welded up is the difference between the factory initial timing for the engine that distributor was for, and the initial timing your engine will want. 18-20* is my guess. There's a guestimate of initial timings from Barry Grant in the Articles.

Get a vacuum canister that only adds advance when the vacuum is 8" or higher.
Its typical that hot cammed engines need enrichment earlier - which means power valve (or step up) at 8.5" to 10.5" Hg. The vacuum advance should go away when mixture goes rich. If you are using an 8.5 PV then vac advance that goes away around 6" is clsoe to what the factroy used in the early 340s. If using a 10.5 PV, which then I think 8" vacuum is closer to the mark. As you know the start and finish are adjustable on many of the vacuum cans. So don't sweat that too much. What is not adjustable is the rate of advance.

Do a search where I wrote "spill the beans" on adjusting a Chrysler distributor.

The timing at cruise at highway rpm should be around 50*. So look at the initial + advance at 2800 or 3000 rpm. Then add to that the vacuum advance needed to bring that to something around 50.

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if its a manual transmission, it might be possible to crutch the initial without shortening the slots by using manifold vacuum source for the cannister. However I suspect the cam is to radical to get away with that crutch.

 

[RustyRatRod]

Put this on it. It's adjustable so you can make it come in at whatever vacuum you want. Good deal and good feedback. Marked 11R which will give the same advance as what you have, but in an adjustable package.
Vacuum Can

 

[Mattax]

At some point I graphed out a '73 manual trans 340 timing, but didn't write down the distr number.

Lets say this is the distributor you have in hand.

Then shorten the slots by 5 distributor degrees (on the inside)
If installed with timing set at 15*, the timing curve would be the same from 1400 rpm up

Turning the spring perch or bending the loop a little will get the adnce started at a lower rpm.
Turn the other perch to get the long loop to engage a little earlier.

Iff you experiment with a bit more initial, timing will look something like this.

Red is where too much mechanical advance can cause pinging when combined with vacuum advance at light throttle, especially when the engine is heat soaked (long trips).
These are just approximations using this example from FSM to give the general concept.

 

[gzig5]

Put this on it. It's adjustable so you can make it come in at whatever vacuum you want. Good deal and good feedback. Marked 11R which will give the same advance as what you have, but in an adjustable package.
Vacuum Can

I already had that one saved from my research this afternoon. It’s supposed to be a direct replacement for the original can that I have. My problem was that I don’t know with this new engine if that’s the best for my new application, but it seems like it should be usable.

 

[gzig5]

At some point I graphed out a '73 manual trans 340 timing, but didn't write down the distr number.

View attachment 1716387459

Lets say this is the distributor you have in hand.

Then shorten the slots by 5 distributor degrees (on the inside)
If installed with timing set at 15*, the timing curve would be the same from 1400 rpm up
View attachment 1716387460

Turning the spring perch or bending the loop a little will get the adnce started at a lower rpm.
Turn the other perch to get the long loop to engage a little earlier.

Iff you experiment with a bit more initial, timing will look something like this.
View attachment 1716387467

Red is where too much mechanical advance can cause pinging when combined with vacuum advance at light throttle, especially when the engine is heat soaked (long trips).
These are just approximations using this example from FSM to give the general concept.

Thanks Mattax. I was hoping you would chime in. You have a knack for clearly explaining a topic that can have multiple intertwined variables. You’ve clarified the initial vacuum question for me. The charts are very helpful.

The car will have a manual transmission but was a 727 originally. It’s a mixmaster though so the pedigree is questionable. Motor and transmission were replaced and we found the motor was a real bastard. Someone put a 318 crank in the 340 block and beat the hell out of it.

 

[RustyRatRod]

I already had that one saved from my research this afternoon. It’s supposed to be a direct replacement for the original can that I have. My problem was that I don’t know with this new engine if that’s the best for my new application, but it seems like it should be usable.

It's adjustable. It can work for virtually anything. I wouldn't have posted something that wasn't a good match.

 

[Bewy]

If the cam is a hydraulic lifter cam, I doubt that you will find a Chrys VA unit that will work with the low vac that the cam will produce. On a low vacuum 440 that I tuned, I had to use the spring out of a GM HEI adj VA unit. Prised the VA units apart & swapped springs. The HEI spring was softer & worked with the lower vacuum.

 

[RustyRatRod]

If the cam is a hydraulic lifter cam, I doubt that you will find a Chrys VA unit that will work with the low vac that the cam will produce. On a low vacuum 440 that I tuned, I had to use the spring out of a GM HEI adj VA unit. Prised the VA units apart & swapped springs. The HEI spring was softer & worked with the lower vacuum.

Mine works with 6hg. It pulls in 20 degrees just fine.

 

[gzig5]

If the cam is a hydraulic lifter cam, I doubt that you will find a Chrys VA unit that will work with the low vac that the cam will produce. On a low vacuum 440 that I tuned, I had to use the spring out of a GM HEI adj VA unit. Prised the VA units apart & swapped springs. The HEI spring was softer & worked with the lower vacuum.

SFT.

 

[Mattax]

 

[Newbomb Turk]

At some point I graphed out a '73 manual trans 340 timing, but didn't write down the distr number.

View attachment 1716387459

Lets say this is the distributor you have in hand.

Then shorten the slots by 5 distributor degrees (on the inside)
If installed with timing set at 15*, the timing curve would be the same from 1400 rpm up
View attachment 1716387460

Turning the spring perch or bending the loop a little will get the adnce started at a lower rpm.
Turn the other perch to get the long loop to engage a little earlier.

Iff you experiment with a bit more initial, timing will look something like this.
View attachment 1716387467

Red is where too much mechanical advance can cause pinging when combined with vacuum advance at light throttle, especially when the engine is heat soaked (long trips).
These are just approximations using this example from FSM to give the general concept.

Pay attention to the curves on this graph.

Study them and consider them. Then figure out how to copy them.