Low vacuum - should I ditch my vacuum advance distributor?

DemonSwede

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So I’ve got a recently built 340 with a Comp cam in it with 274/286 of duration and 488/391 lift. It only makes around 8-10hg of vac at idle, and I’m pretty sure there are no leaks and compression is good. I’ve got it set at around 13-15 initial and 28 total right now, which is as far as my current mechanical will allow right now. That level of vac is about as much as I’ll get.

Now I didn’t build the engine but the car runs good and it has the standard electric conversion kit on it with a blue box and a no-name (proform?) distributor. (Sorry for the bad pics but perhaps someone can confirm this?)

The vac advance is useless at idle as it won’t do anything below like 15hg (I’ve checked this). So I’m now debating with myself what to do in order for me to be able to work with timing a bit more. I think my options are to 1) just skip the vac advance and work with the mechanical advance on my current distributor. (But what springs to use if I don’t even know the brand and version of the distributor!?) 2) get a vac can that triggers at a lower vacuum and keep my current distributor (but those seem hard to find), 3) buy a new all mechanical distributor to replace my no name one and keep the rest, or 4) replace the entire system and go to like MSD.

What would you guys recommend? As mentioned the car runs good, but again I don’t know if I’m leaving HP on the floor by not being able to adjust timing more…?

B5820BBC-9773-411A-BCDE-BCA4239AE42E.jpeg


998058FD-B322-46F8-B50F-B245E4B82A62.jpeg
 

Phreakish

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I've been doing a ton of looking into the same thing for another engine of mine.
The low vac cans are now unobtanium. The choice is either some sort of programmable unit (msd or progression ignition seem to be it - $535-550) or dealing with a poor tuneup.

If the car is meant for track only, or mostly track, then the vac advance won't be missed. But if it's more of a street car and you don't like overheating at every light, then having no vacuum advance kinda sucks. Plus the part throttle response gets sluggish.

Supposedly the Crane canisters were the ones to get. The adjustment inside affected the vacuum operating range, while the amount of advance added could be controlled with a cam inside the distributor. But since holley bought them, the parts seem to be gone.
 

Bewy

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[1] You can lock the timing at say, 28 or 30*. So 30* everywhere. The problems then are:
- might get pinging at lower rpms
- stock starter may not crank, in which case a 3hp mini starter is needed.
[2] If that is an aftermarket dist you have, it should have an Allen Key [ 3/32"] inside the nipple. Screw in fully CW, softest spring setting. This is usually about 5-7" of vacuum, & it should pull the VA actuator in because as it advances the timing, the vacuum increases.
[3] The Torker manifold is a very poor choice. Dual plane would be better suited to the cam.
[4] An MSD system would do nothing for this engine except lighten your wallet.....
[5] The best move is to buy a GM HEI type dist. These have a number of advantages over the one you have now:
- comes with adj vac adv
-bigger cap, less chance of spark scatter
- comes with E core coil. You get more spark energy & can open the plug gaps to 0.060"
- gets rid of the Bal Res, ECU, everything is contained within the dist.
- centri weights easy access to modify the curve

I use the cheap Chinese HEIs that I can buy here for $80 for a Chebby. Have used heaps of them. Well made, lubricated, never had a failure & come with a steel gear for use with steel roller cams.
 

GTX JOHN

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Out of Curiosity = Why not use an early Electronic OEM Distributor and
recurve it or even just take out heavy spring and bend tab out for more tension
on the light one? That is what I have been doing for 50 years. The used mopar ones
are a dime a dozen and easy to find.

It is easy to add or subtract how much the weight move so you can adjust how much
advance is in them.
 

krazykuda

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So I’ve got a recently built 340 with a Comp cam in it with 274/286 of duration and 488/391 lift. It only makes around 8-10hg of vac at idle, and I’m pretty sure there are no leaks and compression is good. I’ve got it set at around 13-15 initial and 28 total right now, which is as far as my current mechanical will allow right now. That level of vac is about as much as I’ll get.

Now I didn’t build the engine but the car runs good and it has the standard electric conversion kit on it with a blue box and a no-name (proform?) distributor. (Sorry for the bad pics but perhaps someone can confirm this?)

The vac advance is useless at idle as it won’t do anything below like 15hg (I’ve checked this). So I’m now debating with myself what to do in order for me to be able to work with timing a bit more. I think my options are to 1) just skip the vac advance and work with the mechanical advance on my current distributor. (But what springs to use if I don’t even know the brand and version of the distributor!?) 2) get a vac can that triggers at a lower vacuum and keep my current distributor (but those seem hard to find), 3) buy a new all mechanical distributor to replace my no name one and keep the rest, or 4) replace the entire system and go to like MSD.

What would you guys recommend? As mentioned the car runs good, but again I don’t know if I’m leaving HP on the floor by not being able to adjust timing more…?

View attachment 1715991364

View attachment 1715991365


Bump up your timing to get 34° total... You need more advance...
 

Kendog 170

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What's your vacuum at hyway cruising. Idle maybe 8 but cruising maybe 15? That's when you need it the most but I'm no expert. I agree about getting 34 total mechanical too. Maybe HallifaxHops will chime in.
 

Ricks70Duster340

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So I’ve got a recently built 340 with a Comp cam in it with 274/286 of duration and 488/391 lift. It only makes around 8-10hg of vac at idle, and I’m pretty sure there are no leaks and compression is good. I’ve got it set at around 13-15 initial and 28 total right now, which is as far as my current mechanical will allow right now. That level of vac is about as much as I’ll get.

Now I didn’t build the engine but the car runs good and it has the standard electric conversion kit on it with a blue box and a no-name (proform?) distributor. (Sorry for the bad pics but perhaps someone can confirm this?)

The vac advance is useless at idle as it won’t do anything below like 15hg (I’ve checked this). So I’m now debating with myself what to do in order for me to be able to work with timing a bit more. I think my options are to 1) just skip the vac advance and work with the mechanical advance on my current distributor. (But what springs to use if I don’t even know the brand and version of the distributor!?) 2) get a vac can that triggers at a lower vacuum and keep my current distributor (but those seem hard to find), 3) buy a new all mechanical distributor to replace my no name one and keep the rest, or 4) replace the entire system and go to like MSD.

What would you guys recommend? As mentioned the car runs good, but again I don’t know if I’m leaving HP on the floor by not being able to adjust timing more…?

View attachment 1715991364

View attachment 1715991365
Your timing needs to be advanced, which is probably why you're seeing such low vacuum. Vacuum advance is important in street cars, so I would not get rid of it but simply tune what you have. You said that the timing is advanced as far as possible. Why can't you advance the initial timing a few degrees?
 

halifaxhops

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You can adjust that vacuum advance for lower vacuum to activate the advance also.

vac1.JPG
 

Professor Fate

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[2] If that is an aftermarket dist you have, it should have an Allen Key [ 3/32"] inside the nipple. Screw in fully CW, softest spring setting. This is usually about 5-7" of vacuum, & it should pull the VA actuator in because as it advances the timing, the vacuum increases.

You can adjust that vacuum advance for lower vacuum to activate the advance also.
^^^Easiest, most effective $0 solutions.
 

AAndrews

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So I’ve got a recently built 340 with a Comp cam in it with 274/286 of duration and 488/391 lift. It only makes around 8-10hg of vac at idle, and I’m pretty sure there are no leaks and compression is good. I’ve got it set at around 13-15 initial and 28 total right now, which is as far as my current mechanical will allow right now. That level of vac is about as much as I’ll get.

Now I didn’t build the engine but the car runs good and it has the standard electric conversion kit on it with a blue box and a no-name (proform?) distributor. (Sorry for the bad pics but perhaps someone can confirm this?)

The vac advance is useless at idle as it won’t do anything below like 15hg (I’ve checked this). So I’m now debating with myself what to do in order for me to be able to work with timing a bit more. I think my options are to 1) just skip the vac advance and work with the mechanical advance on my current distributor. (But what springs to use if I don’t even know the brand and version of the distributor!?) 2) get a vac can that triggers at a lower vacuum and keep my current distributor (but those seem hard to find), 3) buy a new all mechanical distributor to replace my no name one and keep the rest, or 4) replace the entire system and go to like MSD.

What would you guys recommend? As mentioned the car runs good, but again I don’t know if I’m leaving HP on the floor by not being able to adjust timing more…?

View attachment 1715991364

View attachment 1715991365



Have you tried here for a solution?



 

Mattax

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So I’ve got a recently built 340 with a Comp cam in it with 274/286 of duration and 488/391 lift. It only makes around 8-10hg of vac at idle, and I’m pretty sure there are no leaks and compression is good. I’ve got it set at around 13-15 initial and 28 total right now, which is as far as my current mechanical will allow right now. That level of vac is about as much as I’ll get.
That cam in a 340 probably wants 16 to 18* initial, and it that should not advance until 750 or even as much as 850 rpm. Its critical to measure the rpm of the timing.
The vac advance is useless at idle as it won’t do anything below like 15hg (I’ve checked this).
Factory distributors generally were designed so the vacuum advance is used only to increase timing when the mixture is low density under light to moderate load.
Whether the mechanical advance in your distributor will allow you to do that will ahve to be determined by measuring the timing vs rpm and examining the mechanism.

Measure the timing vs rpm from as slow as the engine will go to as high of rpm as you feel safe.
If the advance is too long or too quick and you can't or don't want to make adjustments to it, then sometimes using manifold vacuum at idle can be used to advantage.
You'll have to adjust the vacuum advance as Halifaxhops shows in the How To.

There should be no vacuum advance under heavy load (close to wide open throttle). As Chrysler tech explained, its purpose is to make oup for the slower combustion of low density, lower loads in the combustion chamber. Heavy load is when mixtures are richer, but exactly what the manifold vacuum will be varies from engine to engine. No vacuum advance under 8.5"Hg was typical for hi-performance Chryslers.

see


good luck. I'm not on much, but you can use the search here for examples of hi performance timing.
 
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Phreakish

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[4] An MSD system would do nothing for this engine except lighten your wallet.....

Depends on which system. The 6al-2 programmable would allow for a custom map including vaccum advance.
I wouldn't suggest it for a street car though, since MSD durability is a forever looming question mark.
 

roccodart440

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I don't run VA on my car. 28 total seems really low. IS there a reason you have it set to 28 total?
 

j par

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I've kind of always understood vacuum as an economy thing. And when I hear leave horsepower on the table I'm not thinking of economy nor the cam you described.
Just recently I purchased a summit distributor that was plug and play with the electronic ignition in it for $325. It likely is the same thing as the Edelbrock one for $450. Anyways it gives like the MSD distributor I have in my duster the option to change the mechanical curve whether it be the springs or the amount of timing. Also it has a vacuum advance that's adjustable and can be taken off as well which I did because I don't usually build economy cars.
Also with theforementioned distributor it needs a particular coil that has less than .500 amps... So beware that that may be another $75 investment....
This is just what I did others may have a million different ways but I'm just throwing my hat in and my two cents for another way...
 

Phreakish

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I've kind of always understood vacuum as an economy thing. And when I hear leave horsepower on the table I'm not thinking of economy nor the cam you described.
Just recently I purchased a summit distributor that was plug and play with the electronic ignition in it for $325. It likely is the same thing as the Edelbrock one for $450. Anyways it gives like the MSD distributor I have in my duster the option to change the mechanical curve whether it be the springs or the amount of timing. Also it has a vacuum advance that's adjustable and can be taken off as well which I did because I don't usually build economy cars.
Also with theforementioned distributor it needs a particular coil that has less than .500 amps... So beware that that may be another $75 investment....
This is just what I did others may have a million different ways but I'm just throwing my hat in and my two cents for another way...

Vacuum advance has a lot more effect than just economy. Throttle response, part throttle cruise, gear changes, idle quality and the way a stick car behaves at low rpm are all affected by vacuum (load based) advance. I use my efi to control the timing on my dart, and it's an entirely different car when it's programmed with no additional advance based on MAP reading. With the vacuum advance programmed in, it runs better and has a ton more part-throttle torque and runs through the revs so much faster that it's silly. After a year of experimenting with the programmable ignition map I wouldn't run without some vacuum advance in the mix. I'm able to crank at 15 degrees, idle at 28, and then keep my initial at 24 with an all-in of 34, and cruise with 50 degrees of advance. I don't care what the MPG is, it just plain drives better and pulls harder any time the pedal is pressed. The driving experience is just better all over.

The hard part about doing it with most vacuum pods is making them work with hopped-up parts. Low manifold vac at idle and reduced vac at cruise make OEM style pods behave strangely. Excessive vacuum advance can also be a pain and requires limiting how much can be pulled in over the mechanical. But once it's dialed in, it's amazing. Keeps the pops and farts to a minimum when granny shifting, keeps the plugs clean and happy, keeps the engine cool at idle without having to run a 12 gallon cooling system (yes that's an exaggeration).

The common wisdom I've run across lately is that the vacuum canister should pull completely in at 2" lower vacuum than the engine produces at idle. This helps prevent the vac can from fluctuating at idle and causing instability issues or a 'hanging' idle situation, which makes sense to me.

Seems like most cans available these days operate at much higher vacuum readings (beginning at 8 and all-in by 16-18"). The MSD ones are the worst offenders. The adjustable canisters on most cheap distributors (which used chevy points-style cans) seem to be able to be adjusted down to an all-in vacuum of about 10-12", which is usually just enough to still be funky unless you like 1,000+ rpm idle speeds. The low vacuum 'B28' cans (operates from ~3" to ~8" of vac) for those cheap distributors are no longer available anywhere, and the Crane adjustables which could match them are also long gone :(

There's an excellent article done by @Mattax Here.
 

DemonSwede

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[1] You can lock the timing at say, 28 or 30*. So 30* everywhere. The problems then are:
- might get pinging at lower rpms
- stock starter may not crank, in which case a 3hp mini starter is needed.
[2] If that is an aftermarket dist you have, it should have an Allen Key [ 3/32"] inside the nipple. Screw in fully CW, softest spring setting. This is usually about 5-7" of vacuum, & it should pull the VA actuator in because as it advances the timing, the vacuum increases.
[3] The Torker manifold is a very poor choice. Dual plane would be better suited to the cam.
[4] An MSD system would do nothing for this engine except lighten your wallet.....
[5] The best move is to buy a GM HEI type dist. These have a number of advantages over the one you have now:
- comes with adj vac adv
-bigger cap, less chance of spark scatter
- comes with E core coil. You get more spark energy & can open the plug gaps to 0.060"
- gets rid of the Bal Res, ECU, everything is contained within the dist.
- centri weights easy access to modify the curve

I use the cheap Chinese HEIs that I can buy here for $80 for a Chebby. Have used heaps of them. Well made, lubricated, never had a failure & come with a steel gear for use with steel roller cams.
Thanks! I tried to adjust the canister but didn’t see any change in when it activated. Was testing it with a vacuum pump and no matter which way I turned the hex it still activates at around 13-15hg…‍♂️
 

DemonSwede

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Your timing needs to be advanced, which is probably why you're seeing such low vacuum. Vacuum advance is important in street cars, so I would not get rid of it but simply tune what you have. You said that the timing is advanced as far as possible. Why can't you advance the initial timing a few degrees?
Well I probably can, but to get my total to 34-ish my initial will be around 20 with the current curve. Don’t seem to see much change in vacuum at idle no matter how much I advance initial
 

DemonSwede

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What's your vacuum at hyway cruising. Idle maybe 8 but cruising maybe 15? That's when you need it the most but I'm no expert. I agree about getting 34 total mechanical too. Maybe HallifaxHops will chime in.
Haven’t checked that as I don’t have a vac gauge in the car!
 

DemonSwede

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I don't run VA on my car. 28 total seems really low. IS there a reason you have it set to 28 total?
As mention that’s where it ends up with initial at around 15, to get to 34 or so I would be at 20 initial. Not to say that’s bad…it just seemed like a lot to me!
 

DemonSwede

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Thanks all for your support and ideas, appreciate it! So a few takeaways from the above seem to be:
* Regardless I should increase my advance to at least around 34 total and 18-20 initial
* Vaccum advance is good (if it actually works!) The challenge will be to get it to work from around 10hg and at the right time!
* I might look at a dual plane intake instead to see if that helps with vaccum.

This gives me something to start with, will keep you posted! Thanks!
 

GTX JOHN

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Did I miss why you could not advance Distributor more?
If it hits something just more the wires one position of the cap
and retime it!
 

Phreakish

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Thanks all for your support and ideas, appreciate it! So a few takeaways from the above seem to be:
* Regardless I should increase my advance to at least around 34 total and 18-20 initial
* Vaccum advance is good (if it actually works!) The challenge will be to get it to work from around 10hg and at the right time!
* I might look at a dual plane intake instead to see if that helps with vaccum.

This gives me something to start with, will keep you posted! Thanks!

It's easy to modify the distributor for slightly more advance. There's lots of threads here which describe how the advance works and how to modify it. Going from 28 to 34 degrees will be quite the change in performance too.

The intake swap could add a couple inches of vacuum, which might be enough. Since adjusting your can makes no difference, you may want to swap it. It's possible that the guts are screwed up and the adjustment isn't working properly as a result.
 

crackedback

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XE274H with 18-20 initial timing should pull around 12in of vacuum at 800-850rpm.

Carb? If a carter/ede, make sure the rods are staying down when idling. If they aren't put your fingers on top of them and lightly hold them down, see if it cleans up a but.

That intake... UGH... they should have smelted all those POS and made air gaps out of them! I hate that intake, garbage piece. use them as doorstops before ever putting them on engine. Can you tell my thoughts on the intake.

Get the idle sorted out, then worry about the total number. If you advance the timing a lot, your total might get out of bounds if you try to drive the car.
 

j par

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Vacuum advance has a lot more effect than just economy. Throttle response, part throttle cruise, gear changes, idle quality and the way a stick car behaves at low rpm are all affected by vacuum (load based) advance. I use my efi to control the timing on my dart, and it's an entirely different car when it's programmed with no additional advance based on MAP reading. With the vacuum advance programmed in, it runs better and has a ton more part-throttle torque and runs through the revs so much faster that it's silly. After a year of experimenting with the programmable ignition map I wouldn't run without some vacuum advance in the mix. I'm able to crank at 15 degrees, idle at 28, and then keep my initial at 24 with an all-in of 34, and cruise with 50 degrees of advance. I don't care what the MPG is, it just plain drives better and pulls harder any time the pedal is pressed. The driving experience is just better all over.

The hard part about doing it with most vacuum pods is making them work with hopped-up parts. Low manifold vac at idle and reduced vac at cruise make OEM style pods behave strangely. Excessive vacuum advance can also be a pain and requires limiting how much can be pulled in over the mechanical. But once it's dialed in, it's amazing. Keeps the pops and farts to a minimum when granny shifting, keeps the plugs clean and happy, keeps the engine cool at idle without having to run a 12 gallon cooling system (yes that's an exaggeration).

The common wisdom I've run across lately is that the vacuum canister should pull completely in at 2" lower vacuum than the engine produces at idle. This helps prevent the vac can from fluctuating at idle and causing instability issues or a 'hanging' idle situation, which makes sense to me.

Seems like most cans available these days operate at much higher vacuum readings (beginning at 8 and all-in by 16-18"). The MSD ones are the worst offenders. The adjustable canisters on most cheap distributors (which used chevy points-style cans) seem to be able to be adjusted down to an all-in vacuum of about 10-12", which is usually just enough to still be funky unless you like 1,000+ rpm idle speeds. The low vacuum 'B28' cans (operates from ~3" to ~8" of vac) for those cheap distributors are no longer available anywhere, and the Crane adjustables which could match them are also long gone :(

There's an excellent article done by @Mattax Here.
Well like my last sentence there says this is what works for me and there may be a million other ways to get the job done...
There's probably a reason I have the slowest strokers on the Internet lol...
 

DemonSwede

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XE274H with 18-20 initial timing should pull around 12in of vacuum at 800-850rpm.

Carb? If a carter/ede, make sure the rods are staying down when idling. If they aren't put your fingers on top of them and lightly hold them down, see if it cleans up a but.

That intake... UGH... they should have smelted all those POS and made air gaps out of them! I hate that intake, garbage piece. use them as doorstops before ever putting them on engine. Can you tell my thoughts on the intake.

Get the idle sorted out, then worry about the total number. If you advance the timing a lot, your total might get out of bounds if you try to drive the car.
I’ve got a Holley 4150 on it and it idles well, no problem at all to have it idling at around 650-850, it’s choppy but stable! Wish I could post a video for you!

Yeah definitely sounds like I need to get an air gap intake instead!:))
 
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