Vacuum advance information and myths

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I don't understand why when I go to a car show I see so many cars with the VA can plugged off, no vacuum lines going to them
 
I've done a little bit of reading on this stuff and it seems MOST, not all but MOST videos/articles lean towards manifold vacuum. I don't know enuf about it to give any advice other than each engine is unique and some may or may not run better on MVA or PVA.
 
All the VA does is supply timing. The engine doesn't care how it gets the necessary timing to run as efficiently as possible.....as long as it gets it.
If the engine has a dist that has the timing locked, ie it idles at 36*, has 36* at WOT & everywhere in between, then it is unlikely to benefit much, if at all, from VA.
Once the engine is at cruising speed, MVA & PVA are equal in what they do. The benefit that MVA [ has over PVA ] is that MVA is active at idle & PVA is NOT. Depending on initial timing, MVA can substantially improve idle quality, vacuum & tip response; the engine also runs cooler. Rarely is MVA not a benefit.
 
volaredon said:
I don't understand why when I go to a car show I see so many cars with the VA can plugged off, no vacuum lines going to them
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It's a Chevy guy thing. They all think vacuum advance is not needed, when 99.9% of them drive their junk on the street and they need it. Others look what the Chevy guys do and copy it thinkin it's the right thing to do. It's not. I won't get into ported or manifold vacuum, but if you drive your car on the street, the vacuum advance SHOULD be hooked up.
 
RustyRatRod said:
It's a Chevy guy thing. They all think vacuum advance is not needed, when 99.9% of them drive their junk on the street and they need it. Others look what the Chevy guys do and copy it thinkin it's the right thing to do. It's not. I won't get into ported or manifold vacuum, but if you drive your car on the street, the vacuum advance SHOULD be hooked up.
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Well, I wanna hear Ya get into it. What Do you Prefer? Over or under the throttle blades? And here's an odd one the Holly on my 340 appears to be pulling vacuum from the top vacuum port even at idle.... I haven't actually put my gauge on it but I swear I can feel it with my finger
 
Ironracer said:
Well, I wanna hear Ya get into it. What Do you Prefer? Over or under the throttle blades? And here's an odd one the Holly on my 340 appears to be pulling vacuum from the top vacuum port even at idle.... I haven't actually put my gauge on it but I swear I can feel it with my finger
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It depends totally on the combination. For the most part, I've had the best success with ported vacuum. I "figured" since this slant 6 only pulls 6hg at idle it might benefit from manifold vacuum. I tried it. It didn't run as well so back to ported it went. Most street applications I imagine will respond better to a ported signal, but there are some exceptions. Chevy even used manifold vacuum straight from the factory on a few of their cars. The big block SS Chevelles are but one example.
 
Iron,
At idle, if you can 'feel' suction with your finger on the port above the t/blades, then that is bad news. Blades open too far for proper transition from idle to main cct. What cam?
 
Dig it. I remember reading an article over a decade ago about it. Using a Vacuum gauge to set and adjust things. The Satellite actually responded well. Gave it a really "takita-takita" killer choppy idle. I can't remember the process. I'll try to find the article again and maybe post it up here so everyone can kick me in the teeth, lol. I want to say you set the timing till the motor pulls the highest vacuum possible and then adjust your air idle mix screws until you get even more vacuum. But honestly I can't remember
 
Bewy said:
Iron,
At idle, if you can 'feel' suction with your finger on the port above the t/blades, then that is bad news. Blades open too far for proper transition from idle to main cct. What cam?
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.484 MP shaft. Pulled Carb and flipped her, Z t slots looked, cracked the rear a c hair
 
Cars driven on the street definitely need vac advance and setting it up correctly would solve a lot of peoples cooling issues. When you are running in a lean condition (idle or very light throttle cruising) you need way more advance than you would expect. For years I had my car set up with no vac advance because the muscle car guys say "you dont need it" - well that is true if you are at WOT all the time but not if you drive your car around. This car was chronically hot and could not idle in traffic for more than a few minutes. You could hear the coolant boiling when you turned it off etc. By simply hooking up the vac and making some adjustments so that it was working at idle the problem is solved - it can idle indefinitely and will never get much over the thermostat rating. This also helps a car that runs hot at a steady light cruise on the highway. From what I have researched, the ported vacuum was implemented in the 1970s as an early emissions move. The end effect was increasing the heat in the manifolds as a post-engine afterburner of exhaust prior to developing catalytic converters. The same car I mentioned with no vac advance heated the engine compartments and hood hinges to the point you could not touch them for this reason. The car now has about 13 deg initial, about 30 deg at idle with vac advance, and over 50 degrees going down the highway with the throttle just cracked open.
 
Sweet Deal, I've been Playing with Mopars in TampaBay since '89
Only overheated when I had a leak. But ALWAYS used Vac Advance
 
JTECDN said:
Cars driven on the street definitely need vac advance and setting it up correctly would solve a lot of peoples cooling issues. When you are running in a lean condition (idle or very light throttle cruising) you need way more advance than you would expect. For years I had my car set up with no vac advance because the muscle car guys say "you dont need it" - well that is true if you are at WOT all the time but not if you drive your car around. This car was chronically hot and could not idle in traffic for more than a few minutes. You could hear the coolant boiling when you turned it off etc. By simply hooking up the vac and making some adjustments so that it was working at idle the problem is solved - it can idle indefinitely and will never get much over the thermostat rating. This also helps a car that runs hot at a steady light cruise on the highway. From what I have researched, the ported vacuum was implemented in the 1970s as an early emissions move. The end effect was increasing the heat in the manifolds as a post-engine afterburner of exhaust prior to developing catalytic converters. The same car I mentioned with no vac advance heated the engine compartments and hood hinges to the point you could not touch them for this reason. The car now has about 13 deg initial, about 30 deg at idle with vac advance, and over 50 degrees going down the highway with the throttle just cracked open.
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"By simply hooking up Vac and making adjustments so that it was Working at idle".
I dig that, and get it. So, You Went Manifold Vac, but what Adjustments did you make? Sorry, if I come across rude on that, Totally NOT Intended! Just wanted to know!
 
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Ironracer said:
"By simply hooking up Vac and making adjustments so that it was Working at idle".
I dig that, and get it. So, You Went Manifold Vac, but what Adjustments did you make? Sorry, if I come across rude on that, Totally NOT Intended! Just wanted to know!
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Any vac advance that has a hex shape in the housing can be adjusted. If you insert a 3/32 hex key into the fitting you can thread in-out a plate that is trapped in the hex. This primarily changes the amount of vacuum needed to start the vac advance moving. The amount of max advance is set by the particular part you have but by adjusting the cut-in you can also have some influence on the max amount because you run out of range or do not produce enough vacuum to pull it all the way. Like anything, the first thing you have to establish is what vacuum you have at idle and at light throttle cruise. The best plan is to make carb adjustments to maximize vacuum. Then record what the reading is as you drive the car around. There was an article on another website that gave the step by step on this whole process. In a nutshell, you want probably around 15 deg max on the vac advance and that will come in at idle and at the cruise throttle. By getting the additional advance in these situations, you will burn the fuel in the engine instead of in the exhaust manifolds, producing more power and less heat into the coolant. Just for interest - the max advance in distributor deg is usually stamped in the arm of the vac advance unit - double this for actual crank advance. If yours is too much and creates pinging at cruise you can get another unit (pre-1970 years will have a better range) or you can crimp on a piece of metal to shorten the travel of the arm.
 
JTECDN said:
Any vac advance that has a hex shape in the housing can be adjusted. If you insert a 3/32 hex key into the fitting you can thread in-out a plate that is trapped in the hex. This primarily changes the amount of vacuum needed to start the vac advance moving. The amount of max advance is set by the particular part you have but by adjusting the cut-in you can also have some influence on the max amount because you run out of range or do not produce enough vacuum to pull it all the way. Like anything, the first thing you have to establish is what vacuum you have at idle and at light throttle cruise. The best plan is to make carb adjustments to maximize vacuum. Then record what the reading is as you drive the car around. There was an article on another website that gave the step by step on this whole process. In a nutshell, you want probably around 15 deg max on the vac advance and that will come in at idle and at the cruise throttle. By getting the additional advance in these situations, you will burn the fuel in the engine instead of in the exhaust manifolds, producing more power and less heat into the coolant. Just for interest - the max advance in distributor deg is usually stamped in the arm of the vac advance unit - double this for actual crank advance. If yours is too much and creates pinging at cruise you can get another unit (pre-1970 years will have a better range) or you can crimp on a piece of metal to shorten the travel of the arm.
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One of the best explanations I have read.

Good job.
 
It is good info, but oversimplified. Setting up MVA takes time, but no special tools.
As RustRatRod, found out [ post #8 ], MVA didn't work on his combo because of the low 6" of idle vacuum. MVA will never work with vac that low. At least RRR recognised what the problem was & didn't criticise the MVA concept. About 8" is probably the lowest it will work with & that needs careful calibrating.
Another futile move is trying to get stock VA units to work on engines that have reduced vacuum due to bigger cams installed.
 
I modified my Factory Vcan by filing a portion of the stops off, to get 22*. I think Trailbeast got 24*. My alloy-headed 367 once liked 60 degrees of cruise timing but my current set-up doesn't allow for that.
 
AJ/FormS said:
I modified my Factory Vcan by filing a portion of the stops off, to get 22*. I think Trailbeast got 24*. My alloy-headed 367 once liked 60 degrees of cruise timing but my current set-up doesn't allow for that.
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60 degrees is exactly what Mopar Performance engine book says not to be surprised by including vacuum advance. They go into great detail how to get a correct timing curve and how to include and adjust vacuum advance. I remember when I was a kid, the FIRST cardinal rule when you got a Mopar was to buy either the Direct Connection or Mopar Performance engine and suspension books.......and you know what? We never had any of these prolific problems a lot of these guys are having. Having problems with timing, running hot, drive line vibration from incorrect pinion angle and on and on and on.
 
AJ/FormS said:
I modified my Factory Vcan by filing a portion of the stops off, to get 22*. I think Trailbeast got 24*. My alloy-headed 367 once liked 60 degrees of cruise timing but my current set-up doesn't allow for that.
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Hey AJ, ain't seen You in a minute! Everything OK Brother?
 
RustyRatRod said:
60 degrees is exactly what Mopar Performance engine book says not to be surprised by including vacuum advance. They go into great detail how to get a correct timing curve and how to include and adjust vacuum advance. I remember when I was a kid, the FIRST cardinal rule when you got a Mopar was to buy either the Direct Connection or Mopar Performance engine and suspension books.......and you know what? We never had any of these prolific problems a lot of these guys are having. Having problems with timing, running hot, drive line vibration from incorrect pinion angle and on and on and on.
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hyup, I bought one of those manuals, eons ago.
 
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