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vac advance
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Dieselshine
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OK spill it.What was it you were wanting from me???
I don't have an answer for that question but I thought I'd inform you with a bunch of useless info that you'll use everyday lmao its not my fault I was born this way.... lol
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Lehbs
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Ok so to refresh an old thread, and because I am dumb enough to just start pulling things off without checking vital things like which line has no vacuum at idle; which line off the q-jet is your ported vacuum. I labled all the lines in accordance with the line diagram, and so know which is which, but I can't find anyone who labels if it is the one off the carb port J, B, L, T or H. I mean L is pretty simple, it is the larger port, over to the PCV, but on my setup I had connections to J and B. T was capped off, and H isn't there (this is an 85 diagram, mine is an 84 but from looking under the hood this weekend I think it is just about the same)
Thanks Guys.
Thanks Guys.
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chengny
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I can't really find a question in your post.
Are you asking where to connect the vacuum advance?
Are you trying to re-assemble the system as stock?
Or are you just asking which ports are timed (ported) and which always see manifold pressure (unported)?
To identify which ports are unported, start the engine and (while idling) remove a hose. Put a small piece of paper over the port tube where the hose was connected.
If it stays there, the port is connected below the plates (unported/manifold).
If it falls off, the tap is connected above the plates (unported/timed).
Here are 3 different vacuum hose routing diagrams for a 1984/305/automatic transmission (non California & and not high altitude):
Are you asking where to connect the vacuum advance?
Are you trying to re-assemble the system as stock?
Or are you just asking which ports are timed (ported) and which always see manifold pressure (unported)?
To identify which ports are unported, start the engine and (while idling) remove a hose. Put a small piece of paper over the port tube where the hose was connected.
If it stays there, the port is connected below the plates (unported/manifold).
If it falls off, the tap is connected above the plates (unported/timed).
Here are 3 different vacuum hose routing diagrams for a 1984/305/automatic transmission (non California & and not high altitude):
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Lehbs
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Sorry, but in answering your question, my question is, which line is ported and which are manifold. I have the old one all off right now, and would like to be able to just connect everything up and ready to go. I took a cell phone photo of the vac diagram tonight. will post up tomorrow. It only lists JBLT off the carb. I know L is for the PCV, T was capped at the carb, so which one of J and B is the manifold vac and which one is ported? Or do I just start it up once it is back together with the two lines disconnected and check to see which one draws vacuum under idle?
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If your going to remove anything( which is a watse of time, unless there is a specific problem) put your finger over a vac port, and see which one sucks when you blip the throttle. It usually ends up being one of the ports mounted in the throttle plate.
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Usually so. It has to be above the plates otherwise it would direct manifold vacuum.
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You are absolutley right about one thing and IMO, one thing only. That it is an opinion thing. I've argued this point on several forums, and I find it's about a 50/50 on which way to do it. My way is opposite of yours as I'm sure somewhere in this thread I"ve explained already. Vac Advance is to be hooked to ported vacuum period. Racing app, Street App, Centrifical advance or not. I know I'll never change anyone's opinion who strongly beleive the way they do, but I do explain, if your going to hook to manifold vacuum and not ported, then why not just set your initial timing to what it would be hooked to manifold vacuum and be done, cuz that is essentially all you are doing, is increasing or advancing the timing, then you romp on the motor and lose vacuum, then you are RETARDING the timing while romping on the motor when you should actually want to be ADVANCING timing to romp on the motor and expect performace gain. It's just backwards to hook to manifold.
SO, I've had to stop and give thought to WHY in the hell do some people think hooking it up wrong works??? It has to make sense one way or another or 50% of the people wouldn't think it's right, when clearly ALL auto manufacturers clearly explain to use ported vac, and disconnect it while setting initial timing and for a reason. So I figure what is happening, those who do it this way are setting initial timing. Hook to manifold vac and get advanced timing with a FALSE POSITIVE or performance gain, and when they nail the pedal and romp on it, they actuallly LOSE or RETARD timing which keeps the motor from pinging. So I do see why they think it helps in performance, but who needs more performance at idle, or light throttle??? That is the only scenario I can think of.But, I"ll always use ported vacuum and those who see different and more than welcome to hook to manifold vac. I'm not going to knock anyone for hooking it how they wish. It just makes a good topic argument for gearheads.
EDIT : BTW, For racing applicaitons, there is no sense in hooking vac advance up at all. Usually with a big cam, lots of valve ovelap, you don't have engine vacuum to advance it anway. Best to rely on Centrifical advance only with racing counterweights and springs so you can get more advance, and lighter springs to bring on the advance much quicker.
You Know I have never really given this much thought until the other night and this is exactly the conclusion I came to as well!
Manifold vacuum is highest at idle and progressively drops as the throttle is opened. So manifold vacuum advance is going to be advanced the most at idle and then progressively drop off. WTF. Why would that be a good thing to do?
If any one has actual science to point out why manifold vacuum is correct or that my analysis of the system is incorrect, please by all means share?
There seems to be a lot of different opinions on this subject to the point of name calling. So, lets have it out!
Science only people!
Name calling will only make you look like you have RETARDED timing lol
350runner
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No science to add here, but some advice to anyone... Do not suggest someone just go hooking up the vac advance without knowing what their initial and mechanical timing are.... detonation( silent or not) could be present and ruin that engine. Then who pays for it? I've seen this a ton on that other forum...
Sent from the dust in front of you!
Sent from the dust in front of you!
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rich weyand
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OK, MO, here's the science.
At idle, let's say you are running 16 inches of vacuum. That's half an atmosphere coming in the intake, right? So if you have a stock 350 with 8:1 compression, you only get up to 4 atmospheres of cylinder pressure. This is a thin mixture, and burns slowly. Therefore, you need more advance to have the combustion pressure max out at the right point for pushing on the piston.
Now you open the throttle, and the intake goes to one atmosphere. Now your 8:1 compression can get up to 8 atmospheres of pressure in the cylinder. This thicker mixture burns much faster, so you need to ignite later to get the pressure pulse to hit the piston at the right time.
That is, the vacuum advance is to give extra time to thin mixtures to burn. The rationalizations I see for ported vacuum (as above) never mention the combustion chemistry: thinner mixtures burn more slowly, they need more time.
Centrifugal advance is because the time it takes for the mixture to burn is in fractions of seconds, but the ignition timing is set in degrees. The same number of degrees is a shorter time in seconds when the engine is spinning faster, so the spark needs to be advanced in degrees to give the same seconds for the mixture to burn.
Early vehicles had mechanical advance on the steering wheel, like a turn signal lever. In 1930, Studebaker introduced the vacuum advance to use the measure of how thin the mixture was (the intake manifold vacuum) to advance the timing to give the thin mixture time to burn. It was not needed when the throttle plate was open and the mixture was thicker. This mechanical advance was connected to manifold vacuum, and all vacuum advance modules on production vehicles from 1930 through 1967 were connected to manifold vacuum, because that is the direct measurement of what the cylinder pressure will be when ignition occurs.
On January 1, 1968, new emissions standards came into law. These standards included a measurement of unburned hydrocarbons at idle. Two reasons for that: 1) it was easy to measure when people came in to get their cars tested, because they could just idle there and you stuck the sensor up the tailpipe and measured it, and 2) because unburned hydrocarbons were a big issue in cities that had a lot of traffic jams (people idling) and the resulting smog, which was a huge health issue.
The solution was to pump air into the exhaust manifolds with an AIR pump, to give these unburned hydrocarbons oxygen to burn with, and to switch the idle timing to ported vacuum. This was still the direct measurement of engine cylinder pressure at all throttle openings except at idle, but at idle it was late (lack of advance to accommodate the thinner mixture). The result was that the charge was still burning when the exhaust valve opened, releasing the burning charge into the exhaust manifold where the combination of unburned hydrocarbons, injected air, and a burning charge could continue the combustion process and reduce unburned hydrocarbons. The higher exhaust temperatures also helped primitive early catalytic converters reach their required operating temperature.
It also resulted in higher engine temperatures (because the charge was still burning when it passed out through the exhaust runners in the head), a rough idle, and an off-idle hesitation when the throttle was opened until the vacuum advance caught up to where it would have been if it had been monitoring the manifold vacuum, and therefore the cylinder pressure. Lots of accommodations in base timing and mixture were made to make this work, more or less.
I was 15 years old, car crazy, and working in a machine shop when all this happened in 1968/1969. It was in all the car mags at the time. Some of the guys buying new cars were disgusted with the performance. They machined off the impeller vanes inside the AIR pumps and ran on manifold vacuum. They just had to switch to ported vacuum to pass visual inspection. Actual tailpipe inspection didn't take place where I lived. Most people now don't put AIR pumps on their period cars and trucks when they fix them up, but somehow ported vacuum hung on.
But the old-timers who were around back in the day know all this. For example, the documentation for the Edelbrock 1406 notes that the ported vacuum port is to be used for vacuum advance on emissions-equipped engines, and the manifold vacuum port is to be used for vacuum advance on non-emissions-equipped engines.
One other note. A rich mixture also burns faster than a lean one. If you set the idle mixture on a vehicle with an A/FR meter -- say to 13.5:1 because a thin mixture carbureted at low stack velocity isn't uniform and any cylinder getting over 17:1 won't light -- then switching from ported to manifold vacuum will be a huge difference for the positive.
However, one method for setting idle mixture that I have seen a lot is to adjust for maximum vacuum. On manifold vacuum, this will get you to about the right mixture. But if you are hooked to ported vacuum, and you adjust the idle mixture for maximum vacuum, you will be setting the mixture much richer than 13.5:1. The richer mixture burns faster than the correct mixture, partially making up for the late spark. So it masks some of the negative effects of ported vacuum. Switching this rich idle mixture over to manifold vacuum may idle worse than on ported, but it's a combination of errors.
Thus my one-liner on the other thread that if the other poster's truck ran better on ported than manifold vacuum, he didn't know how to tune a carburetor. Maximum performance will be achieved with proper tuning on manifold vacuum. It is the best indicator of dynamic cylinder pressure at ignition, and how much time the charge needs to burn optimally. That's why manufacturers used manifold vacuum from 1930 through 1967.
I put this link up a lot, but I'm not sure anyone reads it. It's from the Corvette forums, and is an explanation of ignition timing written by a retired GM ignition systems engineer. It's a good read, and covers this whole thing pretty well.
http://www.corvetteactioncenter.com...ported-vs-manifold-source-vacuum-advance.html
And here's the Edelbrock diagram I mentioned.
At idle, let's say you are running 16 inches of vacuum. That's half an atmosphere coming in the intake, right? So if you have a stock 350 with 8:1 compression, you only get up to 4 atmospheres of cylinder pressure. This is a thin mixture, and burns slowly. Therefore, you need more advance to have the combustion pressure max out at the right point for pushing on the piston.
Now you open the throttle, and the intake goes to one atmosphere. Now your 8:1 compression can get up to 8 atmospheres of pressure in the cylinder. This thicker mixture burns much faster, so you need to ignite later to get the pressure pulse to hit the piston at the right time.
That is, the vacuum advance is to give extra time to thin mixtures to burn. The rationalizations I see for ported vacuum (as above) never mention the combustion chemistry: thinner mixtures burn more slowly, they need more time.
Centrifugal advance is because the time it takes for the mixture to burn is in fractions of seconds, but the ignition timing is set in degrees. The same number of degrees is a shorter time in seconds when the engine is spinning faster, so the spark needs to be advanced in degrees to give the same seconds for the mixture to burn.
Early vehicles had mechanical advance on the steering wheel, like a turn signal lever. In 1930, Studebaker introduced the vacuum advance to use the measure of how thin the mixture was (the intake manifold vacuum) to advance the timing to give the thin mixture time to burn. It was not needed when the throttle plate was open and the mixture was thicker. This mechanical advance was connected to manifold vacuum, and all vacuum advance modules on production vehicles from 1930 through 1967 were connected to manifold vacuum, because that is the direct measurement of what the cylinder pressure will be when ignition occurs.
On January 1, 1968, new emissions standards came into law. These standards included a measurement of unburned hydrocarbons at idle. Two reasons for that: 1) it was easy to measure when people came in to get their cars tested, because they could just idle there and you stuck the sensor up the tailpipe and measured it, and 2) because unburned hydrocarbons were a big issue in cities that had a lot of traffic jams (people idling) and the resulting smog, which was a huge health issue.
The solution was to pump air into the exhaust manifolds with an AIR pump, to give these unburned hydrocarbons oxygen to burn with, and to switch the idle timing to ported vacuum. This was still the direct measurement of engine cylinder pressure at all throttle openings except at idle, but at idle it was late (lack of advance to accommodate the thinner mixture). The result was that the charge was still burning when the exhaust valve opened, releasing the burning charge into the exhaust manifold where the combination of unburned hydrocarbons, injected air, and a burning charge could continue the combustion process and reduce unburned hydrocarbons. The higher exhaust temperatures also helped primitive early catalytic converters reach their required operating temperature.
It also resulted in higher engine temperatures (because the charge was still burning when it passed out through the exhaust runners in the head), a rough idle, and an off-idle hesitation when the throttle was opened until the vacuum advance caught up to where it would have been if it had been monitoring the manifold vacuum, and therefore the cylinder pressure. Lots of accommodations in base timing and mixture were made to make this work, more or less.
I was 15 years old, car crazy, and working in a machine shop when all this happened in 1968/1969. It was in all the car mags at the time. Some of the guys buying new cars were disgusted with the performance. They machined off the impeller vanes inside the AIR pumps and ran on manifold vacuum. They just had to switch to ported vacuum to pass visual inspection. Actual tailpipe inspection didn't take place where I lived. Most people now don't put AIR pumps on their period cars and trucks when they fix them up, but somehow ported vacuum hung on.
But the old-timers who were around back in the day know all this. For example, the documentation for the Edelbrock 1406 notes that the ported vacuum port is to be used for vacuum advance on emissions-equipped engines, and the manifold vacuum port is to be used for vacuum advance on non-emissions-equipped engines.
One other note. A rich mixture also burns faster than a lean one. If you set the idle mixture on a vehicle with an A/FR meter -- say to 13.5:1 because a thin mixture carbureted at low stack velocity isn't uniform and any cylinder getting over 17:1 won't light -- then switching from ported to manifold vacuum will be a huge difference for the positive.
However, one method for setting idle mixture that I have seen a lot is to adjust for maximum vacuum. On manifold vacuum, this will get you to about the right mixture. But if you are hooked to ported vacuum, and you adjust the idle mixture for maximum vacuum, you will be setting the mixture much richer than 13.5:1. The richer mixture burns faster than the correct mixture, partially making up for the late spark. So it masks some of the negative effects of ported vacuum. Switching this rich idle mixture over to manifold vacuum may idle worse than on ported, but it's a combination of errors.
Thus my one-liner on the other thread that if the other poster's truck ran better on ported than manifold vacuum, he didn't know how to tune a carburetor. Maximum performance will be achieved with proper tuning on manifold vacuum. It is the best indicator of dynamic cylinder pressure at ignition, and how much time the charge needs to burn optimally. That's why manufacturers used manifold vacuum from 1930 through 1967.
I put this link up a lot, but I'm not sure anyone reads it. It's from the Corvette forums, and is an explanation of ignition timing written by a retired GM ignition systems engineer. It's a good read, and covers this whole thing pretty well.
http://www.corvetteactioncenter.com...ported-vs-manifold-source-vacuum-advance.html
And here's the Edelbrock diagram I mentioned.
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MadOgre
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My 1806 instruction sheet doesn't read like your picture. It says that you need to determine if your distributor is timed or runs off full vacuum.
So are my distributor advance weights different if it is timed rather than full manifold ?
Is the distributor the deciding factor or is it referring to keeping things the way they were ?
Maybe we need to include full knowledge of distributor vacuum advance in this discussion ?
So are my distributor advance weights different if it is timed rather than full manifold ?
Is the distributor the deciding factor or is it referring to keeping things the way they were ?
Maybe we need to include full knowledge of distributor vacuum advance in this discussion ?
rich weyand
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The issue is combustion chemistry. If thinner mixtures didn't burn slower, there would be no reason for vacuum advance at all. The vacuum indicates the cylinder pressure at the point of ignition, which indicates how fast the charge will burn. You can tailor vacuum advance however you want as far as how much advance at vacuum and at what vacuum it comes off and what the ramp is, depending on engine compression ratio and induction system, but there is no distinction between idle and slightly-above-idle throttle position that makes any sense for optimal ignition timing.
I should expand on this. The exact right ignition timing for a given intake manifold vacuum depends on a lot of things: the kind of fuel, the compression ratio of the engine, the geometry of the heads, the total cylinder volume at ignition, the current rpms -- lots of paramters. RPMs is handled by centrifugal advance. Most of the rest for open throttle conditions are handled by the base timing.
At low throttle positions -- high vacuum conditions -- the spark has to be fired earlier because of the slower burn. How much earlier, and at what vacuum advance becomes necessary, varies with all of the above parameters. So adjusting advance curves to get the correct ignition timing for various vacuum settings can be helpful in some applications. For most applications the stock advance curves are OK. Approximate, but OK.
One parameter that makes no difference to correct ignition timing at a given vacuum is whether the throttle plate is at idle or slightly above idle. There should not be a discontinuity in the vacuum advance curve based on the throttle plate position, and with ported vacuum there is. Running base timing at idle (which is what ported vacuum does) is way, way off the correct ignition timing.
I should expand on this. The exact right ignition timing for a given intake manifold vacuum depends on a lot of things: the kind of fuel, the compression ratio of the engine, the geometry of the heads, the total cylinder volume at ignition, the current rpms -- lots of paramters. RPMs is handled by centrifugal advance. Most of the rest for open throttle conditions are handled by the base timing.
At low throttle positions -- high vacuum conditions -- the spark has to be fired earlier because of the slower burn. How much earlier, and at what vacuum advance becomes necessary, varies with all of the above parameters. So adjusting advance curves to get the correct ignition timing for various vacuum settings can be helpful in some applications. For most applications the stock advance curves are OK. Approximate, but OK.
One parameter that makes no difference to correct ignition timing at a given vacuum is whether the throttle plate is at idle or slightly above idle. There should not be a discontinuity in the vacuum advance curve based on the throttle plate position, and with ported vacuum there is. Running base timing at idle (which is what ported vacuum does) is way, way off the correct ignition timing.
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