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  #21  
Old 03-10-2004, 01:04 PM
PhilSine PhilSine is offline
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AS COPIED FROM Vacuum Advance Page

Ignition Advance System

Without some way to adjust the ignition for changing rpm or load, the engine wouldn't make much power, and it'd be pretty hard on a gallon of gas.

These are the reasons for the ignition advance system. Three types of ignition advance are used in most street ignition systems;

Static, vacuum and centrifugal. Together, they account for total ignition advance.

The static advance is the ignition timing that's set at the crankshaft. The vacuum and centrifugal advance systems help optimize ignition timing based on

engine load and speed. When the engine's idling, the spark plug must ignite the air/fuel mixture slightly before the piston reaches the top of its stroke.

This ensures the majority of fuel is ignited and will begin to expand when the piston reaches the top of its stroke, pushing the pistons down. This takes

full advantage of the leverage effect of the connecting rod journal on the crankshaft and is known as the static timing. As engine rpm increases,

piston speed goes up. But the burning and expansion rate of the fuel stays the same. To conpensate for this, ignition of the air/fuel mixture needs

to occur earlier so the fuel can begin expanding when the piston is at the top of its stroke. If ignition begins too late, the piston is already moving

down the cylinder and the pressure against the piston generated by the expanding fuel drops. This causes a drop in cylinder pressure with a resulting

decrease in engine torque. On the other hand, if the ignition timing becomes too far advanced, the fuel begins expanding before the piston reaches the top

of its stroke. The pressure caused by the expanding fuel pushes down violently on the piston as it's still moving to the top of the cylinder, causing the hammering

noise that is typical of detonation. It also slows the momentum of the crankshaft, resulting in a loss of power. As you can see, the ignition must advance at a

certain rate to take full advantage of the lerverage effect of the crankshaft and to prevent engine damage. The vacuum advance system is responsible for increasing

ignition advance based on engine load. The lighter the engine load, the greater the advance. The greater the engine load, the less the advance. The vacuum advance

consist of a diaphragm within a metal housing. Connected to one end of the housing is a source of manifold vacuum. On the other end, a plunger is connected to

the diaphragm, wich, in turn, is attached to the distributor breaker plate. When the engine is lightly loaded, such as at idle and when cruising, manifold vacuum is high

and ignigtion timing can be advanced without causing detonation. The high manifold vacuum pulls on the diaphragm. In turn, the diaphragm plunger pulls

on the breaker plate, causing it to rotate. Since the ignition trigger (breaker points, magnetic switch, and so on) is mounted on the breaker plate,

it begins to open earlier, relative to the distributor shaft. And this causes ignition timing to increase.This increase in ignition timing subsdtantially improves

fuel economy. If the engine is accelarated, manifold vacuum drops. This allows the vacuum diaphragm, and in turn, the breaker plate, to ease back

toward their neutral states, reducing ignition timing. Where the vacuum advance system adjusts ignition timing based on engine load, the

centrifugal advance system adjusts timing based on engine speed. The centrifugal advance system consists of two weights positioned at the

top of the distributor shaft. The weights are held to the distributor drive shaft with springs. As engine rpm increases, centrifugal force pulls the weights away from

the distriburtor shaft causing the springs to expand. The weights are connected to the breaker plate assembly. As the weights move outward, they rotate

the breaker plate within the distributor housing, causing the ignition timing to advance. The advance rate is controlled by the mass of the weights and the tension

of the centrifugal weight springs. The vacuum advance system works in concert with the centrifugal advance system in a type of see-saw routine.

Whenenver engine rpm increases, the vacuum advance system allows the ignition timing to ease back. But when engine rpm increases, the

certrifugal advance system increases ignition timing. So they tend to help balance one another. However, two things combine to make this an uneven trade-off.

First, if the centrifugal advance weights and springs are selected for maximum performance, they won't add any more ignition advance above 2500 rpm

or so. Second, the vacuum advance system is more sensitive than the centrifugal advance system; changes in throttle position that may change engine rpm

only slightly and therefore cause no real change in centrifugal advance rate, can cause more significant changes in the vacuum advance system.

Overall, the more ignition timing you can run without causing detonation, the better the performance and fuel economy.

And these are two good reasons for retaining the vacuum advance system for street operation.

AND ANOTHER PAGE Vacuum Advance Page

VACUUM ADVANCE AND WHY YOU WANT IT FOR YOUR CAR
An often-asked question from many callers relates to whether they really need a vacuum advance mechanism on their distributor. I think this question stems from their observation that many "high performance" distributors do not incorporate vacuum advance and the resulting implication is that it is not desirable or necessary for a "good" ignition. There are a handful of applications where vacuum advance is not of significant benefit:

1) Pure racing engines
2) Severe duty very large trucks
3) Constant speed and load applications (airplanes, generators, pumps)


Other than the above, for normal automotive applications the vacuum advance will benefit the engine as follows.

1) Improved idle cooling
2) Improved idle quality
3) Improved fuel economy
4) Improved throttle response
5) Improved drivability
6) Enables improved spark knock control under full throttle accelerations
7) Enables leaner fuel jetting at light load to further improve fuel economy.


The basic reason for all these improvements is that the vacuum advance mechanism allows the distributor to supply a more optimum spark timing proportional to the load and speed output. Without the vacuum advance the distributor can only vary spark timing in proportion to speed and ignores its need for approximately 20 additional degrees of spark timing ("advance") at light loads: (idle and cruise conditions)


The basic reason for the change in optimum timing at light loads is that when operating at light loads, the mixture is leaner for fuel economy and less dense because of light load. These conditions cause the charge to burn slower, and thus, to reach peak pressure at optimum point in the cycle, the spark must be initiated earlier. Failure to do this will result in "retarded" spark timing and all the aforementioned losses.


All engines are different, and have different spark timing requirements, but they are all the same in that as load is decreased, additional spark timing is required for optimum combustion.


Do yourself a favor - 1) make sure your distributor has a vacuum spark system and 2) experiment to find out what your engine "likes" for timing at idle, light load, and heavy load. Then change the vacuum can to achieve a result closer to the optimum.

[ March 10, 2004, 07:21 PM: Message edited by: PhilSine ]
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  #22  
Old 03-10-2004, 01:05 PM
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Gawdzilla. Gawdzilla. is offline
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Quote:
Originally posted by 1studiac:
</font><blockquote>quote:</font><hr />Originally posted by Gawdzilla:
vacuum advance really only helps when you're not on the gas (when the engine is under very little load)...so when you're cruising or coasting you develop more vacuum. more vacuum gives more advance. More advance allows you to run leaner, etc.
if you do a lot of stop and go driving tho, forget it. your mileage is gonna suck.
I dont think that is the way it works, if you run without a cto then you need to hook the dizzy up to ported vac, ported vac increases as rpm's increase. The only time the dizzy is on manifold vac (which is the vac that drops when you step on it) is when the engine is warming up as it needs the advance to help keep it running while cold. If you hook that dizzy up to ported vac I would almost guarentee you see a couple more miles to the gallon, like said it's like pulling a chute behind you without it. I don't know what my mpg is but it's got to be pushing 13 or 14 and it was somewhere just over half that when I had my vac for the dizzy hooked up to manifold vac by mistake.

</font>[/quote]? did I get it backwards?? EDIT: NOPE!!! I don't have smog so I run off ported vacuum anyway.
Well, either way- the intent of the dist advance is to increase mileage, and if it no workie your mileage will suffer.

[img]smile.gif[/img] Ok, thanks Philsine. That's what I was trying to say! [img]smile.gif[/img]

[ March 10, 2004, 07:14 PM: Message edited by: Gawdzilla ]
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  #23  
Old 03-10-2004, 01:26 PM
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89grand 89grand is offline
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I have my timing set at about 6 degrees BTDC, I'll check into the springs. The distributor is pratically new, but it is a AutoZone rebuild. That is more than likely the problem.
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  #24  
Old 03-10-2004, 04:16 PM
1studiac 1studiac is offline
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Quote:
Originally posted by Gawdzilla:
</font><blockquote>quote:</font><hr />Originally posted by 1studiac:
</font><blockquote>quote:</font><hr />Originally posted by Gawdzilla:
vacuum advance really only helps when you're not on the gas (when the engine is under very little load)...so when you're cruising or coasting you develop more vacuum. more vacuum gives more advance. More advance allows you to run leaner, etc.
if you do a lot of stop and go driving tho, forget it. your mileage is gonna suck.
I dont think that is the way it works, if you run without a cto then you need to hook the dizzy up to ported vac, ported vac increases as rpm's increase. The only time the dizzy is on manifold vac (which is the vac that drops when you step on it) is when the engine is warming up as it needs the advance to help keep it running while cold. If you hook that dizzy up to ported vac I would almost guarentee you see a couple more miles to the gallon, like said it's like pulling a chute behind you without it. I don't know what my mpg is but it's got to be pushing 13 or 14 and it was somewhere just over half that when I had my vac for the dizzy hooked up to manifold vac by mistake.

</font>[/quote]? did I get it backwards?? EDIT: NOPE!!! I don't have smog so I run off ported vacuum anyway.
Well, either way- the intent of the dist advance is to increase mileage, and if it no workie your mileage will suffer.

[img]smile.gif[/img] Ok, thanks Philsine. That's what I was trying to say! [img]smile.gif[/img]
</font>[/quote]I would quote the vacuum advance page also but then this post would get to long, but what the vacuum advance does is advances the timing (hence the name vacuum advance) until the mechnical advance can take over at higher rpm's. The mechnical advance is capable of advancing more than the vac advance because after a certain point you wont gain anymore vacuum, let's say for the sake of arguement you dont gain anymore vacuum after 3300 rpm well what happens after that when your going faster at say 4500 rpm? You dont need anymore advance than at 3300 rpm? If that happened you would be firing way to late and that piston would be a third of the way down before it fired. Thats the purpous of the springs to take over after a certain rpm (adjustable by spring rate). With that said here is some info and definitions of how it works.

VACUUM ADVANCE
Definition: A device that advances the ignition timing in response to increased engine vacuum.

Check the Distributor Vacuum Advance:
Connect your timing light.
Connect hand vacuum pump to the vacuum advance unit.
As you apply vacuum the timing should advance and go back down when vacuum is removed.

PORTED VACUUM
Definition: Engine vacuum that is available above the throttle plates of a throttle body or carburetor. Used to advance ignition timing on older carbureted engines when the throttle is opened above its idle position.

By this info you can see if you had it hooked to manifold vac you would have total advance at idle and not gain anymore when you step on it in fact it would drop for a little bit when you hammer it (that's what manifold vac does) there by losing your advance and with no rpm's yet you wont have mechnical advance either . But your right more vac gives more advance , but more load as I see it is when your trying to get going from a stop, and as for taking a large hill in a high gear your rpm's go down and so does your mech advance therefore no pinging. Now none of this takes into consideration carbon build up/high compression and the need for high octane gas to keep from pinging from predetenation. I think the guy who wrote that article on the chevy board is on crack, but hey I could be wrong and I could be the crack head . In the end do what ya want, think what ya want but this thread contains some ideas to try and try them all and see what works for ya. Just get out there and try something to get those rigs running better
Ok I cant leave it alone, is what that guy saying that your vac advance tries to basicly retard your timing while the mech advance tries to advance it? Also why would you want advance at idel (except when cold), you already set you timing advanced @ 6 or 10 or whatever and with low rpms the spark can get there later and still ignite the fuel in time because the piston is moving slower but when you kick it down and the rpm's go up it needs to send that spark sooner, in fact you could say the difference between the amount of time it has gets cut in half going from idel @ 700rpm or so to 1400 rpm which is "cruzing". I have myself convinced how about you?

[ March 10, 2004, 10:46 PM: Message edited by: 1studiac ]
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  #25  
Old 03-10-2004, 04:30 PM
1studiac 1studiac is offline
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Quote:
Originally posted by 89grand:
I have my timing set at about 6 degrees BTDC, I'll check into the springs. The distributor is pratically new, but it is a AutoZone rebuild. That is more than likely the problem.
Timing sounds right, with it being a AZ rebuild i would definitly check the springs. You can take the vac line off the dizzy and with your timing light watch the timing mark as you slowly rev the engine up, it should advance slow and steady if it jumps real high right away and then doesnt really move for awhile you got weak springs, if it is jumpy it's sticking. You can always take the cap off and make sure it all looks good, I cant remember if I had a spring break or come off but one of those happened when I was a kid.
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  #26  
Old 03-10-2004, 05:11 PM
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Alright guys, got a bit of an off topic question here. Kinda related but kinda not. My motor ('78 258) is running real good, get decent milage (more carb tuning required, different subject ) and I used the HEI dizzy. From what I've heard, and I can believe it, the chebby's advance weights/springs are not really the right ones for the AMC 6. My questions is should they be heavier or lighter? Do they need to advance more/less sooner/later?

Thanks,
Anthony
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  #27  
Old 03-10-2004, 08:19 PM
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Garbage truck has the 360 with all the smog junk gutted, and the only real upgrades are the Edelbrock intake and EB Performer 4bbl carb. TFI coming, but even with those, I usually pull about 15 mpg's out of him when he's all tuned and happy.
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  #28  
Old 03-11-2004, 01:42 AM
1studiac 1studiac is offline
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Now that I am at work and a little more awake I reread my post and want to claify something by saying "mech advance takes over for vac advance" does not mean you dont need it, you need both to get the right amount of total advance. Say at a given rpm I need 35 degrees of advance I get 10 from my inital (which is what my timing is set at) then you get say 13 degrees from vac advance you still need to get another 12 from mech advance to get to the optimim 35 degrees. Make sence?

I will do some checking on your spring /weight question when I get a few min. I dont think ???? there is a huge diff in them though (chevy to amc), unless you are really into the performance like for racing. Then again every little bit adds up.

[ March 11, 2004, 09:50 AM: Message edited by: 1studiac ]
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  #29  
Old 03-11-2004, 03:11 AM
carrotman carrotman is offline
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Cecil 14, now that you have an HEI, the advance springs can be changed very quickly as they are under the rotor. Different weight springs can be bought as a kit for around $10. You can try heavy springs, light springs and combination, since it uses two springs. Every vehicle and driver is different so you have to experiment. Write down your experience with each set of springs and use the ones you like best. By the way, only change one thing at a time.
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  #30  
Old 03-11-2004, 04:15 AM
BIGYELLOW78J10 BIGYELLOW78J10 is offline
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You numbers will also improve via magic, if you correct for your true tire size. Right now, your odometer is setup for a 225/75R15 or so. About 28" tall. You are running a 31" tire, SO you are going farther with less rotations of the tire. So your odometer is underreporting your mileage. Granted, if won't be a huge difference, but it will make some.

Next time you are on the interstate, run for 10 miles, comparing the odometer to the mile markers. Or if you have it, try to correct your speedometer with a GPS. My yota is now reads 5mph SLOW with 31" BFG's vs being dead on with the no name 31"s I used to have. With the 30" tires it came with, the speedometer was just a hair fast.

You'll feel a little better about your mileage if you correct it, I hope.

I am always annoyed with my J-10 and toyota mileage. but I never bother to correct it.

Good luck,

Daniel
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  #31  
Old 03-11-2004, 04:17 AM
BIGYELLOW78J10 BIGYELLOW78J10 is offline
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Unless you regeared. Then never mind.

I've got you up to 7.916 MPG with my guess of your tire size difference.

When in doubt, cook the books
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  #32  
Old 03-11-2004, 04:20 AM
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Quote:
Originally posted by BIGYELLOW78J10:
When in doubt, cook the books
Very good point...it's a start, but that isn't going to get me to the 10mpg I want (sheesh that is pathetic )
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  #33  
Old 03-12-2004, 05:59 PM
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Quote:
Originally posted by 1studiac:
I will do some checking on your spring /weight question when I get a few min. I dont think ???? there is a huge diff in them though (chevy to amc), unless you are really into the performance like for racing. Then again every little bit adds up.
Quote:
Originally posted by carrotman:
Cecil 14, now that you have an HEI, the advance springs can be changed very quickly as they are under the rotor. Different weight springs can be bought as a kit for around $10. You can try heavy springs, light springs and combination, since it uses two springs. Every vehicle and driver is different so you have to experiment. Write down your experience with each set of springs and use the ones you like best. By the way, only change one thing at a time.
That's kinda what I was guessing. So far I've noticed no ill effects or lack of perfomance on either of the two motors I've HEI'ed. The 258 right now has a ton of power and will crank at 3k all day if I want. For now I think I'll leave it be and see what happens.

Thanks,
Anthony
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  #34  
Old 03-13-2004, 02:32 AM
1studiac 1studiac is offline
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There ya go dont try and fix it if it is'nt broke. If you like making graphs, have a dial back timing light, the spring/weight kit like carrotman said and a day or two to play with it you can recurve it just as good as you could on a machine. While there is some truth to advancing your timing by going till it pings then backing it off a little you have to remember that will only work in ideal situation like everything (Vac and Mech)being new and working flawlessly, but in the real world the ideal set up is one that can get you from idel to around 3200 rpm (or where ever your shift point is) the fastest without pinging. That's why I say you can do a better recurve this way than in a machine, this way it is tailored to your rig and it's weight.
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