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FSJunkie
AMC Addicted Joined: Jan/09/2011 Location: Flagstaff, AZ Status: Offline Points: 4742 |
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Depends on what distributor you have.
Some distributors are designed to have little initial timing and a lot of mechanical and others are designed to have a lot of initial and little mechanical, such as: 5* initial and 30* mechanical would be 35* total. 15* initial and 20* mechanical would be 35* total. Both timing curves get you where you need to be but you can't set either distributor to the other's specifications because the design of the distributors only tolerates so much modification. Trying to get around that can result in a really FUBAR distributor. Best thing is to find an AMC technical service manual for your year engine. Timing and advance specs WILL be in there. usually in the "tune up" or "electrical" section. It will either be a graph showing the timing or a values table. |
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1955 Packard
1966 Marlin 1972 Wagoneer 1973 Ambassador 1977 Hornet 1982 Concord D/L 1984 Eagle Limited |
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73hornut
AMC Addicted Joined: Jul/03/2007 Location: Southern Oregon Status: Offline Points: 3130 |
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71 Javelin
74 Gremlin 79 Spirit AMX Rogue Valley Rumblers Like Us on FB https://www.facebook.com/groups/1602825606650796 |
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FSJunkie
AMC Addicted Joined: Jan/09/2011 Location: Flagstaff, AZ Status: Offline Points: 4742 |
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5-10* initial and 15-25* mechanical advance making for 25-30* total @ 3000-4000 RPM is a good general timing curve on the sixes. |
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1955 Packard
1966 Marlin 1972 Wagoneer 1973 Ambassador 1977 Hornet 1982 Concord D/L 1984 Eagle Limited |
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tyrodtom
AMC Addicted Joined: Sep/14/2007 Location: Virginia Status: Offline Points: 6213 |
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The 66 TSM I have states 0-4 degrees at 950 rpm, 14-18 degrees at 1900 rpm and 24-28 degrees at 4400 rpm, that's just mechanical advance. Add your initial advance to that. But that's for a 66 199-232. And it gives no idea how soon you can bring maximum advance in.
A engine doesn't know what distributor is on it, you tailor the distributor to the engine. I know in circle track racing, and in towing you need to be concerned with total timing. You're operating at full power, and well out of the rpm and vacuum range of where vacuum advance has any effect, and the engine is making enough other noise you can't hear pinging if it is occurring. Occasionally I tow to a track down in N.C., when pulling back the mountain coming out of N.C. to Va. past Galax Va. is about a 15 minute hard pull, full power, windows down. I certainly would want to know what my maximum advance is before I attempt that mountain, because the outcome is going to be a great deal different if I have 50 degrees of total advance instead of around 38. |
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66 American SW, 66 American 2dr, 82 J10, 70 Hornet, Pound, Va.
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uncljohn
AMC Addicted Joined: Jan/03/2013 Location: Peoria AZ Status: Offline Points: 5394 |
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Assuming that the Concept of total advance at some given rpm is a benefit with out determining what the object is that you are trying to accomplish is in my opinion a fools mission.
As tyrodtom indicated, in both circle track racing where rpm and load is pretty much predictable and also in towing where load is in itself heavy total advance is again a predictable and maybe an important concept to pursue. But note that there is no reference to accuracy as compared to something else. The following is one of many articles that are available to determine how an engine will react to and how to obtain total advance: http://www.firstfives.org/faq/timing/timing_for_modified_engines.PDF and it consist of a number that includes static advance and that which is obtained by mechanical advance. It is a multiple page document that stresses that the engine is modified and static advance is initially select based on Cam Shaft duration and a curve for mechanical advance is created based on WOT performance. It has a section where partial throttle operation is mentioned and in mentioning it states the vacuum advance is pretty much required to deal with the variables that take place in partial or trailing throttle conditions, but a pure mechanical advance is pretty much required to take advantage of timing when dealing with WOT throttle performance. What is not said clearly, is on a street engine, the vacuum advance characteristics are the important function to be aware of and have operating in a manner that favors performance, fuel economy and engine heat. And that the amount of vacuum advance is not a factor in figuring total advance, at least according this article. So again, I do not find a number of total advance at a given rpm of significant importance, unless I have a need for I. And I don't. |
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70 390 5spd Donohue
74 Hornet In restoration 76 Hornet, 5.7L Mercury Marine Power 80 Fuel Injected I6 Spirit 74 232 I-6, 4bbl, 270HL Isky Cam |
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73hornut
AMC Addicted Joined: Jul/03/2007 Location: Southern Oregon Status: Offline Points: 3130 |
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Uncljohn, nobody is asking you to agree, I just asked a specific question, requesting a specific answer, and some have been more then helpful.
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71 Javelin
74 Gremlin 79 Spirit AMX Rogue Valley Rumblers Like Us on FB https://www.facebook.com/groups/1602825606650796 |
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uncljohn
AMC Addicted Joined: Jan/03/2013 Location: Peoria AZ Status: Offline Points: 5394 |
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Like I said, you can look it up in the TSM. The 1974 TSM says 28.5 -37 degrees at 2000 rpm.
Now, how valid is that information in order to set the timing more accurate? And your question is on an Eagle, which was not manufactured in 1974 although the engine was used then, thus how does it apply? |
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70 390 5spd Donohue
74 Hornet In restoration 76 Hornet, 5.7L Mercury Marine Power 80 Fuel Injected I6 Spirit 74 232 I-6, 4bbl, 270HL Isky Cam |
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amcfool1
AMC Addicted Joined: Jun/18/2011 Location: roanoke va Status: Online Points: 1075 |
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hi, ok same topic sort of, on a brand new engine, on an 84 Eagle 258/auto, trying to tune it. stock BBD got all vacuum hookups right, (i think) looks to me like dist runs of manifold vac, but not sure about this, should it be ported?thanks, gz
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george z
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73hornut
AMC Addicted Joined: Jul/03/2007 Location: Southern Oregon Status: Offline Points: 3130 |
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71 Javelin
74 Gremlin 79 Spirit AMX Rogue Valley Rumblers Like Us on FB https://www.facebook.com/groups/1602825606650796 |
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uncljohn
AMC Addicted Joined: Jan/03/2013 Location: Peoria AZ Status: Offline Points: 5394 |
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No, they are not, at least not as far as the basic engine and it's needs are, but when a question arises asking for timing advance curves, and claiming that some how it is more accurate to know what the total advance is and expecting the answer to include the following 1. Static Advance 2. Vacuum advance 3 mechanical advance, In general mechanical advance is going to be all the way in by about 2000 rpm. Vacuum advance if included assumes that full vacuum is being generated at what ever the rpm claim is for that answer, a condition that rarely is possible and static advance is a function of a number of things, but the important part is the Vacuum advance and the functional use of that statistic is that vacuum advance is used to support partial throttle or trailing throttle engine performance And as loading increases such climbing a hill or acceleration or passing vacuum will fall off causing the ignition to retard until the loading decreases The information that a total advance curve is something is a bit bogus as the factory applies that data differently as to how the car is intended to perform. Thus the 1974 specification of 28.5 - 37 degrees advance at 2000 rpm means the following 1. All the mechanical advance is in by 2000 rpm and the 8 degree window given is the accuracy of the design 2. Vacuum generated by the engine is sufficient at the stated rpm to give full vacuum advance by design. Thus call it 2000 rpm or 4500 rpm, all the mechanical advance is in and vacuum is high enough to generate maximum Vacuum advance. So the RPM number means what? Only that at that rpm all the mechanical advance is in and has been in since the engine passed 2000 rpm. The difference between 1974 and 1985 in the engine is minimal from a functional standpoint. Except That the factory was by that time generating variable levels of vacuum depending on what the engine was being used for and that vacuum was used to generate the timing needed to control the engine so it would pass smog. In general, Manifold vacuum was used on a cold engine start up. Ported Vacuum was used more or less all the time when the engine was hot and for normal driving. there was a third vacuum generated, a mixture of the two which would have been about 1/2 the difference between manifold and ported vacuum for other specific partial throttle control conditions. It was vacuum rather than RPM controlling the timing including the value of timing measured from idle up through WOT. The only constant would have been the mechanical, it is all in by about 2000-2200 rpm. Thus any number that represented what was asked for as total timing advance would be a function of Static timing Mechanical advance and the total timing advance maximum controlled by vacuum. And it would still be a some what bogus specification. AS the specification still is taking into account all the mechanical advance is there by about 2000 or so rpm and all the vacuum needed to generate full vacuum advance is available and the static advance has been taken into account. And that was a far more sophisticated design for vacuum control in 1985. That is what all those vacuum lines and bits and pieces are for. So if the answer needed is to build an advance curve strictly dependent on engine rpm the best answer available for pretty much any engine is all the advance is in by about 2200 rpm and call it good to go. A modified engine will have it's on peculiarities and if you are interested in partial throttle operation, functionally that is laid to waste by eliminating the function of vacuum advance. You asked a simple question, and not for the first time, but the answer is not an easy answer in that it ignores the function of the components used to generate that answer. |
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70 390 5spd Donohue
74 Hornet In restoration 76 Hornet, 5.7L Mercury Marine Power 80 Fuel Injected I6 Spirit 74 232 I-6, 4bbl, 270HL Isky Cam |
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