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Old 01-12-2012, 10:55 AM
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total timing question

can someone help me understand what he means here?
http://www.corvette-restoration.com/...c_Adv_Spec.pdf

"Max Advance
Since the vacuum advance control unit is a part of the distributor, the number of degrees of vacuum advance is
specified in DISTRIBUTOR degrees - NOT crankshaft degrees. When talking about these control units, it is
important that you know whether the person you're talking to is referring to the distributor degrees, or if he's talking
crankshaft degrees. All of the listings shown in the following chart, and in any shop manual & technical spec sheet,
will refer to distributor degrees of vacuum advance. You must DOUBLE this number to obtain crankshaft degrees
(which is what you "see" with your timing light)
. Thus, a vacuum advance control unit with 8 degrees of maximum
advance produces 16 degrees of ignition advance in relationship to the crankshaft. When selecting a unit for max
advance spec, the total centrifugal timing at cruise must be considered. Thus, a car set up to produce 36 degrees of
total mechanical advance at 2500 rpm needs a vacuum advance control unit producing 16 degrees of crankshaft
advance. This would be an 8-degree vacuum advance control unit."


he confuses me with the doubling of numbers? why does it seem hes confusing things, at least for me, by mentioning crankshaft advance?

this is my thought process:

1. i set my initial timing to 12* btdc on the timing mark with the vac adv unplugged at idle using a timing light.

2. i then plug in the vac advance to manifold vac and dial my light back 20* to get the timing mark to 12* before btdc thus adding up that my vac advance gives 20* of advance when fully advanced. does this mean my can is a 10* vac advance can giving 10* of crankshaft of advance? and then do i use the 10 or 20 to calculate total timing??

scenario: using a timing light i set timing to 12* + centrifucal adv is 30* ( assuming the dist gives a total of 30*) = 42* PLUS the vac that adds up to 20* BUT i cut this number in half to 10 and that adds up to 52 degrees of total timing?

help this poor frustrated eager wanting to know individual to understand what it all means!

Mark

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Old 01-12-2012, 01:41 PM
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anyone?
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Old 01-12-2012, 04:55 PM
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Wow, I am even more confused than you after reading your post. First of all remember the dist. turns 1/2 speed of crankshaft, thus the difference in dist. degrees and crankshsft degrees.
Set your timing at 12 degrees. Go full vaccum advance in and check your total CRANKSHAFT degrees, total probably 42 or so. that would be 20 degrees DIST. degrees. Understand? If you have an adjustable vacuum advance you can adjust from there.
Clear as mud?
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Old 01-13-2012, 08:08 AM
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Quote:
Originally Posted by 1972 El Camino
can someone help me understand what he means here?
http://www.corvette-restoration.com/...c_Adv_Spec.pdf

"Max Advance
Since the vacuum advance control unit is a part of the distributor, the number of degrees of vacuum advance is
specified in DISTRIBUTOR degrees - NOT crankshaft degrees. When talking about these control units, it is
important that you know whether the person you're talking to is referring to the distributor degrees, or if he's talking
crankshaft degrees. All of the listings shown in the following chart, and in any shop manual & technical spec sheet,
will refer to distributor degrees of vacuum advance. You must DOUBLE this number to obtain crankshaft degrees
(which is what you "see" with your timing light)
. Thus, a vacuum advance control unit with 8 degrees of maximum
advance produces 16 degrees of ignition advance in relationship to the crankshaft. When selecting a unit for max
advance spec, the total centrifugal timing at cruise must be considered. Thus, a car set up to produce 36 degrees of
total mechanical advance at 2500 rpm needs a vacuum advance control unit producing 16 degrees of crankshaft
advance. This would be an 8-degree vacuum advance control unit."


he confuses me with the doubling of numbers? why does it seem hes confusing things, at least for me, by mentioning crankshaft advance?
Quote:
Originally Posted by 1972 El Camino
this is my thought process:

1. i set my initial timing to 12* btdc on the timing mark with the vac adv unplugged at idle using a timing light.
The vacuum advance hose should be dis-connected from the vacuum advance cannister AND the hose end should be plugged. This is so that no vacuum advance is introduced when checking/setting the timing and the hose is plugged so that you do not induce a major vacuum leak.

Quote:
Originally Posted by 1972 El Camino
2. i then plug in the vac advance to manifold vac and dial my light back 20* to get the timing mark to 12* before btdc thus adding up that my vac advance gives 20* of advance when fully advanced. does this mean my can is a 10* vac advance can giving 10* of crankshaft of advance? and then do i use the 10 or 20 to calculate total timing??
If the vacuum advance is showing an increase of 20 degrees over the initial of 12 degrees at idle, then that is how much your vacuum advance unit is producing with the vacuum available at idle. You can either get a cannister that limits the amount of advance to 10-15 degrees, get an adjustable aftermarket cannister, or limit the travel of the advance unit.

Quote:
Originally Posted by 1972 El Camino
scenario: using a timing light i set timing to 12* + centrifucal adv is 30* ( assuming the dist gives a total of 30*) = 42* PLUS the vac that adds up to 20* BUT i cut this number in half to 10 and that adds up to 52 degrees of total timing?

help this poor frustrated eager wanting to know individual to understand what it all means!

Mark
Most distributors have a mechanical (centrifugal) advance built in that will range from 20-24 degrees.

For most engines the Total Mechanical Timing (initial plus the mechanical) should be set in the 32-38 degree range with NO vacuum advance added. This Total Mechanical Timing should be 'all in' between 2500-3000 RPM for best street performance. For your example using the 12 degrees initial plus 20 degrees mechanical for a GM distributor you have a total 32 with no vacuum advance added.

The vacuum advance will increase the Total Mechanical timing in varying amounts depending on the load on the engine and the vacuum produced. At WOT there is '0' vacuum and thus no additional advance. At idle, at cruise speeds, and on decelleration the vacuum is the highest and the vacuum advance adds aditional timing (up to the mechanical limits of the advance unit).

Other than to aquire additional info pertaining to the vacuum advance unit and its function, the timing is not checked or set with the vacuum advance connected.
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Old 01-13-2012, 10:35 AM
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Want it really simple? Just get a timing tape that matches the diameter of your harmonic damper, clean your damper, and attach the tape as directed. Then do a timing curve according to what your timing light shows you on your new timing tape. Start at about 6 degrees initial, and about 34 degrees by 4000 RPM, plus a 20-degree vacuum can. Then experiment with turning the dizzy a couple of degrees.
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Old 01-13-2012, 03:15 PM
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Quote:
Originally Posted by 1972 El Camino
can someone help me understand what he means here?
http://www.corvette-restoration.com/...c_Adv_Spec.pdf

"Max Advance
Since the vacuum advance control unit is a part of the distributor, the number of degrees of vacuum advance is
specified in DISTRIBUTOR degrees - NOT crankshaft degrees. When talking about these control units, it is
important that you know whether the person you're talking to is referring to the distributor degrees, or if he's talking
crankshaft degrees. All of the listings shown in the following chart, and in any shop manual & technical spec sheet,
will refer to distributor degrees of vacuum advance. You must DOUBLE this number to obtain crankshaft degrees
(which is what you "see" with your timing light)
. Thus, a vacuum advance control unit with 8 degrees of maximum
advance produces 16 degrees of ignition advance in relationship to the crankshaft. When selecting a unit for max
advance spec, the total centrifugal timing at cruise must be considered. Thus, a car set up to produce 36 degrees of
total mechanical advance at 2500 rpm needs a vacuum advance control unit producing 16 degrees of crankshaft
advance. This would be an 8-degree vacuum advance control unit."


he confuses me with the doubling of numbers? why does it seem hes confusing things, at least for me, by mentioning crankshaft advance?

this is my thought process:

1. i set my initial timing to 12* btdc on the timing mark with the vac adv unplugged at idle using a timing light.

2. i then plug in the vac advance to manifold vac and dial my light back 20* to get the timing mark to 12* before btdc thus adding up that my vac advance gives 20* of advance when fully advanced. does this mean my can is a 10* vac advance can giving 10* of crankshaft of advance? and then do i use the 10 or 20 to calculate total timing??

scenario: using a timing light i set timing to 12* + centrifugal adv is 30* ( assuming the dist gives a total of 30*) = 42* PLUS the vac that adds up to 20* BUT i cut this number in half to 10 and that adds up to 52 degrees of total timing?

help this poor frustrated eager wanting to know individual to understand what it all means!

Mark
You are correct the Max Advance explanation only confuses the matter. In a conventional sense ignition systems are set to energize the spark plug at a point where the piston crown is compared to the pistons upper most travel in the cylinder (TDC) and this electrical spark is usually happening before top dead center (BTDC). We need no math considering distributor rotation as compared to crank rotation when using a timing light. However it is mentioned so we know the correlation between distributor shaft vs crank rotation during one 360 degree revolution of either and how the rating of the vacuum advance cans are stamped.

When using a dial back light to set timing it is all a matter of addition and subtraction there are no multipliers involved. Frisco's post give examples of this math

I might add,,,With a dial back timing light you dont need a dampener timing tape/index that indicates up to 60 Deg BTDC. But you need to be sure your are infact at TDC according to the timing tab/pointer & dampener 0 mark.

Things can get out of whack over time and give false readings.

Instructions/explanations for determining this true TDC:

http://www.crankshaftcoalition.com/w...op_dead_center
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Old 01-13-2012, 03:51 PM
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Quote:
Originally Posted by whyholdback
Want it really simple? Just get a timing tape that matches the diameter of your harmonic damper, clean your damper, and attach the tape as directed. Then do a timing curve according to what your timing light shows you on your new timing tape. Start at about 6 degrees initial, and about 34 degrees by 4000 RPM, plus a 20-degree vacuum can. Then experiment with turning the dizzy a couple of degrees.
If you want to do it correctly, read this....
http://www.crankshaftcoalition.com/w...op_dead_center
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Old 01-13-2012, 06:11 PM
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Quote:
Originally Posted by whyholdback
Start at about 6 degrees initial, and about 34 degrees by 4000 RPM, plus a 20-degree vacuum can. Then experiment with turning the dizzy a couple of degrees.
All in by 4000 RPM is much too high for a street engine to get good performance. On the street the engine in high gear will be closer to 1800-2000 RPM for most vehicles.

The use of a vacuum canister with a 20 degree range is also way too high for best street performance. For drag race use only, vacuum advance is not needed and that would be when the all in timing in the 3000-3500 RPM range would be used.

The all in RPM figure can be adjusted by changing the advance weights and/or the advance weight springs to raise or lower when the total mechanical timing is occurring.
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Old 01-14-2012, 08:20 AM
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Quote:
Originally Posted by Frisco
All in by 4000 RPM is much too high for a street engine to get good performance. On the street the engine in high gear will be closer to 1800-2000 RPM for most vehicles.

The use of a vacuum canister with a 20 degree range is also way too high for best street performance. For drag race use only, vacuum advance is not needed and that would be when the all in timing in the 3000-3500 RPM range would be used.

The all in RPM figure can be adjusted by changing the advance weights and/or the advance weight springs to raise or lower when the total mechanical timing is occurring.
All good points it also seems from what I have been reading that the autozone dist gives 24* of advance when all in, isn't this a lot ? Seems stock for 72 was around 14 or 15 full in?
Seems the way to go is an adjustable can as far as the can advance with also weights and springs for the curve but the other thing I have found is that dist would have to be modified by closing the hole up so that it maxes out at 14 or 15 unless there is another way?
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Old 01-14-2012, 08:50 AM
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Quote:
Originally Posted by Frisco
All in by 4000 RPM is much too high for a street engine to get good performance. On the street the engine in high gear will be closer to 1800-2000 RPM for most vehicles.

The use of a vacuum canister with a 20 degree range is also way too high for best street performance. For drag race use only, vacuum advance is not needed and that would be when the all in timing in the 3000-3500 RPM range would be used.

The all in RPM figure can be adjusted by changing the advance weights and/or the advance weight springs to raise or lower when the total mechanical timing is occurring.
All in any sooner risks detonation. That can break pistons. You never want full advance in until 500 RPM after peak torque, which on most mild 350s is 3500. This case might be 3000, for now, but it seems that will change rather soon.
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Old 01-14-2012, 08:57 AM
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Quote:
Originally Posted by 1972 El Camino
from what I have been reading that the autozone dist gives 24* of advance when all in, isn't this a lot ? Seems stock for 72 was around 14 or 15 full in?
What manufacture is the Autozone distributor? The original non-computer controlled GM HEI distributor has a built in mechanical advance of 20 degrees. I believe the GM points distributor also has 20 degrees of built in mechanical advance. MSD and Mallory distributors have a built in mechanical advance of 24 degrees for most models. Gm offers a great number of vacuum advance canisters with the number of degrees of advance with full vacuum ranging from 8 to 20 degrees. Selecting one for your application is suggested. Others and I have posted a list of the GM vacuum canisters on this site but in doing a search I haven't found that thread yet.The use of an aftermarket adjustable vacuum advance canister is also suggested as an alternative.
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Old 01-14-2012, 09:17 AM
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Quote:
Originally Posted by Frisco
What manufacture is the Autozone distributor? The original non-computer controlled GM HEI distributor has a built in mechanical advance of 20 degrees. I believe the GM points distributor also has 20 degrees of built in mechanical advance. MSD and Mallory distributors have a built in mechanical advance of 24 degrees for most models. Gm offers a great number of vacuum advance canisters with the number of degrees of advance with full vacuum ranging from 8 to 20 degrees. Selecting one for your application is suggested. Others and I have posted a list of the GM vacuum canisters on this site but in doing a search I haven't found that thread yet.The use of an aftermarket adjustable vacuum advance canister is also suggested as an alternative.
It's a Cardone points dist. Again it is interesting that my 1972 service manul lists mechanical advance at around 15* full in.

As well if you read this article http://www.corvette-restoration.com/...c_Adv_Spec.pdf it seems with my 16-17 lbs at idle I would go with a b26 can that gives 16* of crankshaft advance max and not the one I have now that came with the dist that gives 20* fully advanced.
I get the idea that the Cardone dist covers all the bases for the wide variety of engine setups but if I want it to match up with my setup I would have to tweak it- adjustable can, dial in the curve and even modify it to give less mechanic advance.
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Old 01-14-2012, 09:43 AM
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Quote:
Originally Posted by 1972 El Camino
It's a Cardone points dist. Again it is interesting that my 1972 service manul lists mechanical advance at around 15* full in.

As well if you read this article http://www.corvette-restoration.com/...c_Adv_Spec.pdf it seems with my 16-17 lbs at idle I would go with a b26 can that gives 16* of crankshaft advance max and not the one I have now that came with the dist that gives 20* fully advanced.
I get the idea that the Cardone dist covers all the bases for the wide variety of engine setups but if I want it to match up with my setup I would have to tweak it- adjustable can, dial in the curve and even modify it to give less mechanic advance.
The link you posted is a very good article. The info about the different vacuum advance canisters is the same as or similar to the one that has been posted on this site in the past.

I am unable to find the specs for the mechanical (centrifugal) advance for the Cardone points distributor. The total mechanical advance can be verified either on a distributor machine (hard to find one anymore) or using a timing light once the distributor has been installed. I would prefer to have that info before purchasing though.
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Old 01-14-2012, 09:51 AM
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Quote:
Originally Posted by Frisco
The link you posted is a very good article. The info about the different vacuum advance canisters is the same as or similar to the one that has been posted on this site in the past.

I am unable to find the specs for the mechanical (centrifugal) advance for the Cardone points distributor. The total mechanical advance can be verified either on a distributor machine (hard to find one anymore) or using a timing light once the distributor has been installed. I would prefer to have that info before purchasing though.
It is a good article!

I did verify my vac adv gives 20* when hooked to manifold vacuum using a dial back timing light but I haven't verified centrifical yet.
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Old 01-14-2012, 09:57 AM
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Do some testing and then post

Power time it to 36 Deg BTDC @ 4000RPM without vacuum advance, then let it idle set up the carb to get 800 rpm and good vacuum. Check and post the base timing that you end up with. You can then do the math to see what you have for base and mechanical, you may not need to change mechanical maximum. If you base timing is less than 14 yes you need to get in there and change the mechanical limits but not yet,,,

Then add manifold vacuum advance at idle, readjust carb and check your idle timing, post it. you now know all your values. The three amigo's base + mech advance + vac advance + vacuum adv = total timing in my book, some just use base + mech when they talk total.

Most adjustable vacuum canisters DO NOT LIMIT THE VACUUM ADVANCE they only allow you to change the range of the diaphragm spring. Be careful when looking at the charts for these cans when it comes to engine hg" vs vacuum advance added. A limiter plate is widely use for limiting not a set screw.

Try to keep the mechanical advance out at idle, sometimes it can creep in above 800 rpm.

There are volumes of threads on this subject. Post some real numbers K
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