What is your interpretation of these advance springs?

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TylerW

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Hey guys:
I got a late model electronic SL6 distributor in with a parts lot that a friend generously donated.

The story goes that it's been "curved" by someone who does Mopar distributors, but of whom I can't find any reference around here. It's a parts-store reman with a new 8.5R vacuum can, an unknown t-bar(the advance is stamped on the bottom I guess) and this spring combo. The slots have not been welded up, and they are long.

What do you make of this? I don't know a lot about how to change the advance curves in these(yet), but that heavy spring looks just like a smog spring with the long opening in it. The light spring looks lighter than the two in my early 60's SL6 distributors. I guess I'm trying to determine if there is anything special here that I can use.

The notations on the enclosed paper read as such:

"Mechanical advance:

2 degrees @ 700rpm
6 degrees @ 1100rpm
17 degrees @ 2000rpm

Vacuum advance:

0 degrees @ 7hg
2 degrees @ 11hg
17 degrees @ 19hg"

"TOTAL 34 degrees @ 2000 rpm"

I KNOW someone will remind me that the best thing is to put it on a distributor machine, but I don't have one of those. I am just a little suspicious of this person, who I will not name, because this unit also arrived well-packed, but with a damaged drive gear, just like somebody laid it directly on the bench while pounding in the roll pin.

Opinions appreciated, thanks

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The heavy one is a stock spring. I kind of think the other one is as well. I would be really surprised if the mechanical is all in at 2000 rpm.
 
I thought that slotted spring was to allow rapid advance to a certain point and then slow it down?
 
I'm gonna say that light spring is a REALLY light spring. Those numbers are probably correct. Why would someone go to all that trouble if they were not accurate? As easy as the slant distributor is to remove and install, throw it in and stick a light on it. But remember, you do not include vacuum advance when timing an engine. Only initial and mechanical. Vacuum just ends up being "whatever it is" on top of mechanical and initial.
 
Like hoppy said, those are some sort of factory springs.
The one with tension on it is the primary spring and the one with the long loop is the secondary spring.
The factory used different combos of primary and secondary springs for almost every year and variation in engine and transmission.

The numbers on that sheet are most likely distributor degrees and distributor rpms. But not everyone does it that way. That's the logical way to do it if you are working on a distributor machine and how the FSM specs the mechanical advance. To convert to crank degrees and rpm, double the numbers. So my guess the last line means 34 crank degrees at 2000 rpm for the camshaft and distributor. Easier to understand as 34* maximum advance at 4000 rpm. This is not total as most people use the term. Total is initial plus maxiumum mechanical advance.

I don't have much slant six stuff plotted but here's two sets of specs from the '68 Plymouth Service Manual.
upload_2019-1-21_10-2-1.png

A 70s distributor will have a curve closer to the CAP advance specs.
 
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"Mechanical advance:

2 degrees @ 700rpm
6 degrees @ 1100rpm
17 degrees @ 2000rpm
Plotting these as distributor degrees and rpm doesn't look right.
Those three points look like a straight line.
The secondary spring should slow the advance as soon as it contacts.
 
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I thought that slotted spring was to allow rapid advance to a certain point and then slow it down?
All of them have a slotted end on heavier second spring. The first lighter spring couldn't function if the other second spring was working against it.
 
I have a combo like that, in my aluminum headed 10.9Scr 367. It provides 14* at 2800 crank degrees and all in of 20* at 3400; which with 14* initial, keeps her out of detonation on 87E10.

When decoded into distributor talk (half crankspeed) this becomes
~0*@ 600 distributor rpm
14* @ 1400
20* @t 1700

Your numbers ,if distributor rpm, look about right for a stock late engine
"Mechanical advance:
2 degrees @ 700rpm.....(1400 engine )
6 degrees @ 1100rpm....(2200 engine )
17 degrees@ 2000rpm...(4000 engine)
With 34 in the mechanical, this makes your initial allowable of about 2*
You asked for opinions;
So without knowing anything about your engine;
As to mine@9.5Scr, in a 1980Volare auto, with 2.45s
I couldn't run that as is; the slots are toooooo long. But if you shorten them, likely, the One-long looper will no longer function. And in my engine neither of those are light enough. I dug thru my pile of Ds and found a light-blue one long looper and a light V8 spring that work pretty good in her at about 12* initial. Yeah I had to weld up the slots. Also the Vcan is too short for mine and too slow.
The point is every engine is different, and you the tuner, have to figure out what she wants, little by little, until she says too much! Sometimes it takes two summers to get it dialed in. And sometimes you have to back up before you can go ahead. Welding up the slots is the easiest part, cuz most engines will want to be limited to ~36* power-timing, and most slantys will want between 6 and 12 initial to maintain the T-slot exposure. So you shoot for 10/34 and Badaboom you are in the ballpark. You can get pretty close by doing a bit of math on the working range of the slot. Find the biggest Vcan you can find, and after you get the mechanical sorted, install that baby and pour the coals to the slanty at cruising speed; she might want 46 to 56 degrees total.
 
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Thanks for the replies. Everything seems legit for a standard curve then...it'll make a nice distributor to use if I run an electronic ignition again. I did notice that the springs in my 1960 and '63 distributors are indeed different than this later distributor. The secondary spring does have a loop, but not as long as the later unit, and the primary spring is heavier.

These distributors would be used on a standard 1bbl engine, so I doubt there is any advantage to trying to get the total timing in earlier. It might just create more issues than it solves but you can let me know. Thanks again.
 
I'm going to post a picture showing the differences in the springs. The rusty stuff on the left is original 1963 225 Slant stuff with an 11.5 mechanical advance, and the clean stuff on the right is the newer reman '70's distributor with a 13 mechanical advance. Notice that the older distributor has a heavier primary spring and a shorter loop on the secondary spring.

How is this going to affect the advance curve?

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The weights and springs do not control how much advance, only how quickly it comes in.

The garage door size spring on the weight is the first thing to go on any distributor I work on.
 
I'm going to post a picture showing the differences in the springs. The rusty stuff on the left is original 1963 225 Slant stuff with an 11.5 mechanical advance, and the clean stuff on the right is the newer reman '70's distributor with a 13 mechanical advance. Notice that the older distributor has a heavier primary spring and a shorter loop on the secondary spring.

How is this going to affect the advance curve?
Pretty much as illustrated in post #6.

The initial position of the weights is all the way in. The spring force holding the weight is determined by the spring rate and how far it is stretched. So even though its a heavier spring, how much force it applies initially can be adjusted by changing the initial tension.
The spring perch is adjustable. If you look close you can see its mounted on an eccentric.
upload_2019-1-26_18-55-22.png

Increasing the primary spring's tension will delay the rpm begins to advance.
The '63 primary spring is made of heavier wire and about the same number of coils, so timing will advance more slowly than the one from the electronic.
Drawn on graph the '63 spring will have an advance something like the light blue lines and the new primary spring like the dark blue from 300 to 800 rpm
upload_2019-1-21_10-2-1-png.png


As the rpms increase, the weights move further out and eventually engage the secondary springs.
The inside length of the secondary spring and the spring perch position will determine how many degrees advanced before it engages.
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Eyeball guess is the new secondary spring is longer and therefore will allow more advance before engaging.
It has less coils so will have a higher spring rate and slow the advance more.
On the graph the secondary springs help slow the advance which will continue until the slots stop the weights at 11.5* for the early distributor and 13* for shiny new one. For a clearer comparison, deleted the minimum specs from the graph..
upload_2019-1-26_19-39-15.png
 
To make sense of why the curves are different its really important to look at the initial timing and rpm for each.
Sticking with the 1968 non-smog and CAP advances, here's what the curves look like on the engine.
upload_2019-1-27_8-36-24-png.1715281476

The quicker and longer primary advance was to make up for the initial needed for idle to meet early emmisions requirements.

Now, IF for sake of discussion, the two sets of spring were the same as used in '68 for CAP and non-CAP distributors, but used the slots that you have of 11.5 and 13, the advances would be cut off early.
11.5 x 2 = 23* advanced from 5* initial = 28*
13 x 2 = 26*
upload_2019-1-27_22-51-4.png


And if the CAP/electronic distributor was installed for initial at 5* BTDC it would look like this.
upload_2019-1-27_22-53-4.png

IMHO not great to be that much earlier timing except maybe for drag racing - if the max isn't too much. Slant six may tolerate the quicker mid advance but just be careful. Tolerance of the earlier timing for street use will depend alot on the vacuum advance. I put a fair amount of weight in the pre-smog ignition curves for points. The factory spent considerable effort in trying to optimize those things for all conditions.
 
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