race distributor advance springs

Anyone know how to i.d. the correct advance springs , that came in the old m.p. race distributor 4120942 ? looks like i have one thats pink and one that has no color/paint marking on it. What springs came in that dissy for full/quick advance ?

Get the lightest set, I use just one to bring back to start, full in at 800 up. Bring up to 1500, then set at where you want full advance degrees to be. Full advance at idle

I like and use the Mr. Gasket real light weight advance springs, part number 925B
Those springs work well on the street and strip only cars with BB motors

Race deal with a big cam? I just lock it out so I don't have to make it idle 13-1500. 800 in gear.

o.k. , anyone know what springs were used in a New chrysler Aluminium race distributor ? 1970's era

IIRC, they were Mallory. You can find them at Summit but they ain't cheap.

From the "Don't Never Throw Nothing Away" dept - This has been in my tool box for longer than I can remember.
I never thought someone might be looking for them.

those look exactly like the Mr. Gasket 925B springs

Same springs hanging on the peg board at the speed shop, ONE company makes them all

Do you happen to know which company that is? I've tried to find out who was making them but never could figure out who is making them.

TIA

There may have been changes in springs used for the tach drive race distributors over the years, but if so, they would be small changes.
Break out your calipers and magnifying glass. Measure the wire, the coil od. and the over length inside the loops.


Spring______Wire Dia. Coil O.D. Coils Spring Rate Length
.Light Spring 1: 0.018", 0.230", 12.5, __1.26 #/in, 0.72"
HeavySpring 2: 0.053", 0.293", _5.0, 174.41 #/in, 1.086"

MP P2932675
Super Light pr: 0.015", 0.225", 11.5, __0.68 #/in, 0.7"

The original combination was designed to:
a. compensate for delays that made the effective timing retard in the higher rpms.
b. provide easy restarting with high compression engine.

I suspect the super light springs were originally for racing with dual point distributors.

Wow, VERY GOOD INFO !!!!!

If it means anything....the color of the springs look like the paint color on the old mopar purple shaft cams . I will end up testing the advance curve , to verify .

If you haven't seen the first before, its from the DC Engine Book.

The second is a small block version of that distributor I ran on Sun 404.

The heavy spring loop is part of the secret sauce that keeps timing steady or slightly rising depending on how quick the pulse amplifier is.

Are you sure the value you give for the heavy spring lbf/in is correct?

Yes. As a calculation anyway. Feel free to run the numbers. I subtracted 1/2 coil for active coils and used 11,000 K for elastic shear modulus.
Measurements make a big difference when there are few coils.

But as a comparison, it should be ballpark correct.
When looking at the effect on the advance, its the rate multimplied by the distance moved. Because of the long loop, it doesn't extend much. And when used with light primary spring, it gets little assistance.

Hard to believe it is 174 lbf/in spring

On springs that I have spring rates from the manufacturers, my calcs based on the measuremens are in the ballpark - often very close.
So I have no reason to suspect these are far off.

As mentioned above, the difference in force between secondary and primary springs is not as great as the difference in spring rates appears.
For example, here's a pair from a semi-generic A1 Cardone rebuild. Who knows what the original application was.
Primary spring 8 lbs/in, 0.72" long
Secondary spring 45 lbs/in, .92 in long.

When the springs are installed, they are stretch to approximately .79"
So here's the force restraining the weight (ignoring the mounting angle which changes as the weights swing out)
Distance - Force Primary - Force Secondary
0.79" - 0.56 lbs - 0 lbs
0.82" - 0.80 lbs -
0.86" - 1.1 lbs -
0.89" - 1.4 lbs
0.95" - 1.8 lbs - 1.4 lbs
1.00" - 2.2 lbs - 3.6 lbs
1.05" - 2.6 lbs - 5.7 lbs


All this only goes so far. What we really want to know is what happens when we spin the distributor up!
But it does save some time when looking at which spring is most likely to cause the change in rate or force wanted.

I guess your confusing me with calculation versus a measured spring rate.

You state the long spring is rated at 175 lbs per inch. I really doubt you could hang a 175 lb weight on it and not have it come apart. This is in the range of Valve springs.

The spring is slotted to allow the lighter spring to control low rpm advance. Higher rpm with greater centrifugal force from the weight and a slower rise of advance required to get to total is where the heavy spring comes in.

I never measured them but I do have a distributor scale for point setting, and an old larger scale for heavier objects so I guess I will do a pull test to see the rating.

Are you sure your not missing a decimal point and it is 17.4lbfs/in. The light spring is only 1-2 lbf/in.

Maybe its the way I wrote the posts. It may take a few posts to straighten out.

The dimensions of the springs were used to calculate the spring rates. That's where all of the spring rates posted come from. I do have a few springs with rates from the manufacturer but did not post those. But they back up the general accuracy of the calculated rates.

In the post immediately above, the numbers in the two right hand columns represent spring force. Pounds force.
This is the force of each spring pulling on the weight.

With the wieghts engage into the advance slots, the primary spring is slightly stretched.
So it provides a restraining force on the wieght.
That force(red arrow) is the spring rate multiplied by distance stretched.
.79"-.72" = 0.07" stretched
8 lbs/in x .07" = .56 lbs force
The secondary spring is free to move, so it applies no force.

Here's the tach drive distributor (small block version) with its original springs.
The first photo shows the loop to loop lengths at rest. That's what I posted early in the thread.
The length effects initial tension, and how many degrees out before the secondary engages.

I follow all that. I would also state the difference between the cardone spring at 45lbf versus 175 is very significant. Granted tighter coils on these secondary springs, but the thickness is not as great as other springs that are in the 35 to 45 lbf range on fuel pumps, let alone a valve spring at 175lbf. I guess I would have to go back to some of my physic course to calculate the maximum force the weight could apply at 3000rpm as an example. Just seems that rate was too high, but I understand what your doing.

As a note, the one picture shows the slot welded up on the inside (closest to shaft), I assume to limit total advance of that 15degree cam stop to something more in the 10 to 12 range. Some have told me it does not matter whether it is welded inside or outside. Which is true as far as total advance is concerned. But it does make a difference on the spring preload. The inside weld puts a preload from the smaller spring which means initial advance as you raise RPM is delayed by that force. That may be what you want if you are heavy on Initial. But is seems to me, it is better to weld up on the outside to limit total advance, while letting the spring combo do its thing.

You understand exactly.

I beleive the idea of limiting the max advance came from the era where the secondary spring was removed, and the vacuum advance pinned.
This worked for drag racing with points. It was pretty easy to do as well.

With electronic amplifiers, this doesn't work well. The first time someone cued me into this was a coworker who had been running stock eliminator. They had tried to improve their times and one of the first things they did was remove that stupid heavy secondary spring from the DC Tach drive distributor. They tried a whole bunch pof different curves with one and two light springs but were not hitting their times and mph. Finally they put that seconday spring back in, and guess what, their mph and times came back.
About 10 years ago Tuner was telling similar stories on Speed-talk, and not just about his own experiences, but noting how all of the factory teams had learned this too.
https://www.speed-talk.com/forum/viewtopic.php?p=201411#p201411 and here
https://www.tapatalk.com/groups/rac...d=5f00c7686ad6e655eaf34962286567e5#p5625
A couple years ago Bill Baldwin posted a video showing this on his distributor machine.
See: https://board.moparts.org/ubbthread...ry-unilite-timing-issue.html#Post2487959
When I tested that tach drive distributor on the sun 404, that was with the Sun amplifier. That's a pretty slow unit - in the same range as the Chrysler ECUs. With a faster ECU, might have seen a slight rise in the timing from 2000 to 4000 rpm. Maybe a degree.

For use with a vacuum advance, and in uses where engines get really heat soaked, the easiest way to get the mechanical advance curve is is shorten the inside and let the secondary spring slow the advance around 2000 rpm plus minus depending on the engine. That's because pretty much all magnetic pickup distributors were made after the implementation of the Clean Air Package. Shortening the inside of the slots makes the curve the way it would have been before CAP and retarded initial timing. If the begining of th e advance is delayed too much because of the increased initial tension, the spring perch can be adjusted.
psst. This is super secret stuff. Only share with people you like.