A stock primary spring will return the advance unless something is hanging up or sticking - it has to. The secondary spring doesn't come into play at low rpm.

Also, the ones I've measured have a spring rate around 8 lbf/in (but will vary with application). I've not done any testing without a second spring, but its probably a reasonable extrapolation that its roughly 2 degrees advance for 300 rpm.

If you need it slower, Mcmaster-carr sells one from Gardner spring thats .75" long and 24#/in. That's a hair longer than the stock and Direct Connection primary springs I've measured. The perch may have to be adjusted outward.

To know if it will hit your 3000 rpm target, need to know the initial timing. The start of the advance can be changed by rotating the spring perch so there is more or less initial spring force. (There's a picture of the perch in this post http://racingfuelsystems.myfunforum.org/viewtopic.php?f=25&t=885&p=8277)


Yes But do yourself a favor and make sure the limiting is done on the inside, not the outside (high rpm). One of the reasons the Tach Drive is the cat's meow is that the secondary spring compensates for 'slew rate' in the electronics. The only downside of that distributor is that timing is varying with rpm at idle. So to solve that, shorten the slot on the idle side. My guess is that when they developed the distributor starting was more important than idle.
I've posted a graph of the curve here
http://racingfuelsystems.myfunforum.org/viewtopic.php?f=21&t=666#p6029
My best 1/4 mile and dyno runs have been with that distributor. I'm trying to replicate the concept into vac advance version, but I'm not giving up tach drive unit, ever!

Robert, Thanks for sharing that. Do the instructions say or show the degrees on a graph? Also, if you have any of the springs, could you take a picture?

One way to save some time on the engine or distributor machine is to measure up the springs. It's a little tedious but so is installing different combos in the distributor.
Spring Rate can be calculated from the following:
> Number of coils; More will reduce spring rate
> Wire Diameter: Larger will increase spring rate
> Coil Diameter (O.D. - wire diameter): Larger will reduce spring rate.

Initial force will vary with length inside the loops, so measure that too.

For calculating spring rate for a coil spring, make a couple of assumptions.
1. Active Coils are 1/2 coil less because the last where it turns out doesn't spring.
2. Elasticity for the material. I've been using 11,000,000 lbs/sq in.

Spring Rate = (11,000,000 x Wire dia)/(8 x Active Coils x Dia^3)

This where the roughly 8 #/in rate for a primary spring cames from. But this is just based on the ones I've measured. Will vary with application.
The P2932675 super light springs work out to 0.68 lb/in each
The DC/MP Aluinum Electronic Tach Drive primary spring 1.3 lb/in

That spring is stiffer than any of the recurve springs are combined. All of the kits are pretty much the same, trans dapt, mopar performance, and mr gasket I have them all. And they all are basically the same spring, which tends to be too weak to keep the advance stable at idle. Unless you've welded your slots up alot already and it pre-loads it enough so that its half stretched. The factory light spring only will give you full advance by about 2500-3000. If you want a little faster you can stretch it a little.

If the factory light spring won't return your advance mechanism you have another issue (something sticking), or its not a factory light spring. That spring pretty much has all of the control in many of the the factory advance curves, except the last 4-8 degrees of advance. The other spring paired with the factory light is usually a slotted HEAVY spring that doesn't do anything at idle or the beginning of the timing curve, but makes the last 4-8 degrees of the timing curve come in REALLY slow, depending on how long the slot is. There was also a medium spring with no slot used in some setups.

The mopar distributer is a good item but you need to understand a few drawbacks compared to MSD and similar.The Mopar distributer is a bushing distributer and any wear can cause timing fluctuation and spark splatter at the cap and rotor.When using light springs the timing can also fluctuate at idle and low RPMs until the weights are fully extended.These are some of the reasons that most early use of the Mopar distributers in performance application the vacume advance was eliminate and the mechanical advance was locked.Another issue is the slop at the slot at the intermediate shaft drive gear.After market distributers usually have a larger and tapered engagement flute and sometime even need filed to fit.

I wrote myself a note to take some pics if I can get the right functions on my Samsung st-72 camera to take a close up. For a spot on dist I would: drive out the lower roll pin & add shims (flat with no beveled edge, like common washers have) such as the speedway motors shim kit (its cheap) to reduce the axial play to .005. Peen the lower dist tang or add weld in the intergear slot to reduce the rotation slop between slot/tang interface to zero. add a side oiler (they're available) to the dist housing or an aluminum nipple & capped with one of those red/green or yellow colored plastic Ace hardware "thread protectors" which are pennies each. As said check bushing/shaft wear & replace/ream the new bushings as needed. check & correct rotor phasing as needed. Spring advance should start no less than 1-200 RPM above your hot in gear idle speed which is what you will see stoplight to stoplight. reduce rotor metal tang tip clearance to dist cap terminal cap clearance to .015" NAPA has an Echlin MO3000 rotor with a .060" longer blade for $8.xx out the door or drill out the rivet on the stock rotor & replace the outer brass piece with a longer piece. IN ORDER set initial with the vac gauge method. set total with the FBO $22 plate (initial+slots) to the accept initial for your eng type (35-SB) for a very close start. mix/match springs so you are just under the pinging point at WOT up thru the gears on your hottest/driest (most likely to ping) day. plug in the can to ported & adj with a 3/32 allen wrench so you are just under the pinging point in everyday driving under varying load (in hg)/RPM conditions on your hottest/driest day (CCW slows the curve/total is set but you can work with the notch on the arm either way or get another can. there is silent ping & gas varies & ain't getting any better and ANY pinging you can hear is EXTREMELY damaging so give yourself a cushion. A 1/2" hole in the top of the cap 2/3 between the center terminal and the #1 terminal lets you check phasing while running and several of these holes (tho unsightly) in the top and sides let ionized air molecules escape which can promote crossfire (misfire) just like when lightning strikes a tree with wide branches the space from the outstretched branches down to the ground is electrified (ionized) in the same way which is why they tell you never to seek shelter under a tree in a rain storm WHEN it is lightning or may be lightning shortly. use a brown or black cap with BRASS inserts. EDIT Oh I forgot about your question, there was no graph but you could
make one. MORE EDIT the crane orange and red springs look identical to theeh OE stout one with the elongated loop on one end that you toss so I didn't even take a pic as I dont think it will be of any use. their light one I could not find but iirc it looked very close to the OE light spring (I'm sure the tension was slightly different on all three of em comparibly).

I use the Mr Gasket kit in my sons stock dist in his Dart. I have had good luck using the Mr Gasket kit in a few street car I used it in. Ron