Distributor Advance Expertise Needed

[Geoff 2]

68,
The VA spring is a parallel wound spring, same as most valve springs. So my description of operation is correct. By adjusting the AK, you are simply compressing the spring, so that there is more tension on it. It will now require more vacuum to start pulling on the spring. You can easily verify this. Turning the AK fully CW expands the spring, softest setting. Do this & push on the actuator. Now go fully CCW [ probably 8-10 turns ] & push again on the actuator. Spring is much stiffer, harder to push actuator in.

I will detail later tonight how to dial in MVA.
This is in a D. Vizard book on Holleys, S-A books # 216. [ Worth buying] , but he talks about ign, Hmmm


'The optimum idle advance is typically about 35-40* for a short cammed street engine & [ though not commonly realized ] as much as 50* for a street/strip engine.'

 

[Geoff 2]

Ok, I am back. I have done the following procedure on dozens of cars, never had a failure. As a general statement, the bigger the cam & the lower the CR, the greater the idle timing that will be reqd.
First step is use factory initial timing or close to it & adjust carb for best idle.
With engine warmed up, idling, in gear if auto, vac adv disconnected, loosen dist clamp & slowly rotate dist to advance timing. RPM will increase. Keep advancing dist until rpm no longer increases; then toggle dist to make sure you have the highest rpm. Now check & note what the timing is. This part of the test is finished, return dist to original position.
Say [ example ] the above test showed 38*. Most adj VA units have a 30* range. You need to select a combination of initial [ static ] & vac adv connected to manifold vacuum that adds up to 38*. If the engine has a high CR, better to select a lower init timing #. In case of detonation.

VA unit adjustment. Set the AK fully CW. Make a stop to limit the VA actuator travel to the desired amount. Check that the timing mark is steady at idle with VA connected, in gear if auto. If car drives ok, no detonation, job done. If you get detonation, there are a few fixes. Try this first: turn AK CCW 2 turns & re-check timing. Keep going 2 turns CCW, checking timing, until timing drops or becomes unsteady. Then go back 3 turns CW.
If this is not enough, you can use heavier spring[ s ] in the centri weights or change the combination amount of static + VA.

 

[Dragon Slayer]

Geoff2 I agree with what you're saying except with your spring rate. Sat is correct the slope is the spring rate. You just preloaded force applied, not the rate as long as you haven't gone past and elastic point on the spring.

You set valve spring height to get the preload force. LBSF. The rate does not change. LBSF/In. IF it was a 350lb/in spring whether you preload it 100lbs or 150 lbs the rate is still 350lbs to move an inch. Since it preloaded with 100lbs against movement, adding 350lbs is only 250 and it moves 250/350 inches open.

Prestolite had different springs lengths and rate, plus the preload via washers. So you can change the position of the curve up or down with respect to vacuum value, and also change the slope. The preload can push your slope to a point where you might not get the full advance of the arm because you run out of vacuum to move it any further.

Since with the modern Chrysler type you only can control preload, not rate you have to live with the rate. Prestolite you can manipulate both, and they had charts to plot the vacuum curve based on that.

 

[68 Sport Satellite]

DragonSlayer - that makes more sense now. Thanks for the clarification.

 

[Geoff 2]

I clarified my comments on spring rate in post #41.

 

[biomedtechguy]

As a general statement, the bigger the cam & the lower the CR, the greater the idle timing that will be reqd.

THIS! So many great posts.
For me and my 70 V-code 440 6bbl Roadrunner, 292°/.509 cam, around 18-20 degrees of ignition advance at idle was the BEST thing that could have happened! Well, along with the epiphany of learning about and how to adjust the outboard carbs idle mixture screws...
I got a complete ignition system from Don at FourSecondsFlat aka FBO and he "pretuned" it for me, and gave me specific instructions based on the HP parts in my 440 6bbl. The higher IDLE advance, along with the TOTAL advance limiter "ring" that is in it made a huge improvement.
I'm going to switch to a Progression Ignition system, which is light years ahead of ANY ignition system that is used with carbureted engines. My FBO system has been flawless, the rev limiter is completely reliable, the ECU has never failed to work, but the Progression Ignition is the ULTIMATE solution, which can only be matched or surpassed by a standalone system like FAST EFI or any of the others.