C3 Dist. shaft question

[bruce11495]

I needed to replace the distributor main shaft in my car. I went to Paragon and bought the HP shaft because the car is an LT1. They only had that and a regular shaft available. On looking in the 72 parts book I see 20-30 different shafts. Does anyone know what is different on these shafts? What shaft should I have used in my car? I'm having a BIG problem with too much timing now, 45-50+ degrees of advance.
Thx in advance.

[Joe L.]

Re: C3 Dist. shaft question

Bruce------

The ONLY difference in the Corvette distributor mainshafts is the "auto cam" brazed to the top of the shaft. This part, in conjunction with the specific weights and springs, determines the centrifugal advance characteristics of the distributor. In fact, the auto cams, weights, springs, and vacuum control are the only really different components of the Corvette distributors. Most of these "auto cams" are not reproduced. Only 2 are and those are the "standard" and "hi-performance" sold in the reproduction.

My recommendation is to get your distributor set up according to the specific advance characteristics that you want. Doing this, the configuration of the auto cam, springs and weights you start with are moot since the distributor servicer will configure the distributor to the specific specs.

[bruce11495]

Re: C3 Dist. shaft question

Joe....That is not so easy to do in this day and age. Number one, trying to find someone to work on a "distributor" is not easy, try and find someone who knows what a distributor is and has a machine capable of running a TI ignition on is all but impossible!!!! TI ignition may be a "household word" in the Vette community, but there are very few mechanics who know and understand it. I'd like to know what the differences are, they must be very subtle, in the autocam to have so many different ones listed, if anyone has that info.

[Joe L.]

Re: C3 Dist. shaft question

Bruce-----

The differences are very subtle in these autocams and there were MANY different configurations of them. The final advance mechanical configuration was a function of the COMBINATION of the autocam, springs and weights. There were a lot more auto cam configurations that there were different springs and weights. However, when you factor in the number of each and consider that the total number of configuration possibilities was the mutiplicative product of the various conmbinations, the totals are in the thousands.

You're right; there aren't too many folks who can set up these distributors anymore. And, sadly, there is one less now with the passing of Dale Pearman. However, Dave Fiedler at TI-Specialties can do it and, I believe, Hi-Tech Innovations in Florida can do it, too. I'm sure that there are others that other folks on the board are familiar with, too.

Dave Fiedler has some information on the autocams that I think he would share with you. The specs on these cams were never published in GM parts literature.

[Duke W.]

Re: C3 Dist. shaft question

The way my SHP SB is setup I have 52-54 degrees total advance at cruise speed over 2500 revs as this condition yields full centrifugal plus full vacuum advance plus initial. The ignition map has to be plotted in three dimensions because there are there are two independent variables - manifold vacuum and revs.

For a SHP engine you want the sum of initial and full centrifugal to equal about 38 degrees, and full vacuum advance should be 16 degrees at 8".

Ask the vendor for the "advance curves" of the two different shaft assemblies they offer, In particular you want to know the amount of advance when the centrifugal is full deployed.

With a dial back timing light one can characterize the advance map in a couple of minutes.

Duke

[William C.]

Re: C3 Dist. shaft question

An added variable is that some distributors used a bushing around the stop pin on the mechanical advance assembly to limit the total advance. In addition to the cam variables mentioned, the use of this stop, plus a myriad of available slot lengths on the distributor point cam assembly coumpound the situation.

[Daniel Grunwald]

Re: C3 Dist. shaft question

Bruce, I understand what you are talking about. I have a 70 454 auto and bought a Paragon distrib shaft, (the HP one as they recommended to me), and when I went to set the timing it would not run at less then about 15 degrees advanced with the vaccum line off. It seems to run up to about 40 degrees when revved to full advanced RPM. I ended up just setting at the 15 degrees so it would stay running and it seems to run well. It seems that it should take a much more technical approach to make it run well but I don't know exactly what that might be. Nothing in the engine specs book remotely resembles the timing readings that I have.
Daniel

[Joe L.]

Re: C3 Dist. shaft question

Daniel-----

Have your distributor re-curved to duplicate the original specs for your distributor. Then, it will work just like an original unit. Dave Fiedler can do it for you.