Another distributor re-curve question

[Steve D.]

I am looking for a tach drive distributor to set up for a 327 SHP application. From answers to previous posts I have a target curve, but my current question is this: are distributor numbers for 350s or even big blocks candidates for recurving to my application?

Steve

[William C.]

Re: Another distributor re-curve question

Within some limits, nearly any distributor can be curved to the '65 SHP specs, some may require changing the distributor cam assembly to limit the total advance as the mid 60's SHP curves use a different slot length in the mechanical advance limit slot to allow the greater initial advance setting as opposed to the later engines which carried initial timing in the low single digits.

[Steve D.]

Re: Another distributor re-curve question

Bill

When you refer to the cam assembly are you talking about the cam that is brazed onto the top of the distributor shaft?

Steve

[William C.]

Re: Another distributor re-curve question

No, the cam that has the lobes that operate the points. That is the one that carries the slot the pin from the cam on the shaft rides in to limit the total advance travel. Sometimes you can work around the issue by changing the size of (or adding if not present) the bushing on that pin. The limits of the total advance built into the distributor are largely controlled here, although the shape of the cam on the top of the shaft also has some effect. Mostly it's amatter of putting the dist on a machine to see what the curve is and then figuring out a plan to get where it needs to be.

[Craig S.]

Re: Another distributor re-curve question

Bill - besides what you mention, the autocam on top of the distributor has an affect to in the way the advance curve functions, the shafts were unique in terms of that autocam brazed on the top. But like you say, you can work around that. I think there were also about 3 sets of weights and the way the weight tangs ride on the autocam affects the advance, along with the actual weights and spring tensions....Craig

[Duke W.]

Re: Another distributor re-curve question

The important issues are to select a vacuum advance that is properly matched to the manifold vacuum characteristics and have the total WOT advance - the sum of initial and full centrifugal in the range of 34-38 degrees.

If you look at '65 distributors the L-76/84 have 24 degrees max centrifugal at 2350, and both used a 8" vacuum can. The recommended initial timing is the range of 10-14 degrees, and they pull about 10" @ 900. The L-79 has the same 8" vacuum can, but the total centrifugal is 30 @ 5100 with initial range of 8-12, which is slower than the mechanical lifter engines. This is because the 30-30 cam has a later closing inlet valve, which yields a lower dynamic CR, so they can tolerate more advance at low revs.

Most distributors have 24-30 degrees of total centrifugal, so you select an initial timing to achieve total WOT advance in the above specified range, and hopefully your engine will tolerate close to 38 without detonation.

In cases where you have a 24 degree distributor, but even with stiff springs the engine detonates at low revs, the solution is to increase the centrifugal and back off the initial so low rev advance is less.

Whatever distributor you end up with, measure the total centrifugal then set the initial to achieve about 38 degrees total WOT advance. If it detonates at high revs - above the point where centrifugal is all in - you will have to back off the initial, but if you have to back it off to the point where total WOT advance is less than 34 degrees you will lose power. If this is the case, you either have to retard initial more and take the power hit or increase fuel octane rating by mixing in some race gas or avgas.

Once the total WOT timing is determined, select springs to get the centrifugal in as quickly as possible without detonation.

The other task is to determine the proper vacuum can. Tie up the centrifugal with a rubber band and set the timing at about 20-25 degrees with a mild cam and 25-30 with a SHP type cam. (This simuilates the desired total idle timing, which is the sum of initial and full vacuum advance.) Go through the idle speed mixture adjustement and use your final manifold vacuum reading to select the vacuum advance. The vacuum advance should provide about 16 degrees maximum advance at (at least) 2" LESS than your vacuum reading. If the idle manifold vacuum is 14" you may use a 12" can. If manifold vacuum is 12", a 10" can is called for, but since none is available, use an 8" can.

Reset the initial timing to the proper value, then install the vacuum can and make sure it is connected to a full time manifold vacuum source. Check total idle timing and verify that is is about equal to the sum of initial plus full vacuum advance. If you have a quick centrifugal curve that starts below idle speed, the total idle timing many also have a few degrees of centrifugal, which may push the total idle timing up to a little over 30 degrees, which is okay if you have a SHP type cam.

Duke

[William C.]

Re: Another distributor re-curve question

Also another point I did not mention is the location of the pivot point for the weights is changed on the newer distributors. This also affects the degree of difficulty of improving the cireve. As Duke says, the system must be kept in Balance, and the curve and vacuum advance must be compatible.