Ignition mapping exercise

[Steve D.]

63 SB NOM
In a post a couple of days ago, Duke listed steps to determine the ignition map. Since I just got my rebuild running, I thought I would go through the exercise. The distributor on the engine prior to rebuild did not have vacuum advance, but I bought one with vacuum advance from an individual. The only mark I can find on the can is “B1”. (Per Duke’s recommendation in previous posts, I ordered the VC1810 can from NAPA and they will have it next week.) The engine is set up like a 340 with domed pistons, 8” balancer, LT-1 cam and solid lifters. The carburetor is a Holley 4160.
Here are the mapping results; I had previously set the total mechanical advance to 36 degrees.

(Note; for some reason the following data are spaced very close together. It is supposed to be a table showing RPM vs Advance in degrees; the first entry is 525 RPM and 8 degrees)

Mechanical
RPM Advance

525 8
800 9
1000 12
1200 15
1400 18
1600 20
1800 22
2000 27
2200 29
2400 32
2600 34
2900 36
3000 36
3200 36

(The following data is supposed to be in three columns; Inches HG, Advance, and Cumulative Advance

Vacuum Can

Inches Advance Cumulative
HG Advance

2 0 0
4 0 0
6 0 0
8 1 1
10 1 2
12 3 5
14 4 9
16 3 12
18 2 14
20 1 15
22 0 15
24 0 15
26 0 15

Any comments? I’m going to change the can anyway, but does this info indicate that this can is mismatch for this application?

Digging through my library of printed posts, I found one in which Bill Clupper had recommended the mechanical curve for the 1965 (327/365hp) #1111069, but I didn’t find the specs for it. Does anyone have them?

I’m also uncertain about which vacuum source on the carburetor is a “full time” source, as opposed to a ported source. In a separate source I will post a photo of the carburetor and maybe someone can point out the correct source.

Steve

[Duke W.]

Re: Ignition mapping exercise

You need to subtract initial timing from your centrifugal measurements to get the true centrifugal curve. From you lowest rev measurement I take it the initial is only 6, so the total centrifugal is about 30, and it's all in by about 2900, which is okay.

As per the specs for the B1 can, the maximum vacuum advance is not in until about 18". Since LT-1 cam will only pull about 12" at idle you really need the VC1810 can.

BASIC RULE: The vacuum can must provide maximum advance not less than 2" less than typical idle vacuum at typical idle speed with somewhere near 25 degrees of total idle timing, which is usually obtained with the sum of initial and full vacuum advance.

I don't know what you mean by "advance" and "cumulative advance".

The advance map works like this.

1. Starting point it initial timing.

2. Advance is added with increasing manifold vacuum

3. Advance is added with increasing RPM.

That's it! Measure the total advance with the vacuum can disconnected and subract out initial to determine the centrigual curve. Get the engine revs low and high enough to determine the start point and point of maximum centrifugal.

Tie up the centrifugal with a stiff rubber band and pump down the vacuum can with a Mighty Vac or equivalent in 2" increments. Do your best to get the exact start point vacuum and vacuum to provide maximum advance. As before, you subtract out the initial to determine the actual vacuum advance curve.

Be sure the vacuum can is connected to a source of full time manifold vacuum, including when the engine is idling.

Duke

[Steve D.]

Re: Ignition mapping exercise

In regard to the vacuum can measurements, I used the term advance for the observed incremental degrees on the dial back light at the indicated applied vacuum, and cumulative advance for the running total of the observed increments.

Steve

[Duke W.]

Re: Ignition mapping exercise

Okay, what you have appears to be 0@7", 15@19". The specs for the "B1" can are 0@9.5, 16@17, so it's a little out of spec. You should always run three measurements and check that they are consistent.

The nominal specs for the "B28" VC1810 are 0@4", 16@8".

Prior to attaching the vacuum signal line to the new can, suggest you bump total WOT timing from 36 to 38. Also rev it to at least 5000 to ensure it does not advance more.

The low rev data indicates that the centrifugal mechanism may be a little sticky, so it would be a good idea to pull the dist. for a teardown, cleaning, inspection, and regreasing of the upper bushing grease well.

You should use the 28-32 oz. breaker arm tension points such as NAPA/Echlin CS89. Your total idle timing should end up in the range of 25-30, which is the correct range and vacuum at 800-900 idle speed should be at least 12".

If it doesn't detonate, you can try some lighter springs, but the map will probably be close to ideal as is with the VC1810.

Duke

[Steve D.]

Re: Ignition mapping exercise

Thanks.
I think I'm getting there, albeit slowly.

Steve