fv64olds98

Hello All,


The engine is a 394ci 4 barrel carb, year 1964, AC car


I would like to know if someone can help me understand the charts below in order to :


1. set the correct Initial Timing


2. set the correct Mechanical Advance (by selecting the correct set of springs for my distributor)


3. verify that my vacuum advance is correct




I have the Oldsmobile factory service manual and I have printed the Tune up charts, but I do not know how to interpret them correctly. Based on the images attached this is my understanding. Please correct me if I am mistaken:


1. According to the chart below (Oldsmobile Tune-up 2). My understanding is that with the vacuum disconnected from the distributor, vacuum port plugged on the engine side, and RPMs set at 850, the timing should be 5 degrees before top dead center


2. Mechanical advance according to the chart below (Oldsmobile Tune-up 1) say 0 to 2 degrees at 400 RPM, 9 to 11 degrees at 1200 RPM, and 12 to 14 at 2000 RPM. These are the questions I have:

How can I test 0 to 2 degrees at 400 RPM, when initial timing is supposed to be set at 5 degrees at 850 RMP?

How can I test 9 to 11 degrees at 1200 RPM? - should the vacuum advance be disconnected? I am assuming it should be disconnected. Should the total timing at 1200 RPM be 9 to 11 degrees or should it be 14 to 16 which includes the initial timing of 5 degrees?


How can I test 12 to 14 degrees at 2000 RPM? - should the vacuum advance be disconnected? I am assuming it should be disconnected. Should the total timing at 1200 RPM be 12 to 14 degrees or should it be 17 to 21 which includes the initial timing of 5 degrees?


Also what should happen above the 2000 RPM range should the timing be advanced any further or should that be the maximum advance?




2. How do I validate the vacuum advance is working correctly (Oldsmobile Tune-up 1).


The table states Start - 9 to 11 In. Hg. 12 degrees to 14 degrees at 19 In. Hg.


I am not clear with what "Start - 9 to 11 In. Hg" means. The second statement states 12 degrees to 14 degrees at 19 In. Hg. Does this mean that when vacuum is 19 In. Hg total timing should be 12 degrees to 14 degrees, or is this added timing to whatever timing the engine would have with the vacuum disconnected from the distributor at the RPM that yields vacuum of 19 In. Hg ?


Any help you guys can provide interpreting the tables attached would be great


I am not sure what this really means

Octania

Yes, the static and Mechanical [MA] and Vacuum [VA] systems are additive.

Beware that the distributor MA may be specified in DISTRIBUTOR RPM [which I shall hereafter call DRPM] which would explain why they do not go above 2000 RPM = 4000 *ENGINE* RPM [ERPM]. Distributor tuning was often done with the dist'r out of the engine, in a special machine, and that machine showed its speed- that of the distributor.

Sounds like you have a pretty good grasp of the three systems.


1. set the correct Initial Timing
===============
Use the chart, vacuum disconnected and engine side of the line plugged [the distributor canister is just an actuator with a diaphragm and not a source of vacuum]. Turn the distributor body to attain the desired setting at idle speed. Why at idle speed? To ensure that the MA system is not active and adding advance.


2. set the correct Mechanical Advance (by selecting the correct set of springs for my distributor)
=================
With VA still inactive, use the distributor/engine rpm vs timing specs in the chart to verify that the MA system adds the proper amount at the specified RPM. Remember that it ADDS TO the static timing. You may find that no spring changing is necessary... unless you have changed distributor parts or altered the basic build parameters of the engine- compression ratio, cam specifications, etc.


3. verify that my vacuum advance is correct
===============
Use a hand vacuum pump with gage. MA inactive because you will be at idle speed. Pull x number inches mercury on the distributor vacuum canister with your hand pump, and observe the timing. Repeat for as many x number in. Hg as you can stand. At least 2-3 readings. Every 2" from 0 to 20 would be nice. Write the results down of course. Compare to the specs chart.




1. According to the chart below (Oldsmobile Tune-up 2). My understanding is that with the vacuum disconnected from the distributor, vacuum port plugged on the engine side, and RPMs set at 850, the timing should be 5 degrees before top dead center
=================
Correct in every way. Set static timing to 5 degrees BTDC with VA disconnected but not leaking vacuum, and ERPM at, in your case, 850.



2. Mechanical advance according to the chart below (Oldsmobile Tune-up 1) say 0 to 2 degrees at 400 RPM,
===============
So, not [or barely] active yet at 800 *ENGINE* RPM = Idling



9 to 11 degrees at 1200 RPM,
=============
adding about 10 degrees for a total of 15 at 2400 ENGINE RPM... mid-range


and 12 to 14 at 2000 RPM.
=============
Maxed out at about 13 [plus your initial 5 of course] [and VA system deactivated] at 4000 ERPM




How can I test 0 to 2 degrees at 400 RPM, when initial timing is supposed to be set at 5 degrees at 850 RPM?
===================
ERPM vs DRPM
Read carefully the chart. Static is separate and additive. You are looking for "5 plus 0 to 2" at 800 ERPM - and frankly 800 is about the same as 850 RPM for this purpose. Go ahead and adjust down to exactly 800 ERPM if you like.



How can I test 9 to 11 degrees at 1200 RPM? -
==================
Dial back timing light is the way to go. E.g.
http://cgi.ebay.com/ebaymotors/NEW-X...item2c85b06cff
In your case, you are looking for 5 initial plus ~10 MA, so set your timing light's dial to 15 and, at the specified 1200 DRPM = 2400 ERPM.... the timing mark should align at 0 = TDC because your dialback light told it to go there. If it's not *exactly* 15 but within a degree, that's OK, close enough. Five degrees off, you might want to look at altering its mechanism [change springs].
If you don't have a dialback light, then you would have to have a 15 degree mark on your timing tab and/or balancer.



I am assuming the vacuum advance should be disconnected?
================
No good can come from trying to measure the effects of several systems at once. So, yes, disable any system that is not under study at the time, to minimize confusion.


Should the total timing at 1200 RPM be 9 to 11 degrees or should it be 14 to 16 which includes the initial timing of 5 degrees?
===============
The systems are additive.
Static does not change, thus the name "static"
So, add your known static to "what your system under study is adding" to get the total at this specified condition under study. In your case, at 2400 ERPM, you expect to see a total of 5 from static plus 10 from MA = 15 degrees, give or take a little. The spring vs centrifugal force MA mechanism is never perfect so some tolerance must be allowed.


Also what should happen above the 2000 RPM range should the timing be advanced any further or should that be the maximum advance?
===============
by 4000 ERPM, you should have all your MA in place. Some say, even earlier, which you can attain by the use of lighter springs. Another way to look at it is to slowly increase ERPM while watching the timing, and note at what ERPM the MA system no longer advances the timing. No point in going above that ERPM then, in regards to checking the timing. Frankly, your engine was not expected to do much duty above 4000 ERPM. It is a family car, right?




2. How do I validate the vacuum advance is working correctly (Oldsmobile Tune-up 1).
The table states Start - 9 to 11 In. Hg.
I am not clear with what "Start - 9 to 11 In. Hg" means.
====================
I means simply that the vacuum canister starts to take effect at 9-11" Hg. With vacuum levels less than that, the spring is more powerful than the vacuum on the diaphragm, and the canister's extending arm does not move, therefore the spark timing is not yet altered by this system.
Watch the timing while you increase vacuum slowly with your hand pump. When timing starts to move, note the vacuum gage reading. Should be ~10 inches eh? Somewhere around 9-11" Hg.


"12 degrees to 14 degrees at 19 In. Hg."
The second part states 12 degrees to 14 degrees at 19 In. Hg.
Does this mean that when vacuum is 19 In. Hg total timing should be 12 degrees to 14 degrees,
================
No, this is the condition where the vacuum canister is maxed out, at which point it has added its 13 +/- 1 degrees of advance to the static reading [which was set to 5] for a total of ~18 degrees BTDC timing. At vacuum levels greater than 19" [not likely much of a concern with an auto engine], the vacuum canister has reached its limit of travel, so no more can be provided. The vacuum has overpowered the internal spring and drawn the moving part as far as it can go, up against its travel limit feature- typically a slot at the business end where the rod bends and engages with the movable part in the distributor.
__Distributor vacuum canisters are built with varying amounts of travel [total timing alteration possible] and various springs inside, affecting the vacuum level at which motion starts due to overcoming the force of the internal spring, and the vacuum level at which maximum travel has been attained by fully overcoming the spring and drawing the internal parts as far as their travel limit feature will allow. Vacuum actuators are usually stamped with a part number to make sure you have the right one. PN last 3 digits and amount of advance possible are common GM stamps- e.g. a 1115361 unit which provides 18 degrees advance will bear "MS 361 18" on the base tang. Looks like yours might say "220 13" if it's original. Aftermarket canisters are somewhat limited in their variations, compared to factory tailored units, and are typically stamped something like "B6."


or is this added timing to whatever timing the engine would have with the vacuum disconnected from the distributor at the RPM that yields vacuum of 19 In. Hg ?
======================
Don't use the engine vacuum to generate distributor testing vacuum. You don't want the MA system in play at all when looking at the VA system, so this VA verification part is done at idle speed, using a hand operated vacuum pump to get the fairly precise control you need over the vacuum present at the timing canister.
e.g.:
http://cgi.ebay.com/ebaymotors/Snap-...item20e840b089

Just remember:
The systems are additive
Test and examine ONE at a time; disable/ ensure inactive the other system
Engine RPM is 2x cam/distributor RPM due to the cam sprocket being 2x as large as the crank sprocket.

fv64olds98

Hello Octania,


Thank you so much for taking the time to answer. Clearly two things:


1. When testing vacuum advance do it at idle using the vacuum pump/gauge and never do it using engine RPM as that will kick in the MA


2. When testing MA disable vacuum advance and keep in mind the distributor is rotating at 1/2 the RPMs of the car. Thus, 2000 DRPM = 4000 ERPM.


Really appreciate you taking the time to explain.

Octania

No worries, hope this helps you and others who run across it. I am surprised that no corrections have been made yet. I doubt that I got it all perfect the first try.

I always found it interesting and confusing that some manuals specify some of the advance figures in DISTRIBUTOR RPM, but other manuals use all ENGINE RPM.

fv64olds98

Hello Octania,

I have looked at the mechanical advance and it looks to be working as the specifications suggest.

There seems to be an issue with the Vacuum advance. This is what i did to test it.

1. I set the car RPMs to 850 and the car in drive (vacuum advance disconnected). I measured the timing using the timing gun. With the advance set to 5 degrees, the timing gun shows the the mark on the crank pulley matching with the 0 timing mark on the engine. That is my initial timing, which matches what is suggested on the Tuning specifications.

2. Using a vaccum pump, I start pulling vacuum from the distributor vacuum advance and obtained the following readings:

at 10 in HG -> total timing = 5 which means vacuum advance is 5 - 5 = 0
at 12 in HG -> total timing = 10 which means vacuum advance is 10 - 5 = 5
at 15 in HG -> total timing = 17 which means vacuum advance is 17 - 5 = 12
at 17 in HG -> total timing = 27 which means vacuum advance is 27 - 5 = 22
at 19 in HG -> total timing = 31 which means vacuum advance is 31 - 5 = 26

According to the specs on the initial tune up table that I attached on my firs post. The advance should start at about 9 to 11 in HG. That seems to be working well. However, at 19 in HG, my readings are twice as much (26 degrees of advance) as what the spec suggests, which is 12 to 14 degrees of advance.

This makes me believe that my vacuum advance is not working properly and is taking too much advance.

One thing that is confusing is that the portion of the chart on the first table where it gives the specs for Vacuum advance and Mechanical Advance, the top row of the table says "based on Distributor RPM." These makes sense for the Mechanical Advance, but why would the vacuum advance settings be shown under "based on distributor RPM"?

What are your thoughts on regards to my vacuum advance?

fv64olds98

Hi Octania,

I found some information online on regards to vacuum advance, and apparently is measured on distributor degrees, which are 1/2 of the crankshaft degrees. This makes me believe that the vacuum advance on my car is working fine.

This is what if found:

Max Advance
Since the vacuum advance control unit is a part of the distributor, the number of degrees of vacuum advance is
specified in DISTRIBUTOR degrees – NOT crankshaft degrees. When talking about these control units, it is
important that you know whether the person you’re talking to is referring to the distributor degrees, or if he’s talking
crankshaft degrees. All of the listings shown in the following chart, and in any shop manual & technical spec sheet,
will refer to distributor degrees of vacuum advance. You must DOUBLE this number to obtain crankshaft degrees
(which is what you “see” with your timing light). Thus, a vacuum advance control unit with 8 degrees of maximum
advance produces 16 degrees of ignition advance in relationship to the crankshaft. When selecting a unit for max
advance spec, the total centrifugal timing at cruise must be considered. Thus, a car set up to produce 36 degrees of
total mechanical advance at 2500 rpm needs a vacuum advance control unit producing 16 degrees of crankshaft
advance. This would be an 8-degree vacuum advance control unit.

Octania

Huh, I knew there is no way I got it all correct the first time.

Is your VA unit stamped not at all, aftermarket "B6" or similar, or "13" for Dist-degrees or "26" for Crank-degrees?

So, yes, because of that 2:1 speed ratio between crank and distributor, there is also a 2:1 position ratio, with 360 degrees of crank motion producing a mere 180 degrees of distributor motion. With that said, I can buy that your observed 26 degrees is actually a nominal or chart value of 13 degrees, but two things bother me about the text just above.

"Thus, a vacuum advance control unit with 8 degrees of maximum advance produces 16 degrees of ignition advance in relationship to the crankshaft. "
==============================
So, a VA canister asserting "8" D-degrees provides you with "16" Crank-degrees. What disturbs me about this is that the GM units I have seen are stamped on the order of 16 to 24 ??-degrees for the pre-HEI era. So, is it up to 48 crank-degrees? That seems like a lot.


"Thus, a car set up to produce 36 degrees of total mechanical advance at 2500 rpm needs a vacuum advance control unit producing 16 degrees of crankshaft advance. This would be an 8-degree vacuum advance control unit."
===================
We must be missing the static timing number and MA portion.
Total 36
VA 16
That leaves 20 crank-degrees to split between static and MA. Back in 1968, a static of 10-12 was fairly common for a High Compression Olds engine, leaving only 8-10 for MA?

Joe P, what say you?