timing problem?

[Dan D.]

Re: timing problem?

Steve,

Dwell is the number of distributor degrees that the points are closed, and the coil is charging. That should remain constant over the RPM range. But as engine speed increases the amount of time the coil has to charge decreases, so the spark energy decrease also. Spark voltage rises when the points open, until the plug fires. Then the spark voltage goes to 0.

Dwell range for 8 cylinder is 28-32, centered around 30*.

You should set points first with a feeler gauge. Then start the engine and check it with a dwell meter and adjust if necessary to 30*. Then set the timing.

-Dan-

[Steven B.]

Re: timing problem?

Steve,

Dwell is the number of distributor degrees that the points are closed, and the coil is charging. That should remain constant over the RPM range. But as engine speed increases the amount of time the coil has to charge decreases, so the spark energy decrease also. Spark voltage rises when the points open, until the plug fires. Then the spark voltage goes to 0.

Dwell range for 8 cylinder is 28-32, centered around 30*.

You should set points first with a feeler gauge. Then start the engine and check it with a dwell meter and adjust if necessary to 30*. Then set the timing.

-Dan-

Dan,

I'm not questioning the dwell angle, I was just wondering about the concept that a lower dwell angle helps low rpm operation and higher dwell supports higher rpm. Since the coil is nothing more than a simple transformer, the margins on either side of 30 degrees should be fairly negligible unless you are racing. The primary collapse of the magnetic field and the resulting movement of the magnetic flux through the secondary windings induces a high enough voltage to overcome the resistance of the plug gap and create a spark, which is the goal. The older coils are capable of producing in the neighborhood of 20-25,000 volts if needed, and they are fairly reliable at creating a spark. The coil will only produce the necessary voltage as you said to create a spark and nothing more, and usually way less than 20,000 volts. As soon as the field collapse induces the high voltage in the secondary, the voltage doesn't immediately return to zero. There is actually a bit of back and forth induced voltage between the primary and secondary windings after the initial discharge of the secondary but dwindles and does eventually return to zero. This is a very short lived event however. The only benefit as I understand it to dwell on the high side is the longer saturation time of the primary in the coil as you indicated. It needs to be balanced by allowing enough time for the primary field to collapse completely. Somewhere in the neighborhood of 28-32 degrees gives that balance (not necessarily equal since 32 degrees only leaves 13 degrees for the points to remain open) . Any higher dwell starts to interfere with the coils ability to collapse the primary field completely because of the shorter time the points are open. I just wondered what the rationale for the high rpm-high dwell vs. the low rpm-low dwell operation was. I wouldn't think that it would be a factor.

Steve

[Gerard F.]

Re: timing problem?

The vacuum line runs from the advance into the carburetor. That's where it's been since I've had the car. If the line leaks enough would it have the symptoms I'm seeing, hard to start and hesitation when starting from a standstill?

William,

When you change the vacuum line, put a tee into it, and put a vacuum gage on it. Then you will know where you are at, and whether you are having vacuum leaks. I had a similar problem with my 67 327, hard to start when hot, hesitation from a standstill and stalling on a hill at low rpm. The main culprit I found was a loose vacuum hose at the vacuum advance fitting to the distributor.

When you set the timing look for the highest vacuum on the gage. On my 67/327/300 I run about 20hg vacuum at idle, 8-10 initial timing, 24-25 with the vacuum hose connected.

[William S.]

Re: timing problem?

Jeff,

Thanks. I will try that. But I don't understand why you disconnect the vacuum advance to set the timing if it isn't getting vacuum at idle with the current set up. I didn't think to see what the timing was with the line hooked up compared to what it is disconnected but I'll do that as well. If nothing else I'm learning a lot from this problem.

[Michael W.]

Re: timing problem?

. But I don't understand why you disconnect the vacuum advance to set the timing if it isn't getting vacuum at idle with the current set up.

There is no real need, it's just standard habit from the era when all engine had full time vacuum. For fun, does your timing change after reattaching the vacuum line?

[Dan D.]

Re: timing problem?

Steve,

Did not mean to insult your intelligence. The coil is an autotransformer, and yes, there will be some ringing after the plug fires, but that will not matter. The ringing usually ends before the points close. The only benefit for increasing dwell a couple of additional degrees is you increase the coil charge time.

If I have done my 5th grade math correctly, an engine running at 7000 RPM is distributor speed of 3500 RPMs, or 58.3RPSec. That equates to 17mSec of time for the distributor to turn 58.3 revolutions (the relationship between speed and time is 1/x). So each distributor revolution is 294uSec in duration. Divide by 8 and that is 36.75uSec for 1 complete ignition cycle. So the coil, with 30* dwell, has 24.6uSec to charge the coil at this speed, and, as you say, a couple more degrees of dwell is not all that much more time.

At this speed, the coil will not fully charge in 24.6uSec, and can in fact be marginal in developing enough voltage to fire the plug. That is why electronic ignition systems that produce much higher voltage allows plug gap to be set wider than the .035 of non-electronic ignition systems.

In the early 70s when pollution control was first being mandated by the government, GM shortened the length of the rotor, and didn't tell anybody. This increased the rotor gap and was done to raise the firing voltage. (firing voltage is determined by the sum of the rotor gap and plug gap). Therefore the plug would see a higher voltage before it fired, resulting in more spark energy and that would burn off more pollutants. The negative results noticed from this was Corvette engines would not rev to red line - they would misfire. There was a Restorer article on this a few years ago.



At idle the charge time is long enough to saturate the coil. This is interesting stuff!

-Dan-

[Mark B.]

Re: timing problem?

What were the points at with the feeler gauge?

[William S.]

Re: timing problem?

Jim,

Did you need to change the vacuum advance unit when you switched to manifold vacuum?

[William S.]

Re: timing problem?

Jeff,

Does this modification require changing the vacuum advance unit?

[William S.]

Re: timing problem?

Thanks Jerry. So you set the timing by finding the max vacuum at idle with the vacuum advance disconnected rather than using a timing light. I've read something about that but haven't tried it. I have a lot of things to try when I get back in town.

[Jeffrey S.]

Re: timing problem?

Jeff,

Does this modification require changing the vacuum advance unit?

William,
I can't say whether or not a change is required but I did as Duke suggested and used a can that adheres to his "2 inch rule" which requires a VAC that gives full advance at 2" less vacuum than the engine supplies. In my case I used a B26 can (NAPA #VC1765) at Duke's recommendation. I suppose you need to see what vacuum your car draws at idle and go from there.
Jeff

[Duke W.]

Re: timing problem?

Jeff - I recommended the 12" B26 because your car has an automatic, which pulls less vacuum idling in Drive than a manual idling in neutral. For the latter I recommend the 15" B22, start at 8", 16@15". The OE VAC starts at 6" and max is 19 @ 17", which might be too much. Typical 350/300 idle behavior in neutral with a manual is 18" @ 500 assuming full time vacuum advance and optimized pre-emission idle mixture. Duke

[William S.]

Re: timing problem?

Thanks. I've got an automatic so I will try the B26 you recommended to Jeff ported to the closed off AIR line as Jeff recommended. But first I'll put the new vacuum line on in the present configuration to make sure that's the problem I'm experiencing. Thanks for all the help.

[Jeffrey S.]

Re: timing problem?

Jeff - I recommended the 12" B26 because your car has an automatic, which pulls less vacuum idling in Drive than a manual idling in neutral. For the latter I recommend the 15" B22, start at 8", 16@15". The OE VAC starts at 6" and max is 19 @ 17", which might be too much. Typical 350/300 idle behavior in neutral with a manual is 18" @ 500 assuming full time vacuum advance and optimized pre-emission idle mixture. Duke

Duke,
Thanks for chiming in. You are right and that's why I said William should check his vacuum before choosing a VAC. There is no way to know if he doesn't have a different cam that draws a lower (or higher) vacuum.
Jeff

[Dan D.]

Re: timing problem?

Steve,

Did not mean to insult your intelligence. The coil is an autotransformer, and yes, there will be some ringing after the plug fires, but that will not matter. The ringing usually ends before the points close. The only benefit for increasing dwell a couple of additional degrees is you increase the coil charge time.

If I have done my 5th grade math correctly, an engine running at 7000 RPM is distributor speed of 3500 RPMs, or 58.3RPSec. That equates to 17mSec of time for the distributor to turn 58.3 revolutions (the relationship between speed and time is 1/x). So each distributor revolution is 294uSec in duration. Divide by 8 and that is 36.75uSec for 1 complete ignition cycle. So the coil, with 30* dwell, has 24.6uSec to charge the coil at this speed, and, as you say, a couple more degrees of dwell is not all that much more time.

At this speed, the coil will not fully charge in 24.6uSec, and can in fact be marginal in developing enough voltage to fire the plug. That is why electronic ignition systems that produce much higher voltage allows plug gap to be set wider than the .035 of non-electronic ignition systems.

In the early 70s when pollution control was first being mandated by the government, GM shortened the length of the rotor, and didn't tell anybody. This increased the rotor gap and was done to raise the firing voltage. (firing voltage is determined by the sum of the rotor gap and plug gap). Therefore the plug would see a higher voltage before it fired, resulting in more spark energy and that would burn off more pollutants. The negative results noticed from this was Corvette engines would not rev to red line - they would misfire. There was a Restorer article on this a few years ago.



At idle the charge time is long enough to saturate the coil. This is interesting stuff!

-Dan-

I have to fix this. I did the calculation wrong. It takes 17 mSec for the distributor to turn 1 revolution. Divide by 8 cylinders and each ignition cycle is 2.14mSec long. Dwell being 2/3 of the 45* cycle is 1.4mSec of coil charge time. Still not enough time to fully charge the coil.

Would Duke or someone please confirm that I did the calculation correctly this time? Fake data - not good.

-Dan-