[DrVett]

I have read a lot about timing and I have a decent understanding of initial timing. The purpose is to have the spark plug fire BTDC at the proper time for proper combustion of the air fuel mixture. Stock engines I have read can be 6-8-10 degrees BTDC. Later model engines I read like 16 degrees or so (mine is at 15 degrees). Too late and fuel is not completely burned which can result in poor performance, overheating, etc. Then there is mechanical advance which is rpm driven with weights and springs. The two together = total timing.
The desired range of total timing is typically 32-36 degrees.
Initial timing: from what I understand one way to determine the proper amount for your engine is to slowly keep advancing until you hear pinging (pre-detonation) under load and the back it off a little until the pinging goes away. I can do that. As for total timing, I don't hear specifics about how to determine what is proper for each engine. What is read is: "my engine likes 34 degrees total timing". My problem is my engine doesn't talk to me and tell me if it is happy. So how do I know what is the best total timing for my engine?
I would love some education.

 

[zray]

Your engine does indeed talk to you. Learning its language is not difficult.

If you can hear pinging when going up a moderate incline with moderate acceleration, that’s the engine shouting at you to retard the timing.

If your acceleration is labored, then that’s another plea for help, usually indicated timing is too advanced too soon (rpm).

Stock timing curves are usually too conservative. Advancing the timing sooner helps acceleration, up to a point.

Total timing is a matter of experimentation. For example, the stock K code distributor calls for 40 degrees total timing. Most SBF engines have best performance with less.

You must experiment with different combinations until your car accelerates smoothly to your target maximum RPM. Smoothly is the key.

One size does not fit all

Z

 

[Gopher428]

Adding on to @zray, the advance curve is also important. So you have initial, total, and the advance curve for when the mechanical advance comes in.

You will need to tune each through trial and feedback as noted above.

Set initial timing first. Then work on total. Then you will know how much advance you need and can work on the curve for how fast it comes in. As you note, you adjust this through different springs typically (depending on your distributor), with lighter springs advancing faster.

You can use a vacuum gauge, connected to manifold vacuum, to guide settingl initial (aiming for maximum vacuum).

You are better off erring on the side of too little rather than too much timing.

 

[poqman]

I do a top down approach. When you don't know what the distributor curve is or which springs are in the distributor, I plug the vacuum advance and rev the engine to 2500-3000 rpm looking for what rpm the total timing is all-in. Let's say all-in is at 2800 rpm. Hold the rpm at 2800 total and set the distributor to 34 degrees. Now drop back to idle and read your initial timing. If the initial timing is around 10 after dropping back to idle, then you know you have 34-10= 24 degrees mechanical advance and 10 degrees initial. Check your manifold vacuum. Now advance it to 12 degrees putting your total at 36 degrees. How's your vacuum? Hook the vacuum advance back up and go for a test drive. Let the engine tell you if it's happy.
If the initial timing wasn't between 10-14 degrees when you dropped back to idle, then the distributor probably needs to be sent to Dan for a recurve.

 

[Woodchuck]

Ignition timing is dependent on multiple factors.... piston position in terms of degrees of crankshaft rotation, engine speed, cylinder pressure, air/fuel mixture, etc., any most of those factors are variable across the operating range of the engine.

Under no or light loads, which typically involve leaner fuel mixtures and lower cylinder pressures, timing needs to be advanced in order for the fuel burn to be complete before the exhaust valve opens to exit the spent mixture from the cylinder. Under load, when fuel mixtures are enriched and cylinder pressures increase, fuel burn is faster and timing retarded to prevent premature spark ignition and/or detonation but as engine speed increases the timing must also then increase as there is less TIME for the fuel/air mixture to be fully combusted.

Most static ignition timing is done with the distributor vacuum advance disconnected and plugged and the setting will depend on whether the vacuum advance signal is from manifold vacuum or "ported" vacuum, also known as "spark port" or "timed vacuum". A full manifold vacuum signal for vacuum advance will add around 20-24* of spark advance to the initial advance at idle so it's not out of the ordinary to see 30^ at the balancer with everything hooked up properly. However, as soon as the throttle is opened, manifold vacuum will decrease proportionally with throttle opening and load so that 30* may immediately drop back to 10* at a heavy press on the accelerator. Both the mechanical (centrifugal) and vacuum advance curves are tuned, accordingly, to recogize that there will be additional vacuum advance at closed throttle to optimize throttle response.

When the vacuum advance signal comes from the carburetor's "spark port", the throttle plate covers the orifice that feeds it and, thus, there is NO vacuum advance signal at closed throttle. Typically, initial timing is a bit more advanced and additional spark advance is added both as engine speed increases, via the mechanical advance, and via vacuum, from the "spark port", depending on the vacuum signal, varying based on throttle position and load. The advance "curves" reflect THAT method of vacuum advance signal, as well.

Added to the timing requirements will be other factors such as cylinder head temperature, inlet air temperature, barometric pressure, humidity and other things such as "quench distance", piston and combustion chamber shape, port flow characteristics, etc.

So, optimal timing will be unique for a particular non-production engine configuration and the establishment of proper spark advance curves should be carefully considered then one can figure out how to best implement them.

 

[cougar70]

Timing advance is mechanically built into the distributor with weights, springs, a vacuum-operated lever, and static timing. The simple design of a stock distributor requires many compromises, and there are "hacks" to get more timing sooner. Make sure you have the correct stock distributor and that it works correctly.

Later model engines use EFI with electronic timing control. My 351C with timing control idles great with 21 degrees at idle. It starts instantly with 15 degrees cranking timing. Timing control wakes up an old engine in ways that are not possible with a mechanical distributor. You need to find the best idle as well as acceleration and cruising within the limitations of a stock distributor.

There are mechanical distributors with far more adjustability than stock distributors available.

 

[DrVett]

Timing advance is mechanically built into the distributor with weights, springs, a vacuum-operated lever, and static timing. The simple design of a stock distributor requires many compromises, and there are "hacks" to get more timing sooner. Make sure you have the correct stock distributor and that it works correctly.

Later model engines use EFI with electronic timing control. My 351C with timing control idles great with 21 degrees at idle. It starts instantly with 15 degrees cranking timing. Timing control wakes up an old engine in ways that are not possible with a mechanical distributor. You need to find the best idle as well as acceleration and cruising within the limitations of a stock distributor.

There are mechanical distributors with far more adjustability than stock distributors available.

View attachment 984935


View attachment 984936

I have an MSD distributor.

 

[DrVett]

Spent the morning reading about and watching videos on timing. I think I understand it a bit more.
I switched to manifold vacuum last October. I find that it idles faster and I pretty much had to back off the idle screw totally with a resulting idle in park of 950 and in gear of 630. The in gear idle is often not smooth and sometimes drops below 600 and the engine wants to die.
My base timing is 15 and when the manifold vacuum is hooked up the idle timing is 30. I’m pretty sure that my mechanical advance timing is 21 with a total timing of around 36. I have an MSD distributor with light blue and silver springs. The mechanical advance based on the MSD graph starts around 1300 rpm and peaks at 2300 rpm.
Woodchuck stated that when the car is switched from ported vacuum to manifold vacuum the distributor may need to be recurved. If I use the lightest springs the mechanical advance kicks in around 800 rpm and reaches max advance at 1800 rpm. I don’t see that being good. An article in Hotrod magazine said in general automatic transmission cars benefit from reaching max advance sooner as it reasonably possible.
Recurving would be increasing the spring thickness so the mechanical advance kicks in the same rpm or later and peak manifold advance takes longer to peak. Is there a science to determining if stiffer springs are needed or change and drive?
If I increase the initial timing a few degrees then when the vacuum line is reconnected the idle timing will increase to above 30. Is this a problem? I know the effect of the manifold vacuum kicks off once the car is driving. Is increasing the timing by 2-3 degrees helpful to the in gear idle rpm?
Is it reasonable to set the initial timing via vacuum then check the timing with the timing light to see where it is?

Thank you.

 

[jlangholzj]

I switched to manifold vacuum last October. I find that it idles faster and I pretty much had to back off the idle screw totally with a resulting idle in park of 950 and in gear of 630. The in gear idle is often not smooth and sometimes drops below 600 and the engine wants to die.

It's been a while since I've looked at anything for an automatic (so there may be some additional considerations here) but the idle screw on the carb should be used primarily to set your transfer slot exposure (and maybe a little wiggle either way to barely fine-tune things). Adjust your timing to adjust the idle RPM.

My base timing is 15 and when the manifold vacuum is hooked up the idle timing is 30. I’m pretty sure that my mechanical advance timing is 21 with a total timing of around 36. I have an MSD distributor with light blue and silver springs.

I'm kind of surprised your idle timing is 30*? Just as thinking out-loud here (which is dangerous) doesn't more advance typically equate to a more-rich mixture due to the relative "density" that the flame front has to get through? Reminder everything is related (timing, mixture, RPM, etc.). Last year when i had mine re-curved i was very surprised at how much of a difference a relatively "small" change made. My point being that it sounds like: a) you've got the curb idle turned way back and 2) feeding it a bunch of timing to stay running. Something doesn't quite add up?

I will also say I seem to remember that just "switching" over to manifold vac for the advance source without changing how the distributor is set up isn't the correct way to do things. I had always worked with ported vac so I'm not much use there; but there are lots of good threads around here that discuss ported vs manifold at length that may help. One thing i seem to remember is that because manifold vac changes with RPM, if the distributor isn't set up appropriately, it's effectively taking timing out when you put the car into gear (because your vac changes with RPM + load) which means in an un-ideal world you might have like ~25* in park but it drops to 15* in gear.