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Ignition Timing Retard /Advance Theory


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I am hoping to get a few pointers to understand the theory behind vacuum and centrifugal advance of distributor spark timing. 

I hear that 100 year old or so cars, the spark timing was manually retarded to start the engine.  Often by turning a lever on the steering wheel. This was to prevent an engine backfire while the crankshaft was slowly turning. The spark was manually advanced by the operator as soon as the engine was running.  

Vacuum: Does an engine make the most vacuum when the throttle valve is mostly closed, such as 1/4 throttle and less? I’m imaging 6 cylinders trying to suck air through the carb. With throttle valve closed, intake manifold vacuum should be high. Correct? 

The vacuum advance pot has a spring in it. It works opposite direction on the vacuum pot action? When the engine is off, the mechanical spring retards the timing. Holds it there. So timing is retarded when the engine first fires up. Like the 100 year old cars mentioned above.  As soon as the engine fires up, vacuum overcomes the spring. The timing is advanced from the earlier retarded position. Correct? As the throttle valve is opened, the engine speeds up. Does manifold vacuum drop? Rendering the vacuum advance less effective? The spring might start to pull the vacuum to the retard direction, however now as engine RPM increases, centrifugal advance takes over. The spark timing is a advanced to account for combustion travel rates within the cylinders, at higher piston travel speeds. 

I think I grasp all that pretty good.

Where I struggle is when the engine is lugging. Say, climbing a steep hill. Engine RPM drops. Centrifugal advance loses speed. Throttle valve is now wide open or close to it. Driver chooses to stay in top gear, lugging the engine, using what torque the engine makes, to climb the hill.

What is the intake manifold vacuum reading in this situation? I assume at least initially, less vacuum, as the throttle is wide open. With engine rpm down, is centrifugal advance doing much? Where should spark timing be in this scenario? Is the mechanical spring inside the vacuum advance pot now overcoming any vacuum? Retarding the spark timing again?


Perhaps a lugging engine at WOT builds up good strong vacuum? Advancing the timing as it lopes up the hill? I’m unsure. 


I sometimes lay in bed as I fall asleep, wondering. So tonight I grabbed the iphone from my night table to ask.


Thanks for any responses. I’ll read

them in morning. 


Edited by keithb7 (see edit history)
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Most 100 year old cars have no vacuum or centrifugal timing advance. Many magneto’s pre WWI also have no way to advance except by driver setting. That said, engine design and fuel of the time made adjustment necessary only at start up. Most cars with decent displacement were basically start and run positions on the steering quadrant. Smaller cars tended to get adjusted more to try and get every bit of power possible.......until accelerator pumps were installed on carburetors, timing didn’t need to be very responsive. Vacuum advance first started to become common in 1937-1938. Then there were the exotic set ups that used oil pressure to advance timing........but it was not common. 

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Lugging the engine under heavy load load with WOT will give you almost zero manifold vacuum, as there is minimal pressure differential between atmospheric and manifold vacuum due to the WOT. With the engine under heavy load, spark advance is not required or desired  due to detonation /  engine damage potential as indicated by pinging or ignition knock. 

Once the engine load is lessened and the throttle closed somewhat, then the vacuum advance can kick in again as the potential threat of Detonation is reduced.  To much total advance  is way more detrimental than not enough, not so much on older vehicles with lower compression ratios but definitely on later models with CR's above the 8.5 :1 ratio....

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Another factor to factor into your factoring is the source of the vacuum.  The distributors' vacuum source was, originally, the intake manifold and is correct (in a fashion) with your scenario until the government got involved to move the vacuum source from the intake manifold to above the throttleblades in the carburetor to create what is called 'Ported Vacuum'.  With ported vacuum at idle you don't have any vacuum and timing is fully retarded to burn the exhaust hotter & lower the CO2.   I believe the mandated change was in the mid to late 60's.  

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To get a grasp on this, centrifugal advance and vacuum advance need to be thought of separately. Also, any Ford or Holley Loadomatic (venturi vacuum) systems as used on some specific Fords in the 40s-50s-60s need to be left completely out of the discussion, and have their own separate discussion. Otherwise the water gets too muddy.


To get the most power out of an engine, you must light the fire while the piston is still on it's way up. This is because the fire takes a little while to get going, and you want the maximum push to take place just past top dead center. Light it too early however and the engine may try to kick back on itself, or slam the rod straight down right at TDC.


The centrifugal advance should be arranged to meet the engine's needs under full power and full throttle. This is the least advance the engine will ever want at any given RPM. Typically an engine needs the advance to rise quickly with engine RPM at lower engine speeds and then advance more slowly at higher engine speeds. The breakover from the fast part of the curve to the slow one often occurs somewhere around 2500 rpm. This is called a "compound curve". An example is shown below:




In this example there are 2 curves shown, indicating tolerance for manufacturing. The actual curve should fall between the two curves shown. Also note the change from the steep/fast part to the slower part occurs at 1000 crankshaft RPM, much sooner than the 2500-ish RPM of a typical car. A common way to achieve this is with 2 springs. One lightweight spring acts on the advance weights first, then a second stronger spring with a big open loop becomes tight (at 1000 RPM in the example above) and both springs act until the mechanism hits the end of it's travel, 6000 RPM in that particular graph. That is higher than most cars.




The breakover point, the strength of the springs that set the slopes on the graph, the total travel of the advance mechanism. etc. are all chosen to make the engine make all the power it can under full load and wide open throttle. Any less advance at a given RPM would make the engine weaker. Any more would create a situation where you could give the engine more throttle and cause the engine to need less advance and try to kick back on itself, reducing power, making pinging noises, and possibly causing permanent damage.


In older cars, sometimes due to engines that turn slow, or flathead engines that typically want less advance than OHV engines, etc. the second half of the curve can become completely unnecessary and is often omitted.


The second half of the curve is also sometimes omitted in hotrodded engines, but that is for different reasons and is beyond the scope of this post. This post mainly applies to street driven engines that will idle smoothly.


I'll tackle vacuum advance in my next post or in an edit of this one



Edited by Bloo (see edit history)
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Bloo has the answer. You can think of it like shooting ducks. The faster the duck is moving the more you have to lead it when you fire your shotgun.

The same applies to lighting a fire in the cylinder. The mixture may seem to explode instantly but in fact it takes time for the flame front to travel from the spark plug ignite all the mixture. It only takes a split second but, when an engine is running 4000 RPM it is firing 33 times a second!

So, when the engine is barely moving as when being turned by the starter you want little or no advance, as the engine speeds up you need more advance for the mixture to burn completely before the piston goes too far down.


Vacuum advance is a separate issue and has more to do with economy and avoiding spark knock.

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Vacuum advance is used because at part throttle the mixture in the cylinders is less dense and burns slower. Therefore, with mechanical advance only, the pressure from the fire occurs late after the piston is already going down. You can get away with only mechanical advance, and many cars have (like some versions of the rear-engine Volkswagen Beetle), but it is not ideal. Since the fire occurs late, some of the energy is wasted. It reduces fuel economy and part-throttle throttle response. It also puts more heat into the exhaust than necessary, because quite a bit of the fuel is still burning when the exhaust valve opens.


Intake manifold vacuum is fairly high at part throttle and at idle. It is highest coming down a hill on compression with the throttle closed. Conversely, manifold vacuum is zero or extremely low at full throttle because the manifold is open to the atmosphere.


You can use this vacuum to advance the timing at part throttle to a more ideal setting, and that is what is generally done. The amount of vaccuum advance needed at different speeds and loads could vary, but generally it is something on the order of 15 or 20 crankshaft degrees on a more modern OHV engine. There will be some limit to how much you can have before too much advance at some particular RPM becomes a problem. Usually it is right around the breakover point of the centrifugal advance. This one spot limits what you can have everywhere else, but usually it doesn't limit you very much.


The vacuum advance will be shown as a straight-ish curve, as shown in the graphic below for an adjustable vacuum advance can. In reality, that curve is pretty steep, and vacuum advance is almost more like a switch, on or off. If you want to see why, drive your car with a vacuum gauge attached and watch how sharply vacuum drops when you go from part throttle cruise to acceleration.




When you step on the throttle hard, the vacuum advance goes away, and sends you back to the only the centrifugal curve, and that is what you want for acceleration.


Vacuum advance can be triggered by either manifold vacuum or ported vacuum. Manifold vacuum is connected directly to the manifold and is on all the time. Ported vacuum comes from a port right by the throttle plate. The port is above the throttle plate at idle and there is no vacuum. As soon as you crack the throttle, the port is below the throttle plate, and there is manifold vacuum.


The only difference, is that with manifold vacuum the vacuum advance is "on" at idle and with ported vacuum the vacuum advance is off at idle. Example: if you have a 20 crankshaft degree vacuum advance, and the static timing is set at 5 degrees, with manifold vacuum the engine will be idling at 25 degrees, and with ported vacuum it will be idling at 5 degrees. Everything else is the same.


Ported vacuum is typically used on street driven engines that idle smoothly, as massive amounts of extra advance at idle just wont work on most of them, and would cause you to have to compromise the advance curve elsewhere.


To avoid any confusion, I will repeat that any Ford or Holley Loadomatic (venturi vacuum) systems as used on some specific Fords in the 40s-50s-60s need to be left completely out of the discussion, and have their own separate discussion. Otherwise the water gets too muddy.



Edited by Bloo (see edit history)
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