Timing questions....?

Can someone explain "degrees of advance/retard" and "cam timing" to me?  

I've read several articles talking about this stuff but have not found a good explaination yet.  
67 Mustang Coupe
302 .040 / C4 / 8.8

Well ok lets keep it simple. For a 4 stroke engine to complete its full cycle it rotates 2x360 degrees = 720 crank degrees. Now everytime the crankshaft goes around the distributor and the cam goes around a half turn(so two turns of the crank = one turn of the distributor and cam). Ok thats a little background, now as each plug fires it does so at about the start of the 4 stroke cycle when the piston is at top dead center(tdc). If an engine is idling this is fine because the gas burns in the cylinder and it pushes the cylinder down. As the engine speeds ups that spark has to come sooner in the cycle because there is a time lag between the time the spark plug fires(and the gas expands) and when the piston is at tdc. So that is expressed in timing degrees as "advance". The spark plug is fired in advance of the piston hitting top dead center. The faster the engine goes the more "advance" is needed. At times this advance might be as much as 50 crank degrees depending on load and engine speed.
Now cam timing is expressed the same way. It all relates to TDC. A cam opens the valves at a set point in the 4 cycle stroke. If you "advance" the cam you move that valve opening in relation to TDC by a few degrees earlier just like you did the distributor for spark advance. You might advance or retard(retard means later in the stroke cycle)depending on what characteristics your looking for(more top end power or more low end).  

Thanks for the response man, I appreciate it!

:
As the engine speeds ups that spark has to come sooner in the cycle because there is a time lag between the time the spark plug fires(and the gas expands) and when the piston is at tdc. So that is expressed in timing degrees as "advance". The spark plug is fired in advance of the piston hitting top dead center. The faster the engine goes the more "advance" is needed. At times this advance might be as much as 50 crank degrees depending on load and engine speed.



I don't understand: why does the spark need to come sooner if the timing of the valves opening and closing is a fixed relation of the crank & cam?  Why does engine speed matter?  How is the lag your talking about created by the engine speed/load?  Is the flame slowed by the piston still moving toward TDC raising camber pressure?

So advancing the timing is moving the spark further to an earlier position/time relative to TDC and retarding the timing is moving further toward a later position/time relative to TDC

You're talking about the plug firing as early as at 310 degrees out of a 720 cycle?  As in 180 degrees down on the intake stroke, then 130 degrees up on compression stroke when the plug fires?  I still dont understand this part of it but it does seem to me if the timing goes that far in advanced the engine is under one hell of a load or at an extreme RPM to get that far.

I do remember my Dad explaining that Fords normally fire at 3 degrees BTDC.  I read in an article some time back, something about engines degree programs being a set of degrees and for some reason in the picture associated with the article TDC was at 15 degrees from this starting point they mentioned.  I don't understand that part of it.  Was that suppose to be a reference of cam/valve activity to spark timing? Or did I just misunderstand all of it?  
67 Mustang Coupe
302 .040 / C4 / 8.8

Ok good questions tanker. Let me see if I can tackle them. First off if an engine is static and the spark fired at a few degrees before tdc it would kick backwards wouldnt it. But the point is, the engine isnt static its turning forward, by a starter or its already running and has some inertia. So what the advance does is build higher pressures in the cylinder and combustion chamber.
Think of a single piston moving up and down where I could change the advance and retard of the spark. When I begin to turn the engine over it cannot tolerate much advance or it will work against itself in turning over(try starting an engine with 30 degrees advance the starter can hardly turn it over) ,,,so I leave it at tdc. After the engine starts it has inertia and the advance will not cause it to work against itself(or kick back) but will cause higher pressures in the cylinder.  In this 0 degree advance position the cylinder sees pretty low pressures and therefore will not develop much power. As The engine speeds up I begin to advance the spark(coming sooner in the cycle) and the pressures go higher and higher because the charge is begining to burn before the piston gets up to tdc. Now the pressures my piston sees are much higher because of this. The higher the pressure in my cylinder the more horsepower it will make (to a point of course). You have to be able to visualize how pressures in the cylinder will go up because of earlier combustion,,,the earlier the combustion starts the higher the pressures in the cylinder. There is a time lag of course as speed gets higher so you start the combustion earlier and earlier to account for piston speed also (like throwing a ball to a man thats running, you throw in front of him) but your main goal is to make high pressures in the cylinder.

Can you see that. I hope thats a little more clear but writing it is harder then showing it to you.  

:
I don't understand: why does the spark need to come sooner if the timing of the valves opening and closing is a fixed relation of the crank & cam?


It's simpler if you remember that the spark starts a burn and not an explosion in the cylinder.  At higher RPM, the flame front needs to start sooner to reach peak cylinder pressure at the right time for the engine to make maximum power.  
1967 Falcon 4 door 351C-4V
1970 Mustang 351C-2V
http://raceabilene.com/kelly/hotrod
Owner built, owner abused.