Long explanation:
Take a crank in an engine block.. Draw a line from the center of a rod journal to the center of the main journal. Draw another line through the center of the cylinder from the head gasket surface to the oil pan.
When the line in the crank is at 90 degrees to the line in the block, the maximum pressure in the combustion chamber is converted to motion at the crank. There's a minimum waste of energy. The silly little piston, rod, and crank drawing shows the point I mean.
Force
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When the line in the crank is parallel to the line in the block, there's a minimum conversion of pressure to motion. Energy waste is at a maximum.
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It turns out that (generally) if maximum cylinder pressure occurs when the rod throw of the crank is around 12 - 15 deg ATDC, you convert the most pressure into motion and waste the least energy. So to get the cylinder pressure to peak with the crank at 12-15 deg ATDC, you just measure the burn time of your fuel, figure out how many degrees the crankshaft spins in that amount of time, and start the fuel burning by firing the plug that many degrees before you need max pressure. This amount of crankshaft degrees, from the point where the ignition begins to the point of maximum cylinder pressure, is what your spark advance is.
But who can actually measure fuel burn time? And how to keep checking it while the car is running? Not too many of us will have the tools for that. What we have is
1) Seat O Pants acceleration meter. Not to accurate, tends to be affected by alcohol and bad attitudes.
2) 1/4 mile times. Works ok if you're a consistent driver, but who can run on a dragstrip whenever they feel like it?
3) Dynomometer. Works good, costs plenty.
4) G-tech type meter. Works surprisingly well and not so spendy.
We can also measure the following:
1) Cylinder knock. Knock is a definite waste of energy.
2) Exhaust gas temperatures. In general, the hotter the exhaust gas, the less efficient the conversion from pressure to motion.
3) Oil. temperatures. Oil cools the bearings. Wasted energy is given off as heat and sound, so the oil will heat more if more energy is wasted.
4) Sound of the exhaust. Louder exhaust can indicate wasted energy.
5) Fuel usage. Excess fuel usage is definitely a sign of wasted energy.
6) Throttle angle. Less throttle angle at a given speed indicates less air needed to get the job done, which can indicate a more efficient energy conversion.
Of the two lists, the items from the second are more consistent, are always with the car, and are relatively cheap to monitor. When all the items from the second list are "right", the measured results in the first list are usually near peak.
So, we have a definition of "what is spark advance" and a rough list of what it affects. Now, how can we get the right amount?
Since the idea is to convert energy efficiently and with no waste, we look for the signs of waste. Easy. In a perfect world, the exhaust would exit the engine at the same temperature that fresh air enters the engine. In this case, all the energy created during combustion would be converted into mechanical motion. In reality the internal combustion engine is a really inefficient pig. So we've got to watch for waste above and beyond what's commonly expected.
What are the signs of waste if the spark advance is right?
Heat. Engine coolant temps stay at or near the thermostat temp, especially when driving. Exhaust temps are relatively low, 700 - 800 deg F for many new cars. Oil temps around 200 deg F for conventional oils are nice. Fuel trims are around 128 or 0%, and knock is zero.
If the timing is advanced?
With the timing advanced, the peak pressure occurs when the rod throw is closer to vertical. More of the peak pressure is directed upward into the block and downward through the rods into the mains. The oiling system has to absorb the wasted downward directed force, and the cooling system, block, and heads have to absorb the upward directed force. Pinging may be noticed first. Oil temps will increase. The more the timing is advanced, the faster the oil temps rise. Coolant temps will increase next. Detonation may occur as chamber temps increase. EGT will increase since any wasted heat that can't get out through the block or rods will escape with the exhaust. Throttle angle needed to get the job done generally decreases as timing is advanced. An O2 sensor may indicate a richer mixture. This is due to less free oxygen in the exhaust gas caused by increased heat and more time for the reaction to occur before a sample of the gas gets to the O2 sensor. Exhaust sound may be "snappier" with more advance.
When the timing is retarded, peak pressure occurs when the crank throw is closer to 90 deg from the cylinder bore axis. Combustion pressure can be converted to motion much easier, but there's less time to react all the fuel / air mix before the crank begins to spin past BDC and the exhaust valve opens. Waste shows up a little differently.
Oil temps are down. There's nowhere near as much wasted energy transferred through the block and crank. Coolant temps are usually down, also. Throttle angle required to get the job done is greater, but knock usually will not occur. An O2 sensor may show a too lean mixture due to more free oxygen in the exhaust. Exhaust temps are lower, often significantly so (below 600 deg F sometimes), and the amount of time for the combustion reaction to occur is lower. Noise and vibration tend to be down, also. Fuel mileage will also be down, and acceleration may be noticeably reduced.
Finally, there is no correct rule such as "more is better" for timing. Fuel blend plays a huuuuge part in what the best timing is for an engine. Increased octane can allow more cylinder pressure and heat without autoignition starting, but it does not indicate the amount of time needed for the fuel to burn. Chamber temps, chamber shape, piston shape, crank speed, rod length, fuel distribution, plug temp all have some effect on when the optimum timing will be. Vehicle size and the average load it sees will dictate the amount of heat the chamber will be regularly subjected to. Most of us hope to find settings which provide increases in power and efficiency without requiring more expensive grades of fuel. Datalog as much as possible and watch for as many signs of wasted energy as you can.
-->Slow
hopefully he's one valuable member that won't get annoyed at this place and leave.
, I've said that for alot of the last couple years! 
