Base Timing Questions

The best way it was "layman termed" to me by our high school auto shop teacher was like this. With retarded timing, some of "the burn" is lost through the exhaust. This is why an engine runs hotter with retarded timing. The burn is still happening in the exhaust manifolds or headers.

Another effect is the engine does not produce all of the down force on the pistons since with retarded timing, the spark and beginning of the power stroke happens later, when the piston is either closer to TDC, at TDC or even after TDC. So, just the same with reduced down force, you have reduced vacuum signal.

Conversely, with advanced timing, the burn is more contained inside the combustion chamber. Exhaust temperatures and engine temperatures are lower, since most of the burn happens inside the engine and most all that's left is spent exhaust gasses.

Also conversely to retarded timing, there is more down force on the pistons, since the spark is now happening well before TDC. There is more time for complete combustion, since the spark is introduced well before TDC. This in turn creates more of a vacuum signal, which is why engine vacuum increases when more initial timing is pulled in.

Retarded timing is why smog era engines tended to run hotter than their earlier cousins. Engines needed more heat in the exhaust to heat the catalytic converters up to operating temperatures, which was generally "around" 650 degrees. As time went on, advances were made that didn't require converters to be so hot. This is why Thermoquads got a bad rap through the smog years. They would get so hot, the plastic main body would warp and leak. It's also why they work well for performance vehicles. Generally, hot rods don't have converters and run at much cooler temperatures.

Hope all that made sense.