Your engine’s timing specification may not be valid.June 26, 2018
Intake, compression, power and exhaust are the names of the four-strokes of an engine that we all learned. Invented by Nicholas Otto in 1876, the proper term for these events is the Otto Cycle.
The correct name for the third stroke is not power but expansion. The flame in a gasoline engine when ignited by the spark plug expands across the cylinder bore and exerts force against the piston.
With both a magneto and a distributor, the spark plug fires as the breaker points separate or with an ignition module, when it shut off.
The position of the housing in relation to the rotor and the distributor cam determines when the firing, measured against the crankshaft’s rotation from the piston being at TDC (top dead center), occurs.
If the plug is fired before the piston reaches TDC, the event is considered to be advanced (BTDC). If ignition occurs when the piston is heading downward, it is retarded (ATDC).
The ideal is to have ignition create peak cylinder pressure in as few degrees of rotation past TDC as possible. The most power from the fuel consumed will be achieved when the combustion pressure peaks just as the piston begins to travel toward BDC.
It is necessary to give the flame a head start since it travels slower than the piston. The scale used to measure flame speed is meters/second and for the piston, feet per minute.
It must be recognized that the piston speed is a function of engine rpm and since it is mechanically linked, does not change; it is not influenced by external factors.
The same can not be said about the flame speed though; almost everything has influence over it.
When an engine is designed the ignition setting and advance curve is based on the amount of cylinder fill for the operating state, the compression ratio, the level of turbulence in the bore, the operating temperature, and the natural burn speed of the fuel.
Internal turbulence caused by the filling of the bore with the air/fuel mixture and then by the piston compressing it, is the most influential on accelerating the natural burn speed of the gasoline.
As an engine ages, the valves do not seal properly along with the piston rings and cylinder wall, carbon deposits limit cylinder fill, and if equipped with a timing chain, stretch alters the camshaft timing from the desired location. The engine runs fine but it is not what it used to be.
Modern gasoline also has a different burn speed than the fuel produced when the engine was new. The speed of the flame’s expansion may be faster or slower. The ignition timing needs to reflect this.
The issue is two fold: an engine due to age has a unique expansion event; fuel with different burn speed characteristics is being used.
A simple answer to a complex equation
It is impossible for anyone to calculate the internal engine events and match them to an unknown fuel burn rate. The good thing is the engine will tell you what it wants by the way it starts, idles, loads and generally how it runs.
The approach is to start at the factory timing specification as a baseline and pay attention to how the engine performs. Then advance the timing two degrees and see how it likes it. Then go back to two degrees from the factory setting. You now have a four degree swing. Depending on the results, add timing in two degree intervals in the direction it liked until you reach the point where the performance degrades. Back the setting up one to two degrees and you found the sweet spot!