Manual Spark Advance & Kick-back

[Ian Curtis]

Alrighty,

I did not pay enough attention in combustion in college, and I am hoping someone who did might be able to clue me in.

I have an old truck (see profile picture) with a manual spark advance. For those following along at home, this means you the driver control when the spark plug makes a spark, which ignites the fuel in the cylinder. For maximum power, you actually want the spark to fire before the piston reaches top dead center as it takes a finite amount of time for the mixture in the cylinder to combust. However, if you were to start the car with the spark firing before top dead center (TDC), there is a chance that the engine would spin backwards and break your starter (or your arm if you were hand cranking it). For this reason, you start the car with the spark retarded which means that spark happens after TDC which should guarantee the engine spins the right way.

I was always taught to retard the spark when starting old vehicles, and always have. But...

I recently purchased a copy of the Service Bulletins from the late 20s/early 30s, and in them Ford recommends starting a warm car with the spark advanced. Is there something about a warm block that speeds up combustion, or did Ford just have some extra starters they needed to sell?

[MrForbes]

I don't know...but generally, in the world of more modern hot rods, we have a substantial amount of advance initially. 10 to 20 degrees BTDC is common. Some guys even lock out the advance mechanism, and run around 30 to 34 degrees all the time, but this is mostly on race cars. Of course, with modern electronics, you can have the ignition amplifier do all the advancing, and pretty much all new cars made since the mid 1980s have had computer control of ignition timing.

Usually you don't get kick back issues with less than 20 degrees initial advance.

Do you have any numbers for the advance lever on your truck? ie what is the timing with it set to full retard, and to full advance?

and it seems that you would want to increase the advance when cold, and not when warm....I know a cold engine will tolerate more advance without pinging than a hot engine, all else being equal