Timing Terminology

[Nixon1]

A quick question... I hear the phrases base timing, initial timing, and several others I believe..what exactly is the difference? Is there some math going on here? My engine, without the computer connector in, was running 18 degrees of timing previously, which is WAY too much for a motor with stock internals..think the only reason it didnt do too much, DIDNT EVEN PING!, is because I was running really rich with a bad Coolant Temp Sensor and 4 shorted injectors dumping fuel..but the point is, is that 18 degrees considered to be base timing?

I'd just like to get some of this stuff nailed down so I'll know how to convert between the different scales here...

[sn95gt19]

base timing is when the spout connector is out

total timing is when your at full advance

[Nixon1]

Isn't there another kind? Just that I've heard before, I think, of cars running somewhere in the 30-40 degree range..but I assume that's race-grade cars running and that number is totaled...

[84LX89GT]

Base/Initial timing is what you set your vehicle at without any kind of vacuum, mechanical (both for carbureted vehicles) or electronic controlled advance. Your engine will adjust timing from that point, or in some race cars the base timing is advanced to full advance because you don't need an advance curve in a certain rpm range.
Total timing is the base(initial timing), plus any kind of ignition advance provided by a computer, a vacuum diaphragm or by advance springs. Total timing can be the same as initial timing if there is no additional timing advance added. alot of vehicles run from 28* to 40* before top dead center total advance because since the engine may be at a higher rpm, it still takes a certain amount of time for the air/fuel to burn. Combustion chamber design, engine rpm, compression ratio as well as camshaft design has alot to do with how much timing you need at the top end of the power band.

[Nixon1]

Thanks for clearing that up..I was wondering about all the terminology. So lemme get this straight now..is all timing advancement occuring BEHIND top dead center then, and timing retarding beyond 0 degrees (which no one would ever do anyways) occurs AFTER top dead center?

[84LX89GT]

Yes, you want to fire the air/fuel before top dead center, so by the time it started to push on the piston/while it's still compressed it's just after top dead center. 10* top dead center is when the piston is slowing down near the top of the stroke, at higher rpm's it just needs to get started earlier so After top dead center the gasses are expanding from the reaction.
After top dead center a car would not run very well (if at all) because the air/fuel would not be compressed (when air compresses it heats up, which eases the burning process) and as a result only a small amount of the air fuel mixture would end up burnt.
Usually when people refer to retarding the timing, they're just talking about getting it closer to top dead center and dropping down the timing advance.
You want to do this when you have pinging (pre-ignition) which is when spark is occuring too soon for the burn (such as running a lower grade/faster burning gas), or sometimes if there is detonation (something in the combustion chamber heats up enough to combust the air fuel too soon). This usually results in rocking the piston because the motion has not been transferred to moving the piston back down the stroke. Also for detonation if the combustion chamber is hot enough it will cause an air/fuel explosion rather than flame travel accross the piston. Flame travel is what pushes the piston down away from the cylinder head. Detonation can be severe enough to melt, or burn holes in pistons.

[Nixon1]

Gotcha... Thanks again..