Originally Posted by Andyman
There is an optimum ignition timing which changes according to engine speed, load, temperature, fuel type, fuel mixture ratio and probably several other factors. Most engines have the ignition timing set a little more retarded than the most efficient setting...
I agree, lots of influences on what 'perfect' is for any particular engine. Pile on fuel octane and altitude to the list.
A little retarded is safe, but if you listen for ping, have a timing light and aren't afraid to experiment, the payoff is worth the trouble if you are chasing every last mpg to advance timing right to the edge, just not over the edge.
I think spark timing and fuel octane are often times misunderstood. The consensus among several top race engine builders, and what I've learn through experience, says to use the lowest octane fuel you can get away with, and advance the timing 'til it pings (then retard it 2 degrees). A higher octane number means the fuel burns a little slower (87 octane has no more energy in it than 92 octane) There is no magic timing advance across the board, only a series of compromises and 'almost perfect' settings.
I've played with old carburated VW's for a long time, this one in particular I was trying to squeak out every last mpg (circa 1981). I wound up recurving the distributor's advance curve through different springs on the flywieghts and bending the stop ears, to match my reduced accelerator pump shot volume and 'egg under my right foot' acceleration style. Through tedious seat of the pants trial and error I found best FE to be right at the edge of pinging. On hot days, I sometimes had to break out the 10mm socket and retart it a couple degrees.
Remember that initial timing and full advance are NOT the same thing. Your engine burns most of it's fuel at full advance. This is key, since not all distributors act the same, not all timing advance computers are the same, your metro might have slightly different initial timing advance needs than mine. I'm a little embarrassed to say, I have no idea what my (or your) Metro uses for an advance curve mechanism (mechanical or computer). I'm a Metro newbe, but I'll find out. Mine pings a little under heavy load when the mercury goes north of 95 deg F, I leave it where it's at since we get 50 days/year over 95F. Either way, a simple test will give full advance and initial advance numbers.
From my race experience, maybe by no coincidence, top torque readings on a normally aspirated race engine is usually a degree or two before it starts to ping (hard to hear with open headers in the dyno room, but you see it in the data, the observed torque curve falls off like a stone). My guess is top FE is at or about the same timing.