spark and MAP

[Edward Hernandez R]

"...keep in mind that a flame in the combustion chamber takes about 3 
ms to burn. Hence, timing has to be advanced with increased rpm..."

That is mostly correct, except that above a certain rpm, the flame 
speed(or burn rate) actually rises with rpm. It turns out that if your
 measure flame speed in crank degrees instead ms, you'll discover that
 it takes the same number of crank degrees to burn at 3000rpm as it 
does at 6000rpm. That is why distributor advance mechanisms and most 
spark maps "top out" and have nearly identical spark advance at the 
two rpms:

 |
s|
p|
a|
r|      **********************************
k|     *
 |    *
a|   *
d|  *
v| *
 |*
____________________________________________
                   rpm

The point at which spark advance tops out varies from engine to
 engine.