Any time a multiple cylinder engine is build, there is a choice of either having all the cylinders fire at once, or having the cylinders fire at intervals. In a four stoke engine, any given cylinder will need teo revolutions of the crank to complete it's cycle, and this becomes the upper limit for the time the engien has to fire each of it's cylinders, because the crankshaft is shared by all the cylinders.
There are three primary factors regarding how the cylinder firing sequence is assigned.
First: They should fire in a sequence that will load the crank as evenly as possible, to avoid shocking it or loading it from one end and then the other.
Second: If the engine is to use an intake manifold with a common plenum or internal volume, as most do, you'll want the cylinders to draw from it in a way that will allow the air/fuel mix to stay as uniform as possible. You don't want adjacent cylinders to rob fuel from one another.
Third: The engine should fire evenly enough so as to make it from one pulse to the next without needing an extremly heavy flywheel.
As it turn out, if one has each cylinder in a V8 fire in a sequence spaced 90 degrees apart, you'll get an even firing sequence that reduces shocks to the crank to a minimum. The is another factor that has some thing to do with even firing, and that is the angle between the banks of cylinders in a "V" arrangement. For most V8s, the banks are arranged at 90 degrees from one another. Also, the rod journals are arranged 90 degrees apart, so that each 90 degree of crank rotation, two pistons will be at TDC Top Dead Center. One on the compression stroke and one on the exaust stroke.
Most V6 engines use a 60 degree angle, but there are some V6s that are arranged at 90 degrees. The angle between the cylider banks can help or hinder even firing. Sixty-degree V6s are even firing, with the crank journals arranged at 120 degrees. If the cylinders are moved out to 90 degrees, typically ot allow better intake manifolding, then crank phasing becomes a problem.
If the crank journals are positioned 120 degrees apart, not all of the pistons will come toTDC in a 120-degree sequence; the engine will need to be compensated by cam lobe relocation and distributor changes. These engine are called "Odd Fire" and have generally been replaced by "Even Fire" 90-degree V6s that employ a clever solution to the problem. By splitting and staggering the rod journals so that each rod has it's own, they can be positioned so that the engien will fire every 120 degrees of crank rotation. This solution is used on the Buick and Chevy 90-degree V6s.
If you study the two firing orders diagrams above, you'll see how the engineers balance the loads from one end of the crankshaft to the other in Chevy (rear distributor) anf Fords (front distributor) small blocks. Ford numbers their cylinders from the front passenger side straight back 1,2,3,4 and continues on the drivers side 5,6,7,8. Chevy numbers their cylinders, 1,3,5,7 on the drivers side and 2,4,6,8 on the passenger side. Why? Got me!
As soon as they are ready, you will be able to find here specific firing orders for various engines.