Originally posted by Dubz
For firing order, you do not want to group the two firing together at the same time until as late as possible.
This would GREATLY reduce flow, as you have 2 times as much air trying to pass through the same size pipe. If they join up further downstream they will have cooled an amount and therefore will require less volume. also i belive that the flow past the other primary tube creates a vaccum which helps scavange exhaust gasses.
well if you think about it, its a chain reaction 1 and 3 don't fire at the same time, it's 1 then 3 so you have fire#1 working its way down with fire #3 right behind it giving it an extra push.