OK I read and re-read this post a couple of times and have a few thoughts about pairing cylinders.
First, let's do some determination of where each cylinder is in the 4-stroke cycle in relation to the other cylinders.
If the firing order is 1-3-4-2, we can assume the following:
1 3 4 2
P _ _ _
_ P _ _
_ _ P _
_ _ _ P
And the blank spaces are representing the remaining cycles of the 4-stroke cycle.
So, with the 4-strokes being Intake Compression Power Exhaust in this order we can fill in the blanks as follows:
1 3 4 2
P E I C
C P E I
I C P E
E I C P
We read bottom to top for each individual cylinder's 4 stroke cycle.
Everyone follow? OK sorry for the Auto Shop 101 but I want to make this as clear as possible.
OK so we ask ourselves what are the best cylinders to pair up? We have seen the 1-3 and 2-4 pairings as well as the 1-4 and 2-3 pairings. So what is happening to the pairs mate as each cylinder is on the exhaust stroke?
Well lets look at the chart-in a 1-3 and 2-4 pairing we see:
when 1 is on the Exhaust the 3 is on the Intake.
when 3 is on the Exhaust the 1 is on the Power.
when 4 is on the Exhaust the 2 is on the Intake.
when 2 is on the exhaust the 4 is on the Power.
Conversely, in the 1-4 and 2-3 pairings, we find the following:
when 1 is on the Exhaust the 4 is on the Compression.
when 3 is on the Exhaust the 2 is on the Compression.
when 4 is on the Exhaust the 1 is on the Compression.
when 2 is on the exhaust the 3 is on the Compression.
Very interesting. Notice that they all are the same (180* apart) as opposed to alternating.
What I am attempting to show here is the natural pairings of 1-4 and 2-3 due to having each partner 180* apart on each revolution of the crankshaft. As each exhaust cycle starts, the partners cycle, the Compression, has had the exhaust valve closed for the cycle previous (Intake) and the cycle following (Power) thus eliminating any chance for reversion (the crossing over of exhaust gases being drawn in to the fresh intake charge) or having both exhaust valves open at the same time. This can happen as, in the case of 1-3 and 2-4 partnerships, as one exhaust valve starts to close the partners starts to open at the end of the power cycle.
What does this all mean: well, from the S&S Header website-quote:"The optimum situation is the four cylinder because of it's firing cycle. Every 180 degree of crankshaft rotation there is one exhaust pulse entering the collector. This is ideal timing because, as one pulse exits the collector, the next exhaust valve is opening and the vacuum created in the system pulls the exhaust from the cylinder. In this ideal 180 degree cycling the collector outlet diameter only needs to be 20% larger than the primary tube diameter. "
So, having the exhaust cycles firing 180 degrees apart would be the ideal situation.
And that, my children, is why I feel say the perfect exhaust pairing for the Ford 1-3-4-2 firing order is 1-4 and 2-3 matched as pairs in a 4-2-1 header.