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four into one (conventional) headers

 0   90     180         270     180  0          720
 1---3---7---2---6---5---4---8---1---3---7
         0       180 90     180          270    720


1-3-7-2-6-5-4-8 1-3-7-2-6-5-4-8
R R   R     R   R R   R     R
    L   L L   L     L   L L   L

firing spacing per bank - both are 90-180-270-180

#3 on right right, #5 on the left, should extend well into the collector to
reduce the effect of overlap from the 1-3 and 6-5 firings

could also enter the collector further down, a'la FlowTech


1-3-7-2-6-5-4-8 1-3-7-2-6-5-4-8
R     R     R   R     R     R
  R               R
    L   L     L     L   L     L
          L               L

180-270-270-180-270-270-180-270-270-180
          odd         odd        odd

running a small (3/4 or 1") balance tube between #3 and #5 might help

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360 degrees:    (two tubes cross from each side)
(this is what NASCAR commentators often call a 180 degree header!)

                                      total     pair
1-3-7-2-6-5-4-8 1-3-7-2-6-5-4-8      spacing  spacing
1       1       1       1            360-360    360
  2       2       2       2          360-360    360
    3       3       3       3        360-360    360
      4       4       4       4      360-360    360
                  
                   1O-----|
|---------O5              |
|                         |
|                  2O--|  |
|    |----O6           |  |
|    |                 |  |
|------------------3O  |  |
|  |------O7           |  |
|  | |--------------------|
|  |---------------4O  |  |
|  |      O8-----------|  |
|  |                   |  |
|  |                   |  |

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180 degrees:   (one tube crosses from each side - #3 and #7)

                                     total    pair
1-3-7-2-6-5-4-8 1-3-7-2-6-5-4-8     spacing  spacing
1   1           1   1               180-540    180
  2   2           2   2             180-540    180
        3   3           3   3       180-540    180
          4    4          4   4     180-540    180

        1   4   2   7
        0   0   0   0

             351W
         0   0   0   0
         6   5   3   8

                1O--|
    |--O5           |
    |               |
    |           2O-----|
 |--|--O6           |  |
 |  |               |  |
 |  |           3O-----|
 |  |  O7-----------|  |
 |  |               |  |
 |--|-----------4O  |  |
 |  |--O8           |  |
 |  |               |  |
 |  |               |  |   groups are 90 deg apart at collector

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Tri-Y - conventional tri-Y headers (Shelby Mustang and Ford 427FE pattern)
(several sources; both tubing and cast iron headers pictured)
(some aftermarket headers had some peculiar pairings!)

427 firing order                   total      pair
1-3-7-2-6-5-4-8 1-3-7-2-6-5-4-8    spacing   spacing
a     a         a     a            270-450    270
  b         b     b         b      270-450    270
    c     c         c     c        270-450    270
        d     d         d     d    270-450    270

        -----   -----
        0   0   0   0
        #1  #2  #3  #4  cylinder #
                                     FE firing order 1-3-7-2-6-5-4-8
         #5  #6  #7  #8 cylinder #
         0   0   0   0
         ---------
             ---------

289 firing order                    total    pair
1-5-4-2-6-3-7-8 1-5-4-2-6-3-7-8    spacing  spacing
a     a         a     a            270-450    270
    b     b         b     b        270-450    270
  c         c     c         c      270-450    270
        d     d         d     d    270-450    270

        -----   -----
        0   0   0   0
        #1  #2  #3  #4  cylinder #
                                     289 firing order 1-5-4-2-6-3-7-8
         #5  #6  #7  #8 cylinder #
         0   0   0   0
         ---------
             ---------


                1O-----|
    |--O5              |
    |                  |
    |           2O-----|
 |--|--O6              |
 |  |                  |
 |  |           3O--|  |
 |  |--O7           |  |
 |  |               |  |
 |  |           4O--|  |
 |---- O8           |  |
 |  |               |  |
 |  |               |  |

     ----------         -----------  ----------         -----------
 0   90     180         270     180  90     180         270     180  90
 1---3---7---2---6---5---4---8---1---3---7---2---6---5---4---8---1---3---7
         0      180  90     180         270     180  90     180         270
         ----------  ----------         -----------  ----------

90,270 pair spacing at second Y

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both 180 and 360 degree headers have pulses hitting the collectors at four 
even 180 degree intervals 

a 270 degree header would have pulses hitting the collector at 180, 180, 270, 
and 90 degrees. (?)  Sharing a common collector won't work well.  That's why 
tri-Ys merge the paired pipes together ahead of the collector; to reduce 
interference from the odd pipe.

it looks like a large collector would work well with a tri-Y