H-Pipe Equalizers
The firing order of all production V8’s, regardless of make, has one cylinder in each bank that will fire within 90 degrees of crankshaft rotation of another cylinder in the same bank.  This occurs twice during completion of the entire firing order.
 
Mustang 4.6L V8
Firing order = 1-3-7-2-6-5-4-8
 
Rear
48
37
26
15
Front
 
The 1-3 and the 6-5 cylinders are on the same bank.   When they fire, the two cylinders exhaust almost simultaneously into the same manifold and fight each other for space in the collector and exhaust pipe.   While the two cylinders are firing, there is no activity in the opposite bank.   The H-Pipe equalizer allows some of the excess pressure on one side to bleed over the quiet side of the exhaust system, resulting in smoother exhaust flow.
None of the above applies to the Mustang 3.8 L V6.
 
Mustang 3.8L V6
Firing order = 1-4-2-5-3-6
 
Rear
36
25
14
Front

 
In every case, when one cylinder fires, the next one to fire is on the opposite bank.  There is no need for an H-pipe equalizer on a Mustang V6.
 
 
Then why do some people still recommend including an H-pipe in their V6 exhaust system?   Here are the arguments I've heard.
 
  • The H-pipe produces a venturi effect.  As each exhaust pulse shoots past the equalizer tube it creates a slight vacuum and a pressure drop on the other side of the tube.   In an even-fire V6 engine, this occurs just as the next exhaust pulse leaves the opposite cylinder bank.   As the pressure drop alternates from side to side it reduces back pressure and allows the exhaust gases to travel more freely through the system.   [An interesting theory, but not one, so far as I know, advanced by any manufacturer of   H- or X-pipes. ]
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  • No exhaust manifold flows perfectly. Even with the V6's even firing sequence, there will still be momentary pressure imbalances between the two banks of cylinders.  [Probably true]
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  • The H-pipe produces a better exhaust sound than straight pipes. [?]
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  • The H-pipe will eliminate the crackle-pop sounds you sometimes get with dual exhaust.  [My system doesn't crackle-pop.]
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    The choice is yours.   I went with straight pipes.