[DeTomaso] X vs.H pipe commentary . . .

[Daniel C Jones]

In Vizard's testing, a properly position cross-over increased power most
of the time and decreased noise nearly all of the time.

> 2) the benefit varies with the size of the crossover, the size of the
> exhaust pipe and the position of the crossover fore or aft in the exhaust
> pipes south of the collector

The length and, too a lesser degree, the diameter of the pipe were relatively
unimportant.  Vizard has pictures of a very long cross-over that was tested
on the dyno.  It went up and over the dyno and back down the other side and
worked fine.  The location of the cross-over is much more important.  H-pipes
need to located near the end of the collector while X-pipes need to half-way
between the end of the primary pipes and the tail pipe end.

Vizard notes that cross-overs work for both open headers and configurations
with mufflers but the mechanism changes depending upon whether or not
mufflers (backpressure) are present.  With mufflers present, the cross-over
permits sharing of the mufflers by each power pulse, reducing the apparent
muffler restriction.  Vizard's testing indicates a cross-over can increase
apparent muffler flow by around 25%.  When headers are open to atmosphere,
little actual flow is shared and the flow in the cross-over moves only a
few inches back and forth.  In that case, the cross-over provides a conduit
for pressure waves.

> he also saw no real differences between an X pipe and an H pipe

In "Performance with Economy" he tested 3 different designs.  The simple
H-pipe was the least effective.  A hybrid X-pipe (standard pipes in parallel
with a very long X-pipe cross-over) was most effective.

Vizard's most recent header article had a favorable mention of the PipeMax
exhaust design software.  I picked up a copy of the software and it predicts
the best and worst header dimensions (and overall exhaust lengths from
collector to tail pipe tips) for reversion for multiple harmonics.  While
the length indicated by a particular harmonic may be best, physical constraints
may prevent that length so another may be chosen.  The software also outputs
the best placement of h and x-pipes (very different locations).  Below, I've
attached a sample output of PipeMax.  The particular output is for a Longchamp
owner building a custom exhaust for his car.

My dyno testing suggests muffler design can have a significant impact on
power.  Some muffler designs were down 50 HP (on a 400 HP street engine)
compared to other more efficient designs (e.g. Magnaflow straight through
mufflers).  Vizard recommends mufflers flow 2.2+ CFM per HP (the point at
which there is a 1% loss in power between mufflers and open pipes) but
be aware that going too large on mufflers increases noise.  Also, the
apparent collector length of a header is a function of the design of the
muffler (whether it acts as a chamber termination or an extension of the
collector).

Dan Jones

1. Street 4-into-1 with mufflers

  --- Single Primary Pipe Specs --- for 408.179 CID from 3500 to 6000 RPM
Diameter= 1.602  Length= 31.4 to 34.2   also  21.7 to 24.5 inches long
         Note> Maximum Primary's Tuned Length= 52.9 to 55.7 inches long

  --- Header Collector Specs  (Conventional Straight Tube) ---
Diameter= 2.957  Tuned Lengths= 19.0 best and 9.5 or 38.0

H-Pipe= 19.0   X-Pipe= 76.0  distance behind end of Primary Tube ends

 -- Total Exhaust System Tuned Lengths (Primary ends to TailPipe end) --
Best HP/TQ Tuned Collector Lengths= 19.0 , 38.0 , 76.0 , 152.1 inches long

Worst HP/TQ Loss Collector Lengths= 28.5 , 57.0 , 114.1 , 228.1 inches long

Note=> measured from where the Primary Pipes end inside the Collector to
      the point the tailpipe exits into the atmosphere.
      H-Pipe is located at end of Collector,   and X-Pipe is located
      usually half-way between end of Primary Pipes -to- tailpipe end

Note-> all Pipe Diameters are OD and based-off .0625 inch Pipe thickness

 ---- Primary Pipe's Harmonics ----
1st Harmonic = 140.5 inches long  ... typically never used
2nd Harmonic = 52.9 inches long  ... longest recommended
3rd Harmonic = 31.4 inches long  ... highly recommended , best Torque Curve
4th Harmonic = 21.7 inches long  ... shortest recommended
5th Harmonic = 16.2 inches long  ... typically never used
6th Harmonic = 12.6 inches long  ... typically never used
7th Harmonic = 10.1 inches long  ... typically never used
8th Harmonic = 8.3 inches long  ... typically never used

 ---- Collector's Harmonics (includes Intermediate, Muffler , TailPipe) ----
1st Harmonic = 152.1 inches long  ... longest with Mufflers and TailPipes
2nd Harmonic = 76.0 inches long  ... longest recommended with Mufflers
3rd Harmonic = 38.0 inches long  ... more bottom-end Torque
4th Harmonic = 19.0 inches long  ... highly recommended , best Torque Curve
5th Harmonic = 9.5 inches long  ... reduced Torque , more top-end HP sometimes
6th Harmonic = 4.8 inches long  ... reduced Torque , not recommended

2. Street Tri-Y with mufflers

 --- Tri-Y Header Design Specs --- for 408.179 CID from 3500 to 6000 RPM

1st Y-Segment Dia.= 1.727  Length= 15.7 to 17.1 inches long
2nd Y-Segment Dia.= 1.852  Length= 15.7 to 17.1 inches long

  --- Header Collector Specs  (Conventional Straight Tube) ---
Diameter= 2.707 to 2.957  Tuned Lengths= 19.0 best and 9.5 or 38.0

H-Pipe= 19.0   X-Pipe= 76.0  distance behind end of Primary Tube ends

 -- Total Exhaust System Tuned Lengths (Primary ends to TailPipe end) --
Best HP/TQ Tuned Collector Lengths= 19.0 , 38.0 , 76.0 , 152.1 inches long

Worst HP/TQ Loss Collector Lengths= 28.5 , 57.0 , 114.1 , 228.1 inches long

Note=> measured from where the Primary Pipes end inside the Collector to
      the point the tailpipe exits into the atmosphere.

Note-> all Pipe Diameters are OD and based-off .0625 inch Pipe thickness

 ---- Primary Pipe's Harmonics ----
3rd Harmonic = 31.4 inches long, both Y-Segments combined Length, best TQ + HP

 ---- Collector's Harmonics (includes Intermediate, Muffler , TailPipe) ----
1st Harmonic = 152.1 inches long  ... longest with Mufflers and TailPipes
2nd Harmonic = 76.0 inches long  ... longest recommended with Mufflers
3rd Harmonic = 38.0 inches long  ... more bottom-end Torque
4th Harmonic = 19.0 inches long  ... highly recommended , best Torque Curve
5th Harmonic = 9.5 inches long  ... reduced Torque , more top-end HP sometimes
6th Harmonic = 4.8 inches long  ... reduced Torque , not recommended