Muffler replacement huge horsepower gains!

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here's a template for an exhaust system for a 427 with 10.5 comp and mild cam.

--- Single Primary Pipe Specs --- for 427.650 CID from 4600 to 7100 RPM
Diameter= 1.873 to 1.998 Length= 29.6 to 32.4 inches long
--- 2-Step Primary Pipe Specs ---
1st Dia. inches= 1.873 Length= 14.8 to 16.2
2nd Dia. inches= 1.998 Length= 14.8 to 16.2
--- 3-Step Primary Pipe Specs ---
1st Dia. inches= 1.873 Length= 14.8 to 16.2
2nd Dia. inches= 1.998 Length= 7.4 to 8.1
3rd Dia. inches= 2.123 Length= 7.4 to 8.1

--- Header Collector Specs (Conventional Straight Tube) ---
Diameter= 3.422 to 3.672 Tuned Lengths= 17.1 best and also 8.6 or 34.2

--- Header Collector Specs (Megaphone or Diffuser Cone Shape) ---
Diameter= 2.922 taper to 3.922 Megaphone/Diffuser Length= 17.1 inches

Best HP/TQ Tuned Collector Lengths= 17.1 , 34.2 , 68.4 , 136.9 inches long

Worst HP/TQ Loss Collector Lengths= 25.7 , 51.3 , 102.7 , 205.3 inches long

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

---- Primary Pipe's Harmonics ----
1st Harmonic = 130.7 inches long ... typically never used
2nd Harmonic = 49.5 inches long ... longest recommended
3rd Harmonic = 29.6 inches long ... highly recommended , best Torque Curve
4th Harmonic = 20.6 inches long ... shortest recommended
5th Harmonic = 15.5 inches long ... typically never used
6th Harmonic = 12.2 inches long ... typically never used
7th Harmonic = 9.9 inches long ... typically never used
8th Harmonic = 8.2 inches long ... typically never used

---- Collector's Harmonics (includes Intermediate, Muffler , TailPipe) ----
1st Harmonic = 136.9 inches long ... longest with Mufflers and TailPipes
2nd Harmonic = 68.4 inches long ... longest recommended with Mufflers
3rd Harmonic = 34.2 inches long ... more bottom-end Torque
4th Harmonic = 17.1 inches long ... highly recommended , best Torque Curve
5th Harmonic = 8.6 inches long ... reduced Torque , more top-end HP sometimes
6th Harmonic = 4.3 inches long ... reduced Torque , not recommended

as you can see it is based on harmonics.