Side Pipe SPL

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since you are playing with exhaust length numbers, here is something that might interest you, it is probably very similiar to your 427. i've read of some significant differences in hp with exhaust length.

--- Single Primary Pipe Specs --- for 427.650 CID from 3500 to 6000 RPM
Diameter= 1.722 to 1.847 Length= 39.4 to 42.2 inches long
--- 2-Step Primary Pipe Specs ---
1st Dia. inches= 1.722 Length= 19.7 to 21.1
2nd Dia. inches= 1.847 Length= 19.7 to 21.1
--- 3-Step Primary Pipe Specs ---
1st Dia. inches= 1.722 Length= 19.7 to 21.1
2nd Dia. inches= 1.847 Length= 9.9 to 10.6
3rd Dia. inches= 1.972 Length= 9.9 to 10.6

--- Header Collector Specs (Conventional Straight Tube) ---
Diameter= 3.163 to 3.413 Tuned Lengths= 21.5 best and also 10.8 or 43.0

--- Header Collector Specs (Megaphone or Diffuser Cone Shape) ---
Diameter= 2.663 taper to 3.663 Megaphone/Diffuser Length= 21.5 inches

Best HP/TQ Tuned Collector Lengths= 21.5 , 43.0 , 86.0 , 172.0 inches long

Worst HP/TQ Loss Collector Lengths= 32.3 , 64.5 , 129.0 , 258.0 inches long

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

---- Primary Pipe's Harmonics ----
1st Harmonic = 170.7 inches long ... typically never used
2nd Harmonic = 65.3 inches long ... longest recommended
3rd Harmonic = 39.4 inches long ... highly recommended , best Torque Curve
4th Harmonic = 27.8 inches long ... shortest recommended
5th Harmonic = 21.1 inches long ... typically never used
6th Harmonic = 16.8 inches long ... typically never used
7th Harmonic = 13.8 inches long ... typically never used
8th Harmonic = 11.6 inches long ... typically never used

---- Collector's Harmonics (includes Intermediate, Muffler , TailPipe) ----
1st Harmonic = 172.0 inches long ... longest with Mufflers and TailPipes
2nd Harmonic = 86.0 inches long ... longest recommended with Mufflers
3rd Harmonic = 43.0 inches long ... more bottom-end Torque
4th Harmonic = 21.5 inches long ... highly recommended , best Torque Curve
5th Harmonic = 10.8 inches long ... reduced Torque , more top-end HP sometimes
6th Harmonic = 5.4 inches long ... reduced Torque , not recommended

the collector measurement is from the inside merge of the 4 pipes to the exit.