I'm planning the build on 2 motors to be built soon, I'm doing some homework on primary tube size. I would like to hear some thoughts. 1st motor just a mild Lake motor 500hp
2nd motor a reliable 625~650 for a single Class 6 boat. I am mainly looking for a flat torque for the rough/choppy races so I'm seeing a few tube sizes available
Thanks

1 3/4" will be fine for both. Same with 2".

Nit picking the 1 3/4" will be better for the 500hp and the 2" for the 625hp.

That said, collector sizing and length (which includes all the tubing going out the boat) is where real power can help or hurt.

Edit in: ,make sure your primary entrances doesn't block off any part of the heads exh port exit. Amazes me no one does this with boats...it seams like.

FYI: The Thunder and Lightnings (2" x 32" primaries) from Diamond Marine will pretty much kick azz...in both motors, especially the 625hp one

The following is from inputs I entered on a 502 with 625 or so HP.

Quoting myself from a earlier post

BTW: The program givea much, much more info than I have posted below. But it will give you a good idea on things.

Remember, the 'collector' is not just the headers collector. If risers and tails are same ID, which most re, then they too are part of the collector.

Do not let yourself end up at a worst length collector. It can/will kill performance.


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


502.176 CID from 3800 to 6300 RPM
--- Single Primary Pipe Specs --- Diameter= 1.899 to 2.024 Length= 33.7 to 36.7 inches long ---

2-Step Primary Pipe Specs ---
1st Dia. inches= 1.899 Length= 16.9 to 18.4
2nd Dia. inches= 2.024 Length= 16.9 to 18.4
--- 3-Step Primary Pipe Specs ---
1st Dia. inches= 1.899 Length= 16.9 to 18.4
2nd Dia. inches= 2.024 Length= 8.4 to 9.2
3rd Dia. inches= 2.149 Length= 8.4 to 9.2

--- Header Collector Specs (Conventional Straight Tube) ---
Diameter= 3.466 to 3.716 Tuned Lengths= 19.4 best and also 9.7 or 38.8

--- Header Collector Specs (Megaphone or Diffuser Cone Shape) ---
Diameter= 2.966 taper to 3.966 Megaphone/Diffuser Length= 19.4 inches

Best HP/TQ Tuned Collector Lengths= 19.4 , 38.8 , 77.6 , 155.2 inches long
Worst HP/TQ Loss Collector Lengths= 29.1 , 58.2 , 116.4 , 232.8 inches long

Note-> all Pipe Diameters are OD and based-off .0625 inch Pipe thickness
---- Primary Pipe's Harmonics ----
1st Harmonic = 149.4 inches long ... typically never used
2nd Harmonic = 56.5 inches long ... longest recommended
3rd Harmonic = 33.7 inches long ... highly recommended , best Torque Curve
4th Harmonic = 23.4 inches long ... shortest recommended
5th Harmonic = 17.5 inches long ... typically never used
6th Harmonic = 13.8 inches long ... typically never used
7th Harmonic = 11.1 inches long ... typically never used
8th Harmonic = 9.2 inches long ... typically never used

---- Collector's Harmonics (includes Intermediate, Muffler , TailPipe) ----
1st Harmonic = 155.2 inches long ... longest with Mufflers and TailPipes
Note: Worst = 116.4 inches long for HP/TQ Loss
2nd Harmonic = 77.6 inches long ... longest recommended with Mufflers
Note: Worst = 58.2 inches long for HP/TQ Loss
3rd Harmonic = 38.8 inches long ... more bottom-end Torque
Note: Worst = 29.1 inches long for HP/TQ Loss
4th Harmonic = 19.4 inches long ... highly recommended , best Torque Curve
5th Harmonic = 9.7 inches long ... reduced Torque , more top-end HP sometimes
6th Harmonic = 4.9 inches long ... reduced Torque , not recommended

With your combination, my input is to get the standard CMI E-tops. Any more efficient products in the CMI product line could add more bling, but won't add much if any more HP under 6000 RPM.

As Sb said the Thunder and lightnings from Bill will give both motors the best performance and they won't crack and destroy them. So it's a win win...