I came across this article on line which is interesting...

https://www.clown-shoe.com/single-post/2015/06/16/What-size-exhaust-is-the-right-size
But basically its this......

In the calculations I'll be doing below, I'll be assuming that we are using standard 16 gauge galvanized steel. This has a typical thickness of .065". This is important, because the size of the tubing is outer diameter and we need to do our calculations on inner diameter to be the most accurate.

Thus you can calculate the max HP supported by a 2" diameter exhaust tube through the following calculations:

Find Radius (D/2 = r)

2"/2 = 1" radius

Account of thinckness of the tube wall (r - .065)

1 - .065 = .935"

Find Area (r^2 * Pi)

.935^2 * 3.14 = 2.75 ci

Find Max CFM (Area * 115)

2.75 * 115 = 316.25

Calculate Max HP given Flow Rate (Max CFM / 2.2)

316.25 / 2.2 = 143.75 HP

So here's the estimates:

note: I used Excel to do the calculations here. That's why the rounding is different than in my example above.

Tube OD Tube Radius Tube ID Radius Tube Area Max CFM Max HP

2.00 1.00 0.94 2.75 315.84 143.56

2.25 1.13 1.06 3.53 405.94 184.52

2.50 1.25 1.19 4.41 507.32 230.60

3.00 1.50 1.44 6.47 743.96 338.17

3.50 1.75 1.69 8.92 1025.76 466.26

4.00 2.00 1.94 11.76 1352.73 614.88

So here is an example: Let us assume we have a motor making 400 flywheel horsepower and we'd like a single pipe exhaust.

A 3" single exhaust is good for about 340 crank horsepower. Our motor is 400 hp at the flywheel, we need an exhaust size larger than 3".

Now I'm not saying that 4" would work fine, either. 4" tubing supports much more power than we are planning to make. While the flow would be adequate, the velocity would be too low because the exhaust pressure would be diminished.

Thus, the rule of thumb with the calculations is to use the smallest tube diameter that supports the horsepower you are planning on making.