I am just making sure my calculations are correct and my thinking.

4in Pipe Volume @ 1in long = 3.14(r2)h = 3.14(4)(1) = 12.56 volume

3in Pipe Volume @ 1in long = 3.14(r2)h = 3.14(2.25)(1) = 7.065 volume

2.5in Pipe Volume @ 1in long = 3.14(r2)h = 3.14(1.5625)(1) = 4.90625 volume

So what i am trying to do is go from 4in off my turbo to (2) 2.5in pipes with cats then back to the 4in pipe

So a 4in pipe has 12.56 of volume and (2) 2.5in pipes has 9.8125 of volume. This is 2.7475 smaller than the 4in pipe.

But this will still be 2.7475 bigger than just a single 3in pipe that i currently have.


I just wanted to make sure my math was right because it just doesn't sound right in my head. I would have thought that two 2.5in pipe would have been bigger than a single 4in.

 

Don't think your math is right. You have the right formula 2r*pi. Or diameter*pi. You did the 4" equation correct but used r*pi for the other two.

4" = 12.56
3" = 9.42
2.5" = 7.85 per pipe 15.7 total

 

This is the break down of how i did it

3in = 3.14(r2)h = 3.14(1.5x1.5)(1) = 3.14(2.25)(1) = 7.065

2.5in = 3.14(r2)h = 3.14(1.25x1.25)(1) = 3.14(1.5625)(1) =4.90625 X 2 = 9.8125

The reason I think you got 9.42 for 3" and 7.85 for 2.5" is because you didn't square the radius. 1.25(Squared)= 1.5625 and 1.25(Times 2)= 2.5

Am i right?

 

Ok! Yeah, I was thinking of the formula for circumference. You were right.

 

Is there some reason you are calculating volume rather than area? If you were intending to calculate area in order to match exhaust gas velocity, then this should help:

The area of a 4" inside diameter pipe is A = pi(r)^2 = 3.14159(2)^2 = 12.56636 sq inches. The equivalent area of 2 pipes is r = 2 * sqrt(A / 2pi) = 2 * sqrt(12.56636 / 6.28318) = 2.82843 inches inside diameter. Therefore, the closest equivalent to one 4" pipe is two 3" pipes.

 

Why go through all of the fabrication work of putting a dual 2.5" cat into a 4" exhaust?

Why not fab up a 4" cat?

 

The main reason is a Super flowing exhuast for my HBP GT42 turbo and as quite as a mostly stock FD.