Second picture
 My new coax muffler.

 Brian

 Looks like the holes in the inner pipe need to be larger
 towards the aft end.

 Paul Lamar

 It's a little too late for that, I already welded the assembly together.
 But, I did pinch in the exit end slightly to increase the pressure toward
 the exit end of the pipe- it's not a huge obstruction, but enough to have
 an effect,I hope.

 Brian Trubee


You say you pinched the exit to increase the pressure towards
the end of the pipe. Now,  I'm not an expert but I'm pretty
sure that pressure decreases in a venturi, so you would be
lowering the pressure. I can't see the pictures though so
I'm not entirely sure.

Wesley B.

That is true but take it to the logical conclusion or limit.
What happens if the inner pipe is completely blocked off?
Obviously the pressure in the inner pipe gets higher. That
is why they call it exhaust back pressure.

When does the pressure change from lower to higher?

The answer is the pressure in  the pipe is higher than
the outside pressure to begin with.

The venturi principle assumes the pressure inside and outside
the pipe are the same to begin with.

Paul Lamar

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