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


The "pinch" will cause a flow restriction at that point. That will cause the
upstream pressure to increase. The flow velocity across the pinch area will
then increase, at that point. If you measure the pressure with a mercury
manometer or gauges above, at the smallest diameter and down stream you will
note the pressure differences. If you have a length of pipe the same diameter
as the initial pipe down stream of the pinch or venturi, the pressure will
return to close to the upstream pressure. This is Bernoulli's law. When the
gas or liquid gets to the end of the pipe as in exhaust, it will resume to be
at atmospheric pressure as it is no longer contained. The fluid inside the
pipe has friction with the inside of the pipe which resists flow. This causes
a back pressure even in a pipe without a venturi, if the pipe is long enough.
There are headers that use a venturi just downstream of the collector to
increase the power.
Pipelines require compressor stations every 100 miles due to the friction
with the pipe or the flow would stop.

Dale Davies

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