Andrew-this is turning into a great talk! Okay here's some rules for
teh gas flow stuff, but I only know the ones for steady flow, not
unsteady, pulsing flow. The average velocity at any point of the pipe =
the mass flow rate of the fluid divided by the cross sectional area at
that point, assuming no leaks out of the pipe. I say AVERAGE velocity
beacuse the fluid travels slower at the wall than at the center, like
this(hope it's readable). Y axis is velocity, X is the measurement
across the pipe. Using some tables for round pipe, there is a bit of
added benefit to the larger pipe, based on the "area under the curve" of
the following graph.
v .
e .
l .
o .
c .
i .
t .
y .
.
-------------------------------
Pipe pipe
center wall
So, back to your question: If you have a very large area, like a cat or
big muffler, average velocity through them is very low. As soon as the
fluid leaves the muffler, it speeds back up to exactly the same speed it
entered the muffler at (closed system - no leaks on our Daks, right?)
This accel/decel takes energy to do and robs power. Smoother transitions
help quite a bit, so why don't we use them? I guess we do on header
collectors, but not many other places that I can see. The rules for
round pipe no longer hold for these devices, but you use a adjusting
constant (based on lab flow tests) to correct for them.
Intuitively, I don't think we should be reducing our pipe diameters, but
maybe we should be doing something with the transitions in and out of
our cats and mufflers... Hmmmmmmm...
Cale
Aajaynefour@cs.com wrote:
>
> I appreciate what happens regarding velocity in a "perfect" exhaust
> situation. This would be unobstructed pipes with no cats and/or mufflers.
> Not exactly what we got here. My question is this: How much does the cat
> and/or the muffler affect the velocity of the exhaust gases and to what
> extent do the rules of pipe diameter still apply? Should we all be using
> smaller diameter pipes than we are to take into account the already
> slowed-down exhaust gas pulses?
>
> My head is not exploding. I love these technical discussions. Thanks Cale.
> Andrew
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