Ok...so here's a prime example of why the CFM ratings on mufflers don't really
mean a whole lot, but do help. (Comments like: "A Straight through will
always flow better than those crappy chambered mufflers." or "If I can't see
through it, it won't flow as well." - I've heard that from quite a few people
at meets.)
The CFM ratings are done on a flowbench...that also checks the CFM ratings of
Carburetors and Throttle Bodies...on a steady stream. Since exhaust gases are
not a simple air stream going out the pipes (which a lot of us already know)
the CFM ratings don't really mean a whole lot. Being able to scavenge the
gasses and keep them flowing out the pipes is what's really needed. This is
the reason for chambered mufflers. (Flowmaster, Ravin, and a few other "no-
names".)
This isn't saying anything bad about the packed, straight through, perforated
core mufflers...they do work better than stock, but what happens in the muffler
when the packing is full of pressure and the exhaust gases (being forced
through the muffler) are "backed up" due to uneven pressure in the straight
tube?
- Bernd
>
> The Physics
>
> It is valuable to keep in mind what is really going on in exhaust systems
> in a modern
> high-speed engine. Don't think of a continuous stream of exhausts gases
> flowing
> smoothly out the pipes; think of a complex fluttering, pulsing, resonating
> system in
> which the overall movement is hugely outbound, but where exhaust gases
> start and
> stop rapidly, bumping into each over, with pressure waves and zones
> continuously
> reflecting off each other and portions of the exhaust system.
>
> If thought of in this way, it is easy to imagine why, by some miracle, at
> certain speeds
> for an unexpectedly good air flow when everything resonates together just
> right whereas
> at other speeds, the exhaust system is, to some degree, fighting
> itself. Predicting the
> complex, interacting behavior of an exhaust system is too complicated for
> easy modeling.
>
<snip>
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