Removing Cats

["Paul C. Waterloo" <74543.407@compuserve.com>]

Eric wrote:

Please explain how I'm loosing HP on my car if I'm decreasing 
back pressure, Increasing Exhaust Flow, Getting the heat out and allowing 
the turbo to spinup faster..... My exhaust emisions are LOWER than the 
state mandated Mins in EVERY State In fact for the most part the exhaust 
gases are as clean as the air that I'm pumping it into.

REPLY (how come I know I am going to get slammed for this?  It doesn't matter if
you are right or wrong, they slam you -- I can take it!)

One of my theories is that you will now have turbulent flow (not laminar flow)
in the cat after removing the material.  The honeycombs provid adequate
passageways for the exhaust to flow in with minimal head loss (friction loss). 

When it was originally engineered, the friction loss coefficient of each section
of the exhaust was calculated (remember, this is MY THEORY).  They also wanted
to maintain air flow velocities and pressures in each section of the exhaust
system, so they designed each section with a given cross section, based on head
loss (friction loss) for that part of the system.  That is why a cat has a much
larger cross section than the rest of the exhaust.  Same with mufflers.

Now if you remove the material from it, as the exhaust flow enters it, it will
spill into a large chamber with very different head loss characteristics as
compared to one with the honeycomb in it.  This will create exhaust SWIRL, there
will be changing velocities all over the place, and the exhaust will not flow
smoothly into the exhaust pipe downstream.  Additionally, the pressure will
decrease in the cat, and then have to be increased again going into the exhaust
pipe downstream due to changes in cross sections.

This creates a lot of exhaust swirl that will have negative effects.  This is
the exact reason why porting and polishing heads, intake and exhaust manifolds
have such positive effects, because the remove the effects of airflow swirl.

This information is based on my real life job.  I work for a company that repair
high energy pumps for utilities.  We concentrate heavily on fluid hydraulics and
there effects on pumps.  They affect the damn things drastically!

Now everybody can slam me!

Paul Waterloo

'87 5KSQ
'95 Subaru Legacy Wagon
'75 Triumph TR6